Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
1
Garnet, corundum and other gem
minerals from Umba, Tanzania
P. C. Zwaan
Zwaan, P. C. Garnet, corundum and other gem minerals from Umba, Tanzania.
Scripta Geol., 20 : 1-41, 24 figs., 10 tables, Leiden, February 1974.
Both rhodolite and almandine garnets occur in Umba. The rhodolites have no
special properties by which they can be distinguished from other garnets in the
pyrope-almandine series, except their rose-red colour. Hence a distinctive name
is unnecessary. Rutile appears to be the most frequent mineral inclusion in
rhodolite, whereas in almandine both apatite and rutile are common. Corundums
of different colours occur. Green corundums owe their colour to iron, while
blue corundums have a high Ti and a low Cr content. Common inclusions are
rutile, pyrrhotite and apatite, whereas graphite especially is found in deep violet
corundums. The inclusions in both the garnets and the corundums are not
characteristic of this locality. The properties of emerald green and orange-brown
tourmalines are given, as well as those of brownish orthopyroxenes, a light green
clinopyroxene, yellow scapolite, reddish brown zircon and turquoise.
P. C. Zwaan, Rijksmuseum van Geologie en Mineralogie, Hooglandse Kerkgracht
17, Leiden, The Netherlands.
Introduction
2
R h o d o l i t e garnet
4
Previous work
4
Properties
5
Inclusions
9
A l m a n d i n e garnet
12
Properties
12
Inclusions
15
Corundum
19
Varieties
19
Properties
21
Inclusions
26
Conclusions
29
Tourmaline
30
2
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Varieties
30
Conclusions
32
O t h e r g e m minerals
32
Pyroxene
32
Scapolite
35
Zircon
36
Turquoise
37
A s s o c i a t e d minerals
Amphibole
38
38
Kyanite
38
Vermiculite
39
Summary
39
References
40
Introduction
T h e m a t e r i a l described here originates f r o m the U m b a g e m m i n e , a s m a l l l o c a t i o n
i n the jungle, situated i n the northeastern part of T a n z a n i a o n the K e n y a b o r d e r .
T h e name of the m i n e is t a k e n f r o m the U m b a river, f l o w i n g i n the area. T h e m i n e
produces a n u m b e r of minerals of g e m q u a l i t y , especially garnet, c o r u n d u m a n d
t o u r m a l i n e . A c c o r d i n g to Solesbury (1967) these minerals o c c u r i n m e t a m o r p h i c
r o c k s of P r e c a m b r i a n age.
Since 1967 a n u m b e r of papers describing minerals f r o m the U m b a area
have been p u b l i s h e d . T h e area, therefore, is w e l l k n o w n n o w to m a n y mineralogists
a n d gemmologists.
D u r i n g a visit to the m i n e i n J u l y 1967, the author was given a n u m b e r of
m i n e r a l specimens, m a i n l y pebbles, for scientific research. T h e author also c o l l e c t e d
garnets a n d c o r u n d u m s , b o t h crystals a n d fragments as w e l l as pebbles, a l l f r o m
secondary deposits.
P r o d u c t i o n at the m i n e is f r o m o p e n pits, d r i l l e d a n d dug to a depth of
about 15 m i n v e r m i c u l i t e pegmatites a n d a m p h i b o l e gneisses.
I n c o n n e c t i o n w i t h theft of m a t e r i a l at the m i n e , a c o m m o n p r o b l e m at s m a l l
gem w o r k i n g s , the manager was anxious to k n o w whether the minerals o c c u r r i n g
at the U m b a m i n e h a d properties that were characteristic of the locality. F r o m
the scientific p o i n t of v i e w the minerals are very interesting indeed. T h e garnets
are b o t h almandines a n d very nice s o - c a l l e d rhodolites. T h e c o r u n d u m s have
different c o l o u r s . Besides r e d a n d blue there are deep violet a n d p a r t i - c o l o u r e d
stones, such as b l u e w i t h y e l l o w , r e d w i t h blue, a n d red w i t h green. O r a n g e c o l o u r e d c o r u n d u m s are also f o u n d , l o c a l l y n a m e d " g o l d e n sapphire". T h e
tourmalines are characterized b y their very nice emerald-green c o l o u r , although
some are y e l l o w i s h b r o w n a n d orange.
O n a s m a l l scale some other minerals are f o u n d : a b r o w n i s h o r t h o p y r o x e n e ,
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
a green c l i n o p y r o x e n e , y e l l o w scapolite, turquoise a n d z i r c o n . T h e y a l l are m o r e
or less of gem quality.
T h i s paper w i l l l o o k into the reasons for the c o l o u r of the t o u r m a l i n e a n d
the c o r u n d u m , a n d consider whether a special name for the " r h o d o l i t e " garnets
is necessary.
A l l specimens as w e l l as the X - r a y p o w d e r photographs are stored a n d
registered i n the R i j k s m u s e u m v a n G e o l o g i e en M i n e r a l o g i e , L e i d e n (numbers
prefixed w i t h R G M ) .
T h e m i n e r a l s have been studied b y o p t i c a l methods a n d b y X - r a y diffraction.
T h e specimens have been p r o v i d e d w i t h one, o r often t w o p o l i s h e d flat faces
to facilitate the o p t i c a l e x a m i n a t i o n a n d to measure the refractive indices o n a
gemstone refractometer. F o r this purpose a R a y n e r standard refractometer was
most often used i n c o m b i n a t i o n w i t h s o d i u m light. I n some cases a refractometer
p r o v i d e d w i t h a d i a m o n d p r i s m was a p p l i e d . T h e m i c r o s c o p i c e x a m i n a t i o n was
d o n e using methylene i o d i d e a n d m o n o b r o m o n a p h t a l e n e as i m m e r s i o n l i q u i d s .
T h i s p r o c e d u r e was also suitable to m a k e p h o t o m i c r o g r a p h s of the inclusions.
T h e a b s o r p t i o n spectra were observed w i t h a R a y n e r p r i s m spectroscope
a n d measured b y means of a H a r t r i d g e R e v e r s i o n Spectroscope. T h e a c c u r a c y of
the last-named instrument is not very great for b r o a d bands, a n d the readings m a y
be i n c o r r e c t up to 10 A .
Specific gravity measurements were c a r r i e d out b y means of a hydrostatic
balance using ethylene d i b r o m i d e as an i m m e r s i o n l i q u i d a n d coils of silver w i r e
to h o l d the specimens.
T h e X - r a y investigation was done b y means of p o w d e r photographs, using
F e - r a d i a t i o n a n d a D e b y e - S c h e r r e r c a m e r a w i t h a diameter of 114.6 m m .
Identification of i n c l u s i o n s was m a d e b y means of X - r a y p o w d e r p h o t o ­
graphs. U s e was m a d e of the s o - c a l l e d " s p h e r e " m e t h o d of H i e m s t r a (1956),
because o n l y very s m a l l amounts of m a t e r i a l were available. T h e samples were
o b t a i n e d b y scraping d o w n w i t h a steel needle part of a n i n c l u s i o n extending to
the surface of the host specimen. I n the past the author has used this m e t h o d w i t h
great succes to identify s o l i d i n c l u s i o n s i n C e y l o n minerals ( Z w a a n , 1965 a n d
1967).
T h e unit cell dimensions of the garnets were c a l c u l a t e d using a m e t h o d
described b y the author i n a paper d e a l i n g w i t h the identification of p y r o p e a l m a n d i n e garnets ( Z w a a n , 1961). T h i s m e t h o d is based u p o n the measurement
of the distance (e) i n m m between the c o r r e s p o n d i n g α !-lines of the reflections
10 4 0, 10 4 2 a n d 880 o n an X - r a y p o w d e r p h o t o g r a p h of the garnet. T h e
function between a a n d e (4 Θ w i t h the c a m e r a used) can be d e r i v e d f r o m the
f o r m u l a , to be used for m i n e r a l s crystallizing i n the c u b i c system:
λ
2
sin2 Θ =
(h2 +
k2 +
12)
4fl2
i n w h i c h : Θ is the g l a n c i n g angle,
λ is the wavelength of the used r a d i a t i o n ,
a is the length of the u n i t c e l l .
3
4
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
A
cknowledgements
T h e author is m u c h indebted to D r C . K i e f t of the W A C O M , a w o r k i n g group for
a n a l y t i c a l geochemistry s u b s i d i z e d b y the N e t h e r l a n d s O r g a n i s a t i o n for the A d vancement of P u r e R e s e a r c h ( Z W O ) at A m s t e r d a m for m i c r o p r o b e analyses of
eight garnets; to D r P . M a a s k a n t of the Institute of E a r t h Sciences of the F r e e
U n i v e r s i t y at A m s t e r d a m for the d e t e r m i n a t i o n of trace elements i n t w o c o r u n d u m s
a n d a t o u r m a l i n e a n d to M r K . M . Stephan of the " G e o l o g i s c h en M i n e r a l o g i s c h
Instituut" of L e i d e n U n i v e r s i t y for his a n a l y t i c a l w o r k o n three a l m a n d i n e garnets
to determine the F e / F e
ratio, o n a n o r t h o p y r o x e n e a n d o n a turquoise.
+ 2
+ 3
Rhodolite garnet
PREVIOUS WORK
T h e n a m e r h o d o l i t e was p r o p o s e d b y H i d d e n & P r a t t (1898) f o r a garnet w i t h a
"delicate rose-like c o l o u r " f r o m N o r t h C a r o l i n a . C h e m i c a l analyses of t w o samples
were g i v e n together w i t h the specific gravity determinations, 3.837 a n d 3.838.
These data i n d i c a t e d that the garnet c o n t a i n e d t w o molecules of p y r o p e o n one
m o l e c u l e of a l m a n d i n e .
T r u m p e r (1952) described this m a t e r i a l as a garnet w i t h a specific gravity
of 3.837 a n d a refractive i n d e x of 1.76, s h o w i n g the characteristic absorption
spectrum of a l m a n d i n e . H e also m e n t i o n e d a f i n d of r h o d o l i t e garnet i n G r e e n l a n d .
A c c o r d i n g to D e e r et a l . (1962) r h o d o l i t e is a r h o d o d e n d r o n p i n k variety of
p y r o p e w i t h M g : F e a p p r o x i m a t e l y 2: 1, k n o w n f r o m N o r t h C a r o l i n a a n d elsewhere. D a t a reported b y W e b s t e r (1972) are very s i m i l a r .
Objections against the name are raised, amongst others, b y A n d e r s o n (1959)
a n d C a m p b e l l (1972), because it is so s i m i l a r i n s o u n d a n d spelling to that o f the
manganese silicate m i n e r a l r h o d o n i t e . C a m p b e l l suggests that the n a m e " r h o d o m a c o n " garnet be considered. T h e first s y l l a b l e relates to the c o l o u r a n d the second
one to the fact that this variety was first d i s c o v e r e d i n M a c o n C o u n t y , N o r t h
Carolina.
M a r t i n (1970) notes that the refractive indices of a n u m b e r of rhodolites
of u n k n o w n o r i g i n ( p r o b a b l y T a n z a n i a ) are w e l l b e l o w that of a specimen f r o m
N o r t h C a r o l i n a . T h e i r average refractive i n d e x is 1.747 w h i l e that of the " C a r o l i n a
stone" is 1.758. T h e average v a l u e is near the m i d d l e of the range for p y r o p e , yet
the stones are of a n entirely different c o l o u r (rose-red to pale v i o l e t ) . Spectroscopy
reveals the three m a i n a b s o r p t i o n bands of a l m a n d i n e , a n d these are of moderate
intensity.
I n a paper o n rose-red garnets f r o m T a n z a n i a , B a n k & N u b e r (1969) state
that since 1964 large numbers of garnets f r o m this area have been s o l d as r h o d o lites. M o s t of the specimens are r o u n d so that n o crystal forms c a n be recognized.
T h e m a t e r i a l is apparently f r o m secondary deposits. T h e refractive indices v a r y
f r o m 1.745 to 1.755; the densities f r o m 3.79 to 3.80. T h e unit c e l l d i m e n s i o n of
these rhodolites is 11.5034 ( ± 0.0003) A . C h e m i c a l e x a m i n a t i o n indicates that
these garnets have a h i g h F e content, little M n a n d traces of R b a n d P b . These
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
authors, too, c o n c l u d e that rhodolites are intermediate types i n the p y r o p e - a l m a n dine series.
C a m p b e l l (1972) made a c o m p a r a t i v e study of what has p r o v e d to be
r h o d o l i t e garnet of k n o w n R h o d e s i a n o r i g i n . H e e x a m i n e d 12 r o u g h stones h a v i n g
n o p a r t i c u l a r f o r m . T h e refractive indices measured are between 1.750 a n d 1.760,
w i t h an average of 1.755. T h e densities lie between 3.83 a n d 3.89, the average
being 3.85. A l l the specimens have an a l m a n d i n e a b s o r p t i o n spectrum, v a r y i n g i n
intensity f r o m very w e a k to m e d i u m . T h e refractive indices of another 5 0 cut
stones also v a r y f r o m 1.750 to 1.760. C o m p a r i n g these results w i t h the data i n the
literature C a m p b e l l notes that the o n l y difference between r h o d o l i t e garnet and
other garnets i n the p y r o p e - a l m a n d i n e series is " c o l o u r a n d c o l o u r a l o n e " .
PROPERTIES
T h e properties of 29 pebbles of r h o d o l i t e are g i v e n i n T a b l e 1 i n ascending o r d e r
of refractive i n d e x . A l l specimens have a pleasant rose-red c o l o u r .
F i r s t it is seen that the properties of these garnets v a r y w i d e l y . T h e lowest
figures for refractive i n d e x a n d specific gravity are f r o m one sample; the same is
true of the highest. A l i n e a r f u n c t i o n between these properties generally h o l d s
true, as was to be expected. A c o m p a r i s o n of these data w i t h those g i v e n b y
previous w o r k e r s indicates that most of t h e m are higher, although a l l lie i n between
the properties of p y r o p e a n d a l m a n d i n e towards the p y r o p e end-member.
F r o m the figures f o r the length of the unit c e l l it c a n be seen that there is a
v a r i a t i o n f r o m 11.497 to 11.532 A , w h i c h cannot be due s i m p l y to the substitution
of F e for M g , because there is no regularity. Samples w i t h l o w refractive i n d e x
a n d density m a y have larger unit cells t h a n samples w i t h higher properties a n d
vice versa. It is a n o b v i o u s fact that C a , a n d to a lesser extent M n , have a great
influence o n the size of the unit c e l l .
T o ascertain this, m i c r o p r o b e analyses of four specimens have been m a d e .
T h e samples selected were those w i t h the lowest a n d highest refractive i n d e x a n d
specific gravity, a n d those w i t h the smallest a n d greatest length of the unit c e l l .
T h e results are given i n T a b l e 2. These specimens appeared to be somewhat
inhomogeneous. V a r i a t i o n s i n the c h e m i c a l c o m p o s i t i o n to a m a x i m u m of relatively
10 per cent were observed. T h e analyses represent the average of three o r four
measurements per sample.
F i r s t it c a n be seen that the garnet w i t h the lowest refractive i n d e x a n d
density has the highest M g a n d the lowest F e content, w h i l e the one w i t h the
highest figures has the highest a l m a n d i n e a n d the lowest p y r o p e content. I n other
w o r d s , the ratio M g : F e c a n be d e r i v e d f r o m the properties m e n t i o n e d .
R e g a r d i n g the size of the unit c e l l it is seen that the r h o d o l i t e w i t h the
highest C a content ( R G M 163 138) has the largest a, w h i l e the one w i t h the
lowest a value ( R G M 163 166) has a l o w C a content i n c o m b i n a t i o n w i t h little
M n . T h e r h o d o l i t e w i t h the lowest C a content ( R G M 163 164) has a larger unit
c e l l , w h i c h p r o b a b l y is due to its higher M n content. T h u s , b o t h C a a n d M n m a y
affect the size of the u n i t cell to a large extent.
B o t h the T i a n d C r
nificant influence o n these
A n o t h e r observation
of these rhodolites have a
contents of these garnets are too l o w to have a sigproperties.
w o r t h m e n t i o n i n g is that a l l X - r a y p o w d e r photographs
pattern characteristic for p y r o p e , that is, almost equal
5
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
6
Table
1.
sample
Properties
no.
of
original
in
29
rhodolite
weight
garnets
from
D
η
Umba,
a in
S
Tanzania.
no.
X-ray
powder
photograph
carats
RGM
163 175
3 .56
1 .749
3 .790
11 .521
"
163 171
4 .65
1 .750
3 .827
11 .511
"
107 196
3 .61
1 .751
3 .809
11 . 5 0 4
t!
201
512
"
163 176
3 .62
1 .751
3 .828
11 . 5 1 4
tt
201
568
"
107 205
4 .08
1 .753
3 .831
11 . 4 9 9
II
201
520
"
107 209
4 .66
1 .753
3
851
11 . 5 1 0
II
201
566
"
163 163
1 .46
1 .753
3 .858
11 . 5 0 6
II
201
523
M
163
137
1 .42
1 .754
3
855
1 1. 5 2 5
11
201
518
"
163 166
1 .75
1 .756
3
861
11 . 4 9 7
II
201
526
"
163 167
2 .01
1 .756
3
863
11 .502
II
201
527
11
163
136
2 .90
1 .757
3
831
11 . 5 0 8
201
517
"
163 134
4
1 .757
3
852
11 .512
It
201
515
11
201
528
201
573
79
"
163 168
1 38
1 .757
3
854
11 .501
M
163
179
2 65
1 .758
3
847
11 . 5 1 8
RGM 201
f » 201
M
"
II
567
531
"
163 178
2 .40
1 .759
3
879
11 . 5 1 5
"
163 162
1 80
1 .759
3
883
11 .501
M
163
133
4
1 .760
3
849
11 . 5 2 0
"
163 132
3
00
1 .760
3
863
"
163 135
3
49
1 .760
3
865
"
163 182
2 04
1 .760
3
871
11 . 5 2 3
II
201
576
"
163 180
1 99
1 .760
3
872
1 1. 5 0 8
II
201
574
"
163 177
4 38
1 .761
3
869
11 . 5 2 3
M
201
569
M
163
181
2 02
1 .'762
3
880
11 . 5 2 4
II
201
575
"
163 138
1 42
1 762
3
883
11 .532
II
201
519
"
163 169
1 40
1 762
3
890
11 . 5 1 0
II
201
529
11
163 161
2 05
1
3 . 891
11 . 5 0 7
II
201
521
II
201
525
16
762
201
572
201
522
II
201
514
11 .521
II
201
513
11 . 5 2 3
II
201
516
"
163 165
1 . 87
1
767
3 . 907
11 . 5 1 4
"
163 170
1 63
1 768
3 . 898
11 . 5 1 4
"
163 164
1 . 34
1 769
3 . 908
11 . 5 0 9
"
II
201
530
201
524
intensities of the diffraction lines 3 3 2 , 4 2 2 a n d 4 3 1 . W i t h a l m a n d i n e garnets the
intensity of 3 3 2 is rather faint i n c o m p a r i s o n w i t h those of 4 2 2 a n d 4 3 1 . T h i s
difference between p y r o p e a n d a l m a n d i n e was first m e n t i o n e d b y S t o c k w e l l (1927).
I n contrast w i t h this observation, the a b s o r p t i o n spectrum is a pattern
characteristic for a l m a n d i n e . A l l rhodolites s h o w the three m a i n absorption bands
of a l m a n d i n e w i t h m o r e o r less intensity depending o n the depth of the c o l o u r of
the sample.
F r o m the data given here, it c a n be c o n c l u d e d that the rhodolites described
are garnets w i t h a c o m p o s i t i o n between p y r o p e a n d a l m a n d i n e . T h e y have n o
special properties b y w h i c h they c a n be distinguished f r o m either p y r o p e or
a l m a n d i n e , except their rose-red c o l o u r . F i n a l l y , this c o n c l u s i o n m a y also be d r a w n
f r o m the X - r a y p o w d e r diffraction data for t w o rhodolites a n d t w o almandines
f r o m U m b a , given i n T a b l e 3. T h e differences, extremely s m a l l , are apparently
due to n o r m a l i s o m o r p h o u s replacement.
Zwaan, Gem minerals from U mba, Tanzania, Scripta Geol. 20 (1974)
Table
2.
Microprobe
analyses
RGM 163
sio
2
Al-O
2 3
Ti0
RGM 163 166
7
40
7
23 0
2°3
Fe 0
3
FeO
RGM 163 138
RGM 163
8
40
7
Tanzania.
39
164
7
22 .7
22 6
0 035
0 03
0.03
0 06
0 21
0 03
- ---
--
- -- -
- -- -
8
21 9
7
16
CaO
f r o m U mba,
0 12
17
5 2
MgO
garnets
0 03
10
MnO
Total
rhodolite
22
r
2
four
41
2
C
175
of
2 8
1 6
9
13 1
12 6
2 0
3 1
1 8
0 6
8
15
2 9
100
17
6
15
99
44
100
895
100
36
η
1 749
1 .756
1 762
1 769
D
3 790
3 861
3 883
3 908
521
11 497
11 532
11 509
α (Ä)
11
Numbers
of
3.03
ions
on t h e b a s i s
of
12
(0)
3.02
2.99
3.03
2.98
3.02
2.99
2.99
0.01
1 .94
1 .99
0.002
0.002
>
2.004
0.002
1.08
1.14
1 .37
0.32
0.04
0.17
0.10
y
3.02
> 3.00
3.02
>
1 .82
1 .74
1.44
1 .41
0.23
0.16
0.25
0.15
21
52
­•­­
7 61
pyrope
60
27
spessartine
10
60
uvarovite
0.01
0.65
Mol.
grossular
0.002
1 .972
1 .995
0.003
>-
andradite
2.00
0.002
1.949
0.007
almandine
1.96'
­­•­
per cent,
35
76
end-members
38.00
5 30
62
1 32
­­­­
45
20
--•­
­­­­
57
3.03
8.33
48.00
5.67
4 97
46
53
3 30
­­•-
R e g a r d i n g the name " r h o d o l i t e " a n d the n e w n a m e " r h o d o m a c o n " p r o p o s e d
b y C a m p b e l l (1972), it is clear that f r o m the m i n e r a l o g i c a l p o i n t of v i e w the
l o g i c a l step w o u l d be to d i s c a r d b o t h , unless it appears f r o m further studies that
one o r another trace element, o n l y o c c u r r i n g i n this type of garnet, is responsible
for the special c o l o u r , something that is very u n l i k e l y . T h i s is a n o m e n c l a t u r e
p r o b l e m w i t h i n the scope o f the C o m m i s s i o n o n N e w M i n e r a l s a n d M i n e r a l N a m e s
of the I n t e r n a t i o n a l M i n e r a l o g i c a l A s s o c i a t i o n .
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Table
3.
X - r a y powder d i f f r a c t i o n
garnets
f r o m Umba,
Tanzania.
rhodolite
almandine
almandine
RGM 163 158
RGM 163 156
I
d
I
d
2
3.32
1
2
3.33
179
hkl
d
222
3 .33
400
2 .88
8
2.88
8
420
2 .58
10
2.57
10
(obs.)
f o r four
RGM 163 138
rhodolite
RGM 163
data
(obs.)
(obs.)
I
I
d
1
3 .32
2.87
8
2 .88
8
2.57
10
2 .58
10
(obs.)
1
2
332
2 .46
6
2.46
6
2.45
4
2 .46
4
422
2 .35
6
2.35
6
2.34
6
2 .35
6
431
2 .26
6
2.26
6
2.25
6
2 .26
6
521
.10
6
2.10
6
2.10
6
2 .11
6
440
2 .04
2
2.04
1
2.03
4
2 .04
2
61 1
1 .870
6
1 .867
7
1 .863
6
1 .870
7
620
1 .822
2
1 .824
1
2
1.818
2
1 .827
1
444
1 .663
5
1 .664
6
1 .660
5
1 .667
6
640
1 .596
8
1 .598
8
1 .595
8
1 .601
8
642
1 .540
9
1 .541
9
1 .537
9
1 .543
9
732
1 .464
1
1 .455
1
2
1 .459
1
1 .465
1
2
800
1 .440
5
1 .441
5
1 .438
4
1 .445
5
741
1 .419
1
1 .420
1
2
1 .417
1
2
1 .425
I
653
1.373
1
1.371
1
2
1 .374
1
2
1 .382
1
2
1 .302
\
1 .302
1
2
1 .308
1
2
1 .287
5
1 .292
6
752
1 .300
1
840
1 .286
6
1 .287
5
842
1 .255
7
1 .256
7
1.255
6
1 .261
7
761
1 .240
1
1 .241
1
2
1 .241
1
1 .245
1
2
664
1 .226
6
1 .227
5
1 .227
5
1 .232
6
851
1 .213
2
1.214
1
1.213
1
1 .219
2
853
1 .162
5
1 .163
4
1.163
4
1 .168
5
1011
1 .139
2
1 . 140
1
2
1 .138
1
2
1 .144
1
2
1020
1 .129
3
1 .130
2
1 .128
3
1 .134
3
943
1 .118
1
1.119
1
2
1.118
1
2
1 .124
1
2
952
1 .097
1
2
1 .097
1
2
1 .096
1
2
1 .102
1
2
1 .069
8
1 .071
8
1 .070
8
1 .075
8
1040
1042
1 .052
7
1 .053
7
1 .052
7
1 .057
7
880
1 .019
7
1 .020
7
1 .018
7
1 .023
7
972
0 .9956
I.
0.9948
1
2
1 .002
\
1060
0 .9880
1
2
0.9876
\
0 .9928
1
2
1141
0 .9809
i
0.9807
1
2
1062
0 .9739
1
2
0.9734
\
F r o m the c o m m e r c i a l p o i n t of v i e w it w i l l be p r a c t i c a l l y i m p o s s i b l e to
eliminate the n a m e " r h o d o l i t e " , because it has b e c o m e part a n d p a r c e l o f the
language. It w o u l d therefore create c o n f u s i o n to i n t r o d u c e another n a m e designating the specific c o l o u r . M o r e o v e r , there is every chance that a n e w n a m e w o u l d
be used to i n t r o d u c e a " n e w gemstone". It is advisable that the name " r h o d o m a c o n " s h o u l d be c o n s i d e r e d b y the I n t e r n a t i o n a l C o n f e d e r a t i o n of Jewelry,
Silverware, D i a m o n d s , Pearls a n d Stones ( C I B J O ) .
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
9
INCLUSIONS
M i c r o s c o p i c e x a m i n a t i o n o f a l l rhodolites indicates that they are generally rather
clean. I n this respect they have m u c h i n c o m m o n w i t h p y r o p e garnets.
R u t i l e appears to be the most frequent s o l i d i n c l u s i o n . It occurs i n b o t h
l o n g a n d short p r i s m a t i c needles, either r u n n i n g i n three directions m a k i n g a n
angle o f 6 0 ° , o r isolated (see F i g s . 1, 2 a n d 3 ) .
I n specimen R G M 163 1 7 1 , b l a c k , somewhat r o u n d e d crystals are f o u n d .
T h e y have a short p r i s m a t i c habit a n d , therefore, l o o k as i f they are i s o m e t r i c
(see F i g s . 4, 5 a n d 6 ) . I n reflected light a s u b m e t a l l i c lustre m a y be observed.
O n e o f them, extending to the surface o f the garnet, was identified as rutile b y
means o f a n X - r a y p o w d e r p h o t o g r a p h ( R G M 2 0 1 3 3 7 ) .
I n specimen R G M 107 2 0 5 i n c l u s i o n s are present that resemble the " s m o o t h
tubes w i t h r o u n d e d e n d s " described b y C a m p b e l l (1972). T h e y have p a r a l l e l
e x t i n c t i o n , although they resemble two-phase i n c l u s i o n s (see F i g . 7 ) . T h e i r identity
is n o t certain but they c a n be rutile needles because they have m u c h higher
refractive indices than the s u r r o u n d i n g garnet a n d rather h i g h interference c o l o u r s .
I n three specimens z i r c o n s w i t h haloes c a n be seen. M o r e o r less
crystals o c c u r i n t w o other samples w h i c h f r o m their habit a n d interference
c o u l d be apatite. I n specimen R G M 163 1 6 1 , there are i n c l u s i o n s w i t h
habit that m i g h t b e l o n g to the m i c a group o f minerals. T w o r h o d o l i t e
c o n t a i n l i q u i d feathers s i m i l a r to those o c c u r r i n g i n C e y l o n garnets.
rounded
colours
a platy
garnets
F r o m these observations one m a y c o n c l u d e that the i n c l u s i o n s f o u n d i n these
r h o d o l i t e garnets are not unique f o r this l o c a l i t y . E v e n pseudo-isometric rutile
crystals, as detected i n specimen R G M 163 171, have been f o u n d i n a n a l m a n d i n e
garnet f r o m C e y l o n ( Z w a a n , 1967). T h e other inclusions frequently o c c u r i n
garnets f r o m other localities.
Fig. 1. Rutile needles in rhodolite garnet R G M 163 178, 60x.
10
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 2. Rutile needles in rhodolite garnet RGM 163 178, 60x, reflected light.
Fig. 3. Rutile needles in rhodolite garnet RGM 107 209, 80x, reflected light.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 4. Rutile crystals in rhodolite garnet R G M 163 171, 65x, reflected light.
Fig. 5. Rutile crystal in rhodolite garnet
R G M 163 171, 150x, both reflected and
transmitted light.
Fig. 6. Rutile crystal in rhodolite garnet
RGM 163 171, 150x, reflected light.
11
12
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 7. Smooth tubes with rounded ends, probably rutile, in rhodolite garnet, R G M
107 205, 120x.
Almandine garnet
PROPERTIES
M a n y specimens of a l m a n d i n e garnet were available for e x a m i n a t i o n . A l l have a
pleasant red to b r o w n i s h red c o l o u r a n d most of t h e m are of gem quality. A l l
samples are parts of crystals, but n o crystal forms c o u l d be observed. T h o s e
h a v i n g interesting inclusions were selected for d e s c r i p t i o n . T h e i r properties are
given i n T a b l e 4 i n ascending o r d e r of refractive indices.
A l t h o u g h t w o o f these garnets have properties l y i n g i n the field of the
rhodolites, the r e m a i n i n g five show higher values for n, D a n d a. A linear
function between
refractive i n d e x a n d specific gravity is present,
but
there is n o consistent r e l a t i o n between these data a n d the length of
the unit c e l l ; i n other w o r d s the v a r i a t i o n of a is not due alone to the substitution
of F e for M g . It is o b v i o u s that b o t h the C a - i o n a n d the M n - i o n p l a y an i m p o r t a n t
part here. T h i s c a n be seen i n T a b l e 5 i n w h i c h m i c r o p r o b e analyses of four
a l m a n d i n e garnets are given. A s w i t h the rhodolites, a selection was made of those
specimens h a v i n g the lowest a n d the highest refractive indices a n d densities, as
w e l l as those w i t h the m i n i m u m a n d m a x i m u m values for the size of the unit c e l l .
Since the elements C r a n d T i , as w e l l as trivalent F e were not determined, the
m o l e c u l a r percentages of the end-members andradite a n d u v a r o v i t e are not given.
T h e analyses represent the average of three measurements per sample. N o i n d i c a tions of i n h o m o g e n e i t y are f o u n d i n these garnets.
It is seen f r o m these analyses that the garnet w i t h the highest C a content
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Table
4.
sample
Properties
no.
of
seven
almandine
garnets
o r i g i n a l weight
in carats
η
D
α
f r o m Umba,
S
in
no.
13
Tanzania.
X - r a y powder
photograph
RGM
163 155
30.00
1 .750
3 798
11.511
RGM
201
"
163 157
39.25
1 .758
3 856
11 .493
"
201
718
"
107 203
19.43
1 .771
3 910
11 .543
"
201
333
719
"
163 158
6.73
1 .772
3 931
11.523
"
201
"
163 156
3.53
1 .775
3 929
11.576
"
201 717
"
163 160
7.56
1 .775
3 964
11 .528
n
201
721,
201
723
"
163 159
9.38
1 .787
4 037
11.537
"
201
720,
201
722
four
almandine
Table
5.
Microprobe
analyses
RGM 163
sio
2
2°3
Fe 0
A
716
1
2
RGM 163
f r o m Umba,
RGM 163
156
Tanzania.
RGM 163
40 . 3
38
7
38 .15
23 .45
23 .2
21
9
21 .75
- --
- --
15 .0
18 .8
MnO
1 .1
MgO
17 .0
CaO
99 .60
27 .15
65
3 .4
05
16 .35
7 9
6 .85
0 .6
7 5
2 .65
99
99 .55
159
-
•22
0 .3
2 .65
Total
157
garnets
40 .4
3
FeO
155
of
99 .95
70
η
1 .750
1 . 758
1 775
1 .787
D
3 .798
3 .856
3 929
4 .037
11 .511
11 . 4 9 3
a (S)
Numbers
Si
Λ1
Ι
>
0 . 025 ,
Al
2 .00
of
2 .96
Fe
+
3
Fe
+
2
Mn
on
2 98
2. 9 S S
•
-
ions
2. 00
0 .92
1
•
0 01
J
2 00
I
-- --
basi s
of
12
(0)
2.98
'
1.99"
*
>
0.90
Ca
0 . 205 ,
0 045 .
0.62
cent,
3.00
•
2.005
2 .005 "
0 .22
3.02
3.05
>
1 80
•
1 .78
3.025
1 .85
per
\
1 .99
Ί
0 .005 .
1 .46
Mg
Mol.
2.98
0.07
3. 04
11 . 5 3 7
2 .995
Y
.
2.00
0 02
-
576
2.99
'
1 16
0 .065
the
11
0 .80
0 .22
,
,
end-members
47
87
58 .95
almandine
30 .26
38 .35
andradite
- --
- --
grossular
6 .75
1 .49
20 .33
7 .28
60 .85
59 .50
29 .50
26 .49
spessartine
2 .14
0 .66
2.30
uvarovite
-
-
pyrope
--
- --
7 .28
-
14
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Table
6.
(Analyst:
Chemical
Miss
analyses
of
RGM 163
sio
2
garnets
from
Umba,
Tanzania.
RGM 163
155
RGM 163
157
41 .01
39.04
22.93
22.40
21.14
0.00
0.64
1.14
4.22
13.88
14.88
2
2
2°3
FeO
F
almandine
41 . 8 8
A1 0.
Ti0
three
II.Bontje).
e
0.00
3.01
23.78
0.37
3.39
16.51
15.46
6.83
CaO
2.89
0.65
2.79
Na 0
0.20
0.20
0.23
0.00
0.00
0.00
0.03
0.05
0.00
MnO
1 .32
MgO
2
κ ο
2
P
2°5
Total
100.21
99.88
100.78
159
η
1 .750
1 .758
1 .787
D
3.798
3.856
4.037
α
(S)
11
11.511
Numbers
Si
3
'
0 3
}
of
ions
on
the
basis
}
3.01
·
3.03
3
2.00
0.04
0
1
11.537
.493
of
12
CO)
3
• }
3.02
0 2
Al
Al
1.95η
Ti
0.00
Fe
0.05
Fe
+?
-
Μη
0.03
I
J
2.00
0 00
0 .07
1.11*
1 .64 '
0.08
0.02
0 .22
1.78
Ca
0.23
Na
0.01
V-
2.98
I
1 .69
2.88
0.01
.
Mol.
per
cent,
0
79
.
end-members
56.7
andradite
2.4
3.8
3.8
grossular
5.1
---
4.5
60. 1
2.7
2.89
0 .01 .
38.5
spessartine
V
2.00
0 .23
0.05
29.7
pyrope
I
J
0.88^
Mg
almandine
1
1 .93 Ί
i
J
57.0
27.4
0.7
7.7
uvarovite
( R G M 163 156) has the highest a, w h i l e the one w i t h the lowest value for a has
the lowest C a content ( R G M 163 157).
T h e t w o a l m a n d i n e garnets w i t h e q u a l C a content ( R G M 163 155 a n d
163 159) have different unit c e l l dimensions, w h i c h m a y be a s c r i b e d to their
different M n content.
T h e c o n v e n t i o n a l analyses given i n T a b l e 6 d o not differ v e r y m u c h f r o m
the analyses o b t a i n e d b y the m i c r o p r o b e . It is seen that the andradite percentage
is not h i g h i n these garnets, a n d that the F e content is m a i n l y due to divalent i r o n .
Zwaan, Gem minerals from U mba, Tanzania, Scripta Geol. 20 (1974)
T h e r e l a t i o n between the M g or F e content a n d the figures of η a n d D c a n
be observed i n b o t h T a b l e s 5 a n d 6. T h e higher the p y r o p e content the l o w e r η
a n d D , the higher the a l m a n d i n e content the higher these properties.
A l t h o u g h the X ­ r a y p o w d e r photographs of these a l m a n d i n e garnets are
very similar, it is seen that t w o of them, m a d e f r o m the samples R G M 163 155
a n d 163 157 have a pattern characteristic for p y r o p e . T h e r e m a i n i n g five h a v e a n
a l m a n d i n e pattern, w h i c h means that the intensity of the diffraction line 3 3 2 is
rather faint i n c o m p a r i s o n w i t h those of 442 a n d 4 3 1 .
I n T a b l e 3 X ­ r a y p o w d e r diffraction data for t w o a l m a n d i n e garnets f r o m
U m b a are g i v e n . A s m i g h t be expected the m u t u a l differences are o n l y s m a l l , just
l i k e the differences between t h e m a n d the p o w d e r data for the rhodolites, also
given i n this table.
R e g a r d i n g the a b s o r p t i o n spectra, a l l specimens have an a l m a n d i n e spectrum.
T h e three m a i n a b s o r p t i o n bands of a l m a n d i n e c a n easily be observed. It is
striking that this spectrum also dominates i n the t w o garnets h a v i n g properties i n
the p y r o p e field. A p p a r e n t l y even a l o w F e content i n garnets o f the p y r o p e ­
a l m a n d i n e series is sufficient to eliminate the b r o a d a b s o r p t i o n b a n d i n the
yellow­green centred near 5 7 5 0 Â , characteristic for p y r o p e . M o r e o v e r it s h o u l d
be interesting to examine the relationship between the intensity of the reflection
3 3 2 o n X ­ r a y p o w d e r photographs a n d the M g / F e ratio. T h e data f o u n d indicate
that these garnets, except t w o of them, have higher properties t h a n the r h o d o l i t e s .
T h e y b e l o n g to the p y r o p e ­ a l m a n d i n e series a n d are generally situated i n the
region towards the a l m a n d i n e end­member.
INCLUSIONS
S a m p l e R G M 107 203 contains numerous m o r e o r less r o u n d e d crystals, b o t h
large a n d s m a l l , h a v i n g l o w p o l a r i z a t i o n colours, p a r a l l e l e x t i n c t i o n a n d a l o w e r
r é f r i n g e n c e t h a n the s u r r o u n d i n g garnet ( F i g . 8). O n e of them, extending to the
p o l i s h e d surface of the a l m a n d i n e , was identified b y X ­ r a y p o w d e r p h o t o g r a p h
R G M 201 3 4 0 as apatite. N o o r i e n t a t i o n of these apatite crystals was observed.
I n specimen R G M 163 155 groups of apatite crystals o c c u r ( X ­ r a y p o w d e r
p h o t o g r a p h R G M 201 334). I n F i g . 9 some of t h e m c a n be seen. T h e y have the
same habit as those o c c u r r i n g i n the garnet described above. I n a d d i t i o n , rutile
c a n be seen, oriented i n three directions m a k i n g an angle of 6 0 ° w i t h one another.
T h e needles are sparsely spread t h r o u g h the stone but concentrated a r o u n d the
clusters of apatite. S m a l l z i r c o n crystals w i t h haloes c a n be seen throughout the
garnet. F i n a l l y , a w e l l ­ d e v e l o p e d l i q u i d feather is present.
R a t h e r s m a l l r o u n d e d crystals of apatite ( X ­ r a y p o w d e r p h o t o g r a p h R G M
201 335) o c c u r i n sample R G M 163 156 ( F i g . 10). T h e y are concentrated i n one
part of the garnet. T h e characteristic habit of apatite crystals, w h e n i n c l u d e d i n
other minerals, is s h o w n i n F i g . 11. I n a d d i t i o n , a few rutile needles c a n be
observed i n this garnet.
S p e c i m e n R G M 163 157 contains a great n u m b e r of b l a c k opaque, m o r e o r
less r o u n d e d crystals, arranged i n groups w i t h o u t any o r i e n t a t i o n ( F i g . 13). I n
reflected light they have a s u b m e t a l l i c h i g h lustre. ( F i g . 12). B y means of an X ­ r a y
p o w d e r p h o t o g r a p h ( R G M 201 336) one of t h e m was f o u n d to be rutile. I n
r h o d o l i t e garnet R G M 163 171, s i m i l a r crystals were i n c l u d e d . Besides the rutile
crystals, the a l m a n d i n e garnet contains a few l o n g rutile needles.
15
16
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 8. Apatite crystals in almandine garnet R G M 107 203, 65x.
Fig. 9. Apatite crystals in almandine garnet
R G M 163 155, 80x.
Fig. 10. Apatite crystals in almandine garnet RGM 163 156, 180x.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 11. Apatite crystal in almandine garnet RGM 163 156, HOx.
Fig. 12. Rutile crystal in almandine garnet
R G M 163 157, HOx, both reflected and
transmitted light.
Fig. 13. Rutile crystals in almandine garnet RGM 163 157, 40x.
17
18
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 14. Pyrrhotite and apatite crystals in almandine garnet R G M 163 158, 40x.
Fig. 15. Pyrrhotite crystals in almandine
garnet RGM 163 158, 90x, both reflected
and transmitted light.
B l a c k opaque crystals, most often r o u n d e d a n d sometimes w i t h a h e x a g o n a l
shape, o c c u r i n sample R G M 163 158 (Figs. 14 a n d 15). I n reflected light they have a
b r o n z y y e l l o w c o l o u r . O n e of t h e m , t o u c h i n g the surface o f the garnet, was
identified as p y r r h o t i t e ( X - r a y p o w d e r p h o t o g r a p h R G M 201 3 2 4 ) . I n a d d i t i o n ,
b o t h l o n g a n d short p r i s m a t i c colourless transparent crystals o c c u r w i t h o u t any
o r i e n t a t i o n throughout the stone. T h e y resemble the apatite crystals i n c l u d e d i n
the above m e n t i o n e d a l m a n d i n e garnets, h a v i n g the same o p t i c a l properties as far
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
as these c a n be e x a m i n e d . Needles of rutile, arranged i n the w e l l - k n o w n three
directions, are concentrated a r o u n d the larger crystals of pyrrhotite a n d apatite.
I n specimen R G M 163 159 some groups of apatite crystals are present, a l l
crystals h a v i n g properties s i m i l a r to those f o u n d i n the other a l m a n d i n e garnets
described here. M o r e o v e r , some b l a c k crystals w i t h a h i g h lustre are i n c l u d e d ,
w h i c h resemble the rutile crystals f o u n d i n sample R G M 163 157.
I n garnet R G M 163 160 n o s o l i d inclusions c a n be observed but t w o v e r y
w e l l developed l i q u i d feathers o c c u r .
F r o m these data it is o b v i o u s that apatite a n d short p r i s m a t i c rutile crystals
are c o m m o n inclusions i n a l m a n d i n e garnets f r o m U m b a . These inclusions cannot
be considered diagnostic for the l o c a l i t y , as the same i n c l u s i o n s are f o u n d i n
a l m a n d i n e garnets f r o m C e y l o n . P y r r h o t i t e is also k n o w n f r o m C e y l o n stones
( Z w a a n , 1967), w h i l e there is n o question that z i r c o n s w i t h haloes are c o m m o n l y
f o u n d i n garnets f r o m other sources.
Corundum
VARIETIES
T h e most s t r i k i n g property of the c o r u n d u m s f r o m U m b a is the v a r i a t i o n i n c o l o u r .
Therefore, the available m a t e r i a l w i l l be described first a c c o r d i n g to the colours
a n d subsequently a c c o r d i n g to the properties.
Ruby - ( R G M 107 193, 107 2 0 2 , 107 2 0 7 , 107 2 1 0 , 107 2 2 5 , 163 172, 163 173
a n d 163 183). R e d c o r u n d u m was available i n rather large quantity, the c o l o u r
being v i o l e t - r e d to a paler orange-red. M o s t of the m a t e r i a l consists of crystal
fragments, sometimes w i t h one o r t w o basal p i n a c o i d faces. T h e r e are a few
specimens, however, h a v i n g distinct crystal forms. S a m p l e R G M 107 2 2 5 , for
instance, is a v e r m i c u l i t e - b e a r i n g r o c k i n w h i c h four r u b y crystals are to be seen.
These are h e x a g o n a l prisms w i t h basal p i n a c o i d faces. T h e largest one has a length
of 15 m m a n d a diameter of 13 m m .
R G M 107 202 consists of t w o p a r a l l e l - g r o w n h e x a g o n a l prisms, the j o i n i n g
plane b e i n g i n the d i r e c t i o n of the b a s a l p i n a c o i d . B o t h prisms have a length of
5 m m a p p r o x i m a t e l y , the diameters b e i n g 9 a n d 8 m m respectively.
A l t h o u g h the c o l o u r s of these rubies are rather g o o d , they are t o o brittle
a n d so i m p u r e as to be almost translucent, a n d therefore are not of g e m quality.
Sapphire - ( R G M 163 188 to 163 191). These four crystals have a pleasant blue
c o l o u r w i t h a light v i o l e t hue. T h e y are h e x a g o n a l prisms w i t h basal faces, a
tabular h a b i t w h i c h is quite u n u s u a l for sapphire. T h e size of R G M 163 188 is
17 χ 11 m m , R G M 163 189 is l l /
χ 1 4 / m m , R G M 163 190 is 6 χ 18 m m a n d
R G M 163 1 9 1 i s 4 x l 8 m m , the first figure c o r r e s p o n d i n g to the length a n d the
second to the diameter. T h e p r i s m faces are c o r r o d e d , the pits being f i l l e d w i t h
v e r m i c u l i t e flakes. It is very l i k e l y therefore that these crystals originate f r o m a
v e r m i c u l i t e r o c k s i m i l a r to that described above ( R G M 107 225).
2
Greenish
corundum
2
- R G M 107 198, 163 141 to 163 144). M a n y large crystal
19
20
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
fragments w i t h basal p i n a c o i d faces were available for investigation. T h e y have a
greenish to y e l l o w - b r o w n c o l o u r a n d tend to be r e d d i s h i n artificial light, a c o l o u r
change very m u c h l i k e that of an alexandrite. T h e specimens are m a i n l y almost
translucent, hence are not of g e m quality.
Pinkish corundum - ( R G M 107 2 0 6 , 107 214 a n d 107 224). S a m p l e R G M 107 206
consists of a n u m b e r of rather large crystal fragments, a l l i n t e r g r o w n w i t h a
greenish b l a c k a m p h i b o l e w h i c h w i l l be described later o n . T h e c o r u n d u m s have
a flattened habit b y w h i c h the basal p i n a c o i d s are w e l l developed. D i a m e t e r s up
to 20 m m are observed.
I n sample R G M 107 214 a n u m b e r of crystal fragments w i t h o u t any crystal
faces are present. T h e y are p i n k i s h w i t h a distinct violet hue. S o m e of t h e m have
a p i n k core s u r r o u n d e d b y violet i n h e x a g o n a l z o n i n g . Parts of the stones are
almost colourless.
R G M 107 224 is a r o c k specimen i n w h i c h large crystal fragments, up to a
size of 50 χ 4 0 χ 20 m m , are i n t e r g r o w n w i t h kyanite.
Violet corundum - ( R G M 107 2 0 1 , 107 204, 163 153 a n d 163 154). T h r e e speci­
mens have a deep violet c o l o u r resembling that of g o o d q u a l i t y amethyst. O n e of
them, R G M 107 2 0 1 , is a c o r r o d e d h e x a g o n a l p r i s m of 8 m m w i t h a diameter of
15 m m . I n c l u d e d b l a c k s u b m e t a l l i c flakes m a y be seen w i t h the n a k e d eye.
A n o t h e r one, R G M 163 153, also has a h e x a g o n a l p r i s m habit w i t h a length
of 10 m m a n d a diameter of 11 m m . Its o r i g i n a l weight is 14.62 carats. T h e same
inclusions as seen i n the above m e n t i o n e d crystal m a y be observed. T h e y are
oriented p a r a l l e l to the basal p i n a c o i d .
T h e t h i r d one, R G M 163 154, has an irregular shape w i t h basal p i n a c o i d s
o n l y , its o r i g i n a l weight being 8.82 carats. W i t h the n a k e d eye numerous i n c l u d e d
transparent colourless crystals m a y be seen together w i t h the same b l a c k flakes as
i n c l u d e d i n the other two specimens m e n t i o n e d above.
R G M 107 204 consists of about 20 c o r r o d e d h e x a g o n a l prisms, p a r t l y
covered w i t h v e r m i c u l i t e . A l l crystals have a tabular habit a n d are m u c h lighter
violet than the above m e n t i o n e d c o r u n d u m s . M o r e o v e r they are almost translucent,
due to i m p u r i t i e s . T h e largest one has a length of 7 m m a n d a diameter of 19 m m .
Yellow-orange
corundum
- ( R G M 107 208 a n d 163 174). O n e of the t w o speci­
mens, R G M 107 2 0 8 , is a p a r a l l e l g r o w t h of t w o w e l l crystallized flat h e x a g o n a l
prisms. T h e total size is 6 x 1 3 m m , the latter figure being the diameter.
R G M 163 174 is a crystal fragment w h i c h , o n account of its c o l o u r , is c a l l e d
" g o l d e n s a p p h i r e " i n the U m b a area.
Parti-coloured
corundum
- ( R G M 107 199, 107 217, 163 145 to 163 148 a n d
163 184 to 163 187). A m o n g t h e m are five specimens w i t h v i o l e t - p i n k a n d blue
c o l o u r s i n w h i c h some areas are almost colourless. R G M 163 145 is a c o r r o d e d
h e x a g o n a l p r i s m , c o v e r e d w i t h v e r m i c u l i t e , w i t h a length of 15 m m a n d a diameter
of 30 m m . T h i s crystal is sapphire blue w i t h a p i n k i s h c o l o u r e d core.
T h r e e others ( R G M 163 146 to 163 148) are crystal fragments w i t h a b l u e
core changing to p i n k towards the outside of the specimens.
S a m p l e R G M 107 199 is a fragment of a crystal that o r i g i n a l l y h a d a v i o l e t p i n k i s h core changing into pale y e l l o w a n d almost colourless, a n d then into blue
at the exterior. E l e c t r o n m i c r o p r o b e determinations have been c a r r i e d out i n this
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
specimen, the results of w h i c h w i l l be g i v e n b e l o w o n page 2 2 a n d i n T a b l e 7.
R G M 107 217 is a c o r r o d e d h e x a g o n a l p r i s m w i t h a length of 10 m m a n d a
diameter of 20 m m . It is m a i n l y blue w i t h an orange-pink core.
R G M 163 184 is a flat c o r r o d e d slab of a h e x a g o n a l p r i s m , about 55 m m
thick, w i t h a diameter of 21 m m . T h e m a i n c o l o u r is blue, the edges a n d a s m a l l
part of the core are y e l l o w to colourless.
R G M 163 185 is a part of a h e x a g o n a l p r i s m w i t h a thickness of almost
11 m m . P e r p e n d i c u l a r to the c-axis it has a v i o l e t - p i n k c o l o u r . P a r a l l e l to this
axis the centre is blue but the rest of the stone has orange edges changing to
y e l l o w . T h e specimen is c o v e r e d b y v e r m i c u l i t e , w h i l e the same m i n e r a l is also
included.
R G M 163 186 is about half of a h e x a g o n a l p r i s m w i t h a thickness of 9 m m
a n d a diameter of 14 m m . T h e c o r r o d e d p r i s m faces are c o v e r e d w i t h v e r m i c u l i t e .
P e r p e n d i c u l a r to the c-axis its c o l o u r is m a i n l y o r a n g e - y e l l o w but p a r a l l e l to this
axis the centre is y e l l o w i s h a n d the outside is v i o l e t - p i n k . C o l o u r z o n i n g is very
distinct.
R G M 163 187 is an irregular shaped crystal fragment h a v i n g basal p i n a c o i d
faces o n l y . Its thickness is about 7 m m . T h e different colours that are visible are
orange, blue a n d p i n k , the blue being p r e d o m i n a n t .
PROPERTIES
Density - T h e specific gravity of several c o r u n d u m s was measured, especially of
those w h i c h m a y be confused w i t h other minerals. T h e green c o r u n d u m R G M
163 143 was f o u n d to have the lowest density (3.975), w h i l e another green one
( R G M 163 142) has the highest figure of 3.993. A l l other densities are between
these t w o figures.
Optical properties - T h e refractive indices lie between 1.760 a n d 1.765 f o r ε a n d
between 1.768 a n d 1.774 for ω. These figures are o b t a i n e d f r o m a large n u m b e r
of stones. T h e y d o not deviate f r o m data given b y B a n k (1972). T h e higher values,
m e n t i o n e d b y B a n k (1970) for orange c o r u n d u m s f r o m U m b a were not f o u n d
w i t h R G M 163 174. T h i s orange s p e c i m e n has ε 1.765 a n d ω 1.774.
A l t h o u g h m a n y specimens have n o r m a l e x t i n c t i o n , there are m a n y s h o w i n g
a n o m a l o u s interference c o l o u r s i n p o l a r i z e d light, w h i c h apparently is due to
impurities, cracks, p a r t i n g , etc. D e p e n d i n g o n the depths of their c o l o u r s , a l l
c o r u n d u m s have distinct to strong d i c h r o i s m , the rubies i n tones of y e l l o w - o r a n g e
a n d deep red, the sapphires i n pale blue a n d deep blue, the orange stones i n
y e l l o w a n d orange-red. T h e greenish c o r u n d u m s ( R G M 107 198 a n d 163 141 to
163 144) have b o t h b l u i s h a n d y e l l o w i s h tints; i n artificial light the y e l l o w changes
i n t o a r e d d i s h y e l l o w c o l o u r . T h e c o r u n d u m s of other c o l o u r s are d i c h r o i c i n pale
a n d deep tones of the same c o l o u r .
A s m i g h t be expected, the nature of the a b s o r p t i o n spectra is strongly
dependent o n the c o l o u r of the stone. T h e r e d c o r u n d u m s have an a b s o r p t i o n
spectrum w h i c h is apparently due to C r , w i t h lines a n d bands, characteristic of
rubies, a n d therefore, their spectrum w i l l be c a l l e d the r u b y spectrum.
T h e blue c o r u n d u m s have the characteristic sapphire a b s o r p t i o n spectrum,
due to F e . T h e same s p e c t r u m is s h o w n b y the y e l l o w a n d the green m a t e r i a l , as
w e l l as the b l u e w i t h y e l l o w a n d the green w i t h y e l l o w p a r t i - c o l o u r e d c o r u n d u m s .
21
22
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
T h e orange m a t e r i a l has a m o r e c o m p l e x spectrum, being a c o m b i n e d r u b y
a n d sapphire spectrum. B o t h C r a n d F e lines m a y be observed. T h e deep violet
c o r u n d u m s have a s i m i l a r a b s o r p t i o n spectrum.
I n a n u m b e r o f the p a r t i - c o l o u r e d stones the a b s o r p t i o n s p e c t r u m changes
w i t h the c o l o u r f r o m a r u b y to a sapphire pattern b y m o v i n g the stone f r o m one
side to the other u n d e r the spectroscope, f o r instance f r o m r e d to blue a n d f r o m
r e d to green.
W i t h respect to the intensities, the c o r u n d u m s w i t h deep c o l o u r s have the
most distinct spectra, w h i l e those o f paler-tinted stones have moderate to w e a k
intensity. I n the pale violet c o r u n d u m s the sapphire s p e c t r u m dominates w h i l e
the r u b y s p e c t r u m is weak. T h e same is true w i t h the orange m a t e r i a l .
U n d e r b o t h l o n g - a n d short-wave ultra-violet light most o f the c o r u n d u m s
are inert. T h e y e l l o w variety specially is n o t fluorescent, w h i c h is i n contrast w i t h
y e l l o w c o r u n d u m s f r o m C e y l o n . T h e latter have a n apricot y e l l o w c o l o u r under
l o n g - w a v e u l t r a - v i o l e t light. U n d e r this i r r a d i a t i o n the r e d variety o f the U m b a
c o r u n d u m s w i l l give a r e d fluorescence w h i l e the violet to p i n k c o r u n d u m s are
w e a k orange-red. T h e deep violet stones are distinct r e d , the paler violets give a
d u l l r e d . U n d e r short-wave ultra-violet light they have the same c o l o u r , h o w e v e r
w i t h less intensity.
Chemical properties - T o f i n d out w h i c h elements are responsible f o r the c o l o u r
of the c o r u n d u m s , electron m i c r o p r o b e determinations have been c a r r i e d out b y
D r P . M a a s k a n t o n the green c o r u n d u m R G M 163 144 a n d the p a r t i - c o l o u r e d
specimen R G M 107 199. P a r t i c u l a r attention was p a i d to those elements w h i c h
m a y o c c u r i n c o r u n d u m , as c a n be seen i n analyses g i v e n b y D e e r et a l . (1962).
Spectrometer 2 0 - s c a n n i n g o n l y revealed the presence o f F e , T i a n d C r , w h i c h
elements are, a c c o r d i n g to H a r d e r (1969), responsible f o r the c o l o u r s o f c o r undums.
Table
one
7.
Electron
tourmaline
corundum
RGM
163
144
green
Fe
1 .08
0.015
RGM
107
199
part
<0.05
Ca
< 0 . 001 *
Mn
0.002*
*
of
(in weight
two
corundums
corundum
RGM
107
blue
and
%).
199
part
tourmaline
RGM
0.005
0 .35
(0.010-0.025)
0.16
0.020
(0.014-0.030)
(0.012-0.030)
<0.05
< 0.05
*
197
(0.41-0.43)
0.019
(0.009-0.018)
107
emerald-green
0.42
(0.41-0.43)
0.023
V
present
corundum
0.012
0.005
if
determinations
Tanzania
0.42
(0.011-0.019)
Cr
Umba,
red
(1.06-1 .10)
Ti
microprobe
from
0.46
*
< 0 . 001 *
<0.001*
<0.0005*
<0.0005*
1 .46
+
0.010
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
A s these c o r u n d u m s were e m b e d d e d i n the same m o u l d as t o u r m a l i n e R G M
107 197, M n , C a , a n d V were also analysed i n the c o r u n d u m s . E l e c t r o n m i c r o probe determinations were p e r f o r m e d w i t h a C a m b r i d g e Instrument C o . G e o s c a n
operated at accelerating potentials of 25 a n d 30 k V . A s standards were used
diopside ( F e ) , T i O a n d p y r o p e ( T i ) , p y r o p e ( C r ) , o l i v i n e ( M n ) , diopside ( C a ) a n d
V 0 O 5 for V a n a d i u m . A p p a r e n t concentrations, o n l y corrected for dead-time a n d
b a c k g r o u n d , o b t a i n e d f r o m the average value of three a r b i t r a r i l y chosen spots,
are g i v e n i n T a b l e 7.
T h e F e content of the green c o r u n d u m R G M 163 144 is extremely h i g h i n
c o m p a r i s o n w i t h the data g i v e n b y H a r d e r (1969), for a green sapphire f r o m U m b a .
M o r e o v e r , the T i a n d C r contents are about twice as h i g h as those m e n t i o n e d b y
H a r d e r . T h e h i g h F e content of this c o r u n d u m m a y be the cause of its green
c o l o u r , w h i l e its r e d d i s h hue i n artificial light m a y be due to the rather h i g h
C r content, t h o u g h it is m u c h l o w e r than that of the p a r t i - c o l o u r e d c o r u n d u m
R G M 107 199. T h e data for the r e d d i s h part of the latter indicate a h i g h F e
content i n c o m p a r i s o n w i t h results g i v e n by H a r d e r for rubies.
A s already m e n t i o n e d , however, this c o r u n d u m is not deep ruby-red but m o r e
p i n k i s h b r o w n - r e d changing into pale o r a n g e - y e l l o w a n d finally pale b l u e to
violet-blue. H e n c e the h i g h F e content of the r e d d i s h part is e x p l i c a b l e . T h e T i
a n d C r contents of this part are i n g o o d agreement w i t h those f o u n d b y H a r d e r .
T h e b l u i s h part of this c o r u n d u m has a h i g h C r content c o m p a r e d w i t h data
given b y H a r d e r (1969) for a b l u e sapphire f r o m U m b a . It fits better, however,
w i t h his data o n v i o l e t c o r u n d u m s f r o m the same area. T h i s m a y be due to the
fact that there is distinct v i o l e t present i n the blue part of the specimen. T h e data
for b o t h F e a n d T i are w i t h i n the range given b y H a r d e r for b l u e sapphires.
I n d i v i d u a l spot analyses of simultaneously measured T i a n d C r contents i n
the r e d part (left) a n d the blue part (right) of the p a r t i - c o l o u r e d c o r u n d u m
i n d i c a t e d a possible relationship between these two elements. A m o r e detailed
investigation of this relationship is s h o w n i n F i g . 16: six continuous scanning
profiles for T i - a n d Cr-Ka r a d i a t i o n across the p a r t i - c o l o u r e d c o r u n d u m a n d m o r e
o r less p e r p e n d i c u l a r to the red-blue b o u n d a r y .
T h e six sections have a s t r i k i n g similarity, especially w h e n the C r content is
t a k e n into consideration. T h e numbers i n the figure represent c o m p a r a b l e situations i n the sections: 1 corresponds w i t h the first C r m i n i m u m , 2 is the second
C r m i n i m u m . 3 is a C r m i n i m u m just before one o r t w o C r m a x i m u m s , f o l l o w e d
i m m e d i a t e l y b y a C r level at 4. T h e end of this level is 5, w h i l e 6 is the second
C r m a x i m u m after the C r level. A t the p o i n t of intersection between A a n d F the
C r level i n section A is missing. H e r e the r e d part of the c o r u n d u m is penetrating
into the blue.
F r o m the data obtained, it m a y be c o n c l u d e d that the b l u e c o l o u r of this
specimen m a y be due to a higher T i a n d a l o w e r C r content, whereas i n the r e d
part the C r content is higher a n d the T i content is l o w e r .
H a r d e r (1969) points out that X - r a y fluorescence is m o r e convenient than
the m i c r o p r o b e to detect trace elements. Differences i n concentration, however,
c a n better be measured w i t h the a i d of the last-named instrument. A disadvantage
of the m i c r o p r o b e is that the F e / F e
ratio cannot be detected.
It is very u n l i k e l y that blue sapphires owe their c o l o u r to C o , as m e n t i o n e d
b y Solesbury (1967). H a r d e r (1969) does not m e n t i o n C o . W e d i d not f i n d this
element i n the blue c o r u n d u m s described here. T h e i r a b s o r p t i o n spectra, moreover,
d o not have any absorption bands w h i c h c a n be ascribed to cobalt.
+ 3
+ 2
23
24
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
R G M
C O R U N D U M
Umba,
107
199
Tanzania
\2>
\ 2
1/
3\
)5
(2
6>
\5
\6
2^
c
\2
D
/2
\4
3/
\6
5N
5/
/6
I
Ti
ppm
1 1 mm
Cr
ppm
600
200 -4
200
Ti
ppm
Cr
ppm
200
h 400
100
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 16. Continuous scanning profiles for Ti- and Cr-Ka radiation across the corundum
R G M 107 199.
25
26
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
INCLUSIONS
A s u s u a l w i t h c o r u n d u m , m a n y i n c l u s i o n s c a n be observed. M o s t of t h e m
s o l i d a n d represent various minerals.
are
Rutile - R u t i l e is a c o m m o n i n c l u s i o n i n m a n y of the c o r u n d u m s . I n the green
stone R G M 163 141 it occurs as l o n g needles, oriented i n t w o directions ( F i g . 17).
I n the same w a y needles are f o u n d i n rubies R G M 163 172, 163 173 a n d 107 2 1 0 .
M o r e o v e r , short needles of rutile, oriented i n three directions, are present i n m a n y
other specimens, as i n the deep violet c o r u n d u m R G M 163 154 a n d concentrated
at several places i n the p a r t i - c o l o u r e d stone R G M 163 184.
A c c o r d i n g to E p p l e r (1972) elongated needles o c c u r i n c o r u n d u m s f r o m
U m b a , w h i c h seem to be s i m i l a r to rutile needles but i n fact are c o r u n d u m . It is
difficult to identify these needles b y use of o p t i c a l methods o n l y . It is quite
possible, therefore, that the m a t e r i a l described here also contains c o r u n d u m needles
together w i t h o r instead of rutile needles.
I n the four sapphire crystals ( R G M 163 188 to 163 191), 'needles' o c c u r
w h i c h m a y be ascribed to a k i n d of d i s l o c a t i o n , a p o s s i b i l i t y m e n t i o n e d by
S c h u b n e l (1972).
Pyrrhotite - A rather c o m m o n i n c l u s i o n i n r u b y is pyrrhotite. I n F i g . 18 somewhat
r o u n d e d crystals m a y be seen i n c l u d e d i n r u b y R G M 163 172. T h e y have a b r o n z y
y e l l o w c o l o u r w i t h a m e t a l l i c lustre. X - r a y p o w d e r photographs ( R G M 201 2 7 7 ,
201 278 a n d 201 330) indicate that it is pyrrhotite. M a n y of the rubies b e l o n g i n g
to sample R G M 163 173, a n d the rubies R G M 107 2 1 0 a n d 163 183 c o n t a i n
s i m i l a r crystals. M a t e r i a l scraped f r o m the latter was f o u n d to be strongly magnetic.
Apatite - A l m o s t colourless crystals w i t h the same habit as the apatite crystals
f o u n d i n the described a l m a n d i n e garnets f r o m U m b a o c c u r i n a n u m b e r of
c o r u n d u m s . F i g . 21 illustrates these apatite crystals ( X - r a y p a w d e r p h o t o g r a p h
R G M 201 328), i n c l u d e d i n the deep v i o l e t c o r u n d u m R G M 163 154. I n the green
c o r u n d u m s R G M 163 141 a n d 163 142 these apatite crystals are w e l l developed
( X - r a y p o w d e r photographs R G M 201 339 a n d 201 332). I n F i g s . 19 a n d 20 some
of the apatite inclusions i n these c o r u n d u m s m a y be seen.
I n a n u m b e r of the specimens b e l o n g i n g to the p a r c e l of rubies R G M
163 173, as w e l l as i n the p a r t i - c o l o u r e d c o r u n d u m R G M 163 186, s i m i l a r crystals
are i n c l u d e d . T h e i r identity has not been c h e c k e d b y X - r a y analysis but their
o p t i c a l properties give strong indications that they are apatite crystals as w e l l .
Graphite - I n some of the c o r u n d u m s , especially i n the deep v i o l e t specimens,
m a n y b l a c k s u b m e t a l l i c flakes, visible to the m a k e d eye, have been identified as
graphite ( X - r a y p o w d e r photographs R G M 201 325 a n d 201 329), T h e y o c c u r i n
layers p a r a l l e l to the basal face. F i g . 2 2 illustrates these i n c l u s i o n s i n sample
R G M 163 153.
S i m i l a r flakes o c c u r to a lesser extent i n violet c o r u n d u m R G M 163 154,
a n d the v i o l e t crystal R G M 107 2 0 1 . F i n a l l y , i n a n u m b e r of the stones i n the
p a r c e l of rubies R G M 163 173, flakes c a n be observed, w h i c h are p r o b a b l y
graphite.
Zircon - M a n y somewhat r o u n d e d , colourless a n d transparent crystals o c c u r i n
the y e l l o w - o r a n g e c o r u n d u m R G M 163 174. I n p o l a r i z e d light they have haloes.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 17. Rutile needles in green corundum R G M 163 141, 70x, reflected light.
Fig. 18. Pyrrhotite crystals in ruby R G M
163 172, 60x, both reflected and transmitted
light.
Fig. 19. Apatite crystal in green corundum
R G M 163 142, 150x, both reflected and
transmitted light.
27
28
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 20. Apatite crystal in green corundum RGM 163 142, 90x.
Fig. 21. Apatite crystals in violet corundum R G M 163 154, 45x.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 22. Graphite flakes in deep violet corundum R G M 163 153, 60x.
O n e of them, t o u c h i n g the surface of the c o r u n d u m was identified as z i r c o n
( X - r a y p o w d e r p h o t o g r a p h R G M 201 3 4 5 ) . S i m i l a r crystals w i t h haloes are i n c l u d e d i n the four sapphire crystals R G M 163 188 to 163 1 9 1 .
R e c e n t l y , S c h u b n e l (1972) has also m e n t i o n e d z i r c o n i n c l u s i o n s i n violet
a n d blue c o r u n d u m s f r o m this area.
Spinel - I n r u b y R G M 107 193 some b l a c k to d a r k greenish b l u e i n c l u s i o n s c a n
be seen, w i t h a habit that is generally isometric, t h o u g h elongated a n d flattened
crystals also o c c u r . O n e of t h e m was f o u n d to be spinel ( X - r a y p o w d e r p h o t o g r a p h
R G M 201 7 7 5 ) .
Vermiculite
- M a n y of the c o r u n d u m s , but especially the violet to p i n k specimens
of sample R G M 107 214, i n c l u d e flakes of a m i c a - l i k e m i n e r a l , i n planes p a r a l l e l
to the basal face of the host crystal. T h e i r c o l o u r is b r o w n i s h green to a pleasant
olive green. T h e y are p r o b a b l y v e r m i c u l i t e because they are very s i m i l a r to the
material c o v e r i n g m a n y of the c o r u n d u m s that is identified b y X - r a y p o w d e r
photographs R G M 201 331 a n d R G M 201 3 3 8 . T h e p a r t i - c o l o u r e d specimen
R G M 163 185 also contains a n u m b e r of v e r m i c u l i t e flakes.
Liquid feathers - L i q u i d feathers are a c o m m o n feature i n m a n y of the U m b a
c o r u n d u m s described here. A w e l l d e v e l o p e d one c a n be observed i n the p a r t i c o l o u r e d stone R G M 163 187. These feathers resemble closely those u s u a l l y
found i n C e y l o n sapphires.
CONCLUSIONS
F r o m these observations it is o b v i o u s that rutile a n d apatite are c o m m o n i n c l u s i o n s
in c o r u n d u m s f r o m U m b a , w h i l e p y r r h o t i t e is c o m m o n i n the rubies. I n c l u d e d
graphite seems to be distinctive for the deep violet c o r u n d u m s .
29
30
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
These i n c l u s i o n s , w i t h the exception of graphite, cannot be considered diagnostic
for the l o c a l i t y as the same i n c l u s i o n s m a y o c c u r i n C e y l o n c o r u n d u m s ( Z w a a n ,
1967). G r a p h i t e is, up to n o w , o n l y f o u n d i n U m b a c o r u n d u m s .
T h e o r i e n t a t i o n a n d the habit of the i n c l u s i o n s might be characteristic of a
locality, but for U m b a c o r u n d u m s it has not yet been recognized.
Tourmaline
VARIETIES
O n l y eleven tourmalines were available for investigation. O n the basis of c o l o u r
they c a n be d i v i d e d into t w o groups, one w i t h e m e r a l d green c o l o u r a n d the other
w i t h r e d d i s h b r o w n a n d orange c o l o u r (see T a b l e 8). T h e stones, listed i n this
table, were cut f r o m r o u g h m a t e r i a l presented b y the manager of the U m b a m i n e .
A s the tourmalines described here were f o u n d i n the weathered p o r t i o n of
the deposit, their p r i m a r y o r i g i n is u n k n o w n . T h e author c o l l e c t e d a crystalline
limestone ( R G M 107 190) i n w h i c h s m a l l graphite flakes o c c u r together w i t h
m i n o r amounts of pale green t o u r m a l i n e ( X - r a y p o w d e r p h o t o g r a p h R G M
201 2 0 9 ) . It is quite possible that the e m e r a l d green crystals originate f r o m the
same type of r o c k . A s the e m e r a l d green specimens are of p a r t i c u l a r interest
because of their c o l o u r , they w i l l be described first.
Emerald
green tourmaline
- F r o m the c r y s t a l l o g r a p h i c p o i n t of v i e w the r o u g h
specimens are most interesting. R G M 107 219 has a flattened p r i s m a t i c habit.
T h e best developed f o r m is { 0 2 2 1 } . Subsequently b o t h
{1120} and
{1010}
are
w e l l developed. O t h e r forms present are { 0 0 0 1 } , { 1 0 1 1 } a n d { 0 1 1 2 } . T h e crystal is about 10 m m across the p r i s m a n d about 7 m m i n depth.
R G M 107 2 2 0 has
forms is also the same.
d i c u l a r to the c-axis, the
T h e crystal R G M
the same u n u s u a l habit. T h e o r d e r of development of the
T h e basal p i n a c o i d faces are absent, however. P e r p e n size is 30 m m , w h i l e the depth is about 21 m m .
107 197 has a p r i s m a t i c habit, the p r i s m faces being w e l l
d e v e l o p e d a n d v e r t i c a l l y striated. M o r e o v e r , the f o r m { 0 2 2 1 } c a n be observed.
T h e length of the crystal is 7 m m , across the p r i s m it measures 4 m m .
S o m e properties of b o t h the r o u g h specimens a n d the cut stone are given
i n T a b l e 8. D u e to the perfect c r y s t a l l i z a t i o n , it was possible to measure the
refractive indices of the crystals o n a refractometer using the r h o m b o h e d r a l faces.
T h e refractive indices a n d the densities are i n g o o d agreement w i t h those given
by W e b s t e r (1961) a n d B a n k & B e r d e s i n s k i (1967) f o r green tourmalines f r o m
Tanzania.
A l l f o u r specimens have a strong d i c h r o i s m . T h e d i c h r o i c c o l o u r s , y e l l o w green a n d deep b l u i s h green, closely resemble those of a g o o d q u a l i t y e m e r a l d .
U n d e r the C h e l s e a c o l o u r filter they s h o w a deep r e d c o l o u r , just as e m e r a l d
n o r m a l l y does. T h e b i g crystal R G M 107 2 2 0 even has a r e d c o l o u r i n transmitted
artificial light, a p h e n o m e n o n also m e n t i o n e d b y W e b s t e r (1961) for a d a r k green
tourmaline from Tanzania.
R e g a r d i n g the a b s o r p t i o n s p e c t r u m a l l four crystals have the same pattern,
the intensity b e i n g the highest i n R G M 107 2 2 0 . I n the r e d part, a w e a k line m a y
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Table
8. P r o p e r t i e s
s a m p l e no.
o f t e n t o u r m a l i n e s f r o m Umba, T a n z a n i a .
colour
original
in
RGM 107 197
31
weight
carats
ε
ω
remarks
D
crystal
emerald
green
1 .01
1 .620
1 .642
3 027
"
107 219
emerald
green
4. 01
1 .620
1 .642
3 040
crystal
"
107 220
emerald
green
115. 42
1 .620
1 .642
3 051
crystal
"
151 262
emerald
green
1 .02
1 .618
1 .638
3 054
emerald c u t
"
107 192
orange
5. 31
1 .620
1 .642
3 .053
pebble
pebble
"
163 1 30
orange
2 .74
1 .621
1 .643
3 055
"
151 259
yellow-orange
2. 12
1 .627
1 .648
3 057
o v a l mixed c u t
"
163 131
yellow-brown
1 .15
1 .621
1 .646
3 .036
pebble
"
151 260
reddish
brown
1 .93
1 .620
1 .643
3 044
o v a l mixed c u t
"
151 261
reddish
brown
13. 53
1 .626
1 .648
3 042
cushion cut
be observed at 6995 A , w h i l e at about 6765 A a doublet is present. A b r o a d
a b s o r p t i o n b a n d centred between 6 1 0 5 a n d 6 0 7 0 A is s h o w n b y these stones as
w e l l as a general a b s o r p t i o n of the violet part of the spectrum. These observations
are very s i m i l a r to data g i v e n b y C r o w n i n g s h i e l d (1967/68) a n d A n d e r s o n (1971).
T h e author agrees that the a b s o r p t i o n spectrum m a y be due to C r , although V as
a source s h o u l d also be t a k e n into account ( A n d e r s o n , 1971).
F o r that reason D r P . M a a s k a n t c a r r i e d out a p a r t i a l electron m i c r o p r o b e
analysis o n sample R G M 107 197. T h e results are g i v e n i n T a b l e 7. F r o m the
data, it is v e r y l i k e l y that b o t h v a n a d i u m a n d c h r o m i u m cause the e m e r a l d green
c o l o u r of this t o u r m a l i n e , although it is not i m p o s s i b l e that the h i g h T i content
might also influence this c o l o u r i n one w a y o r another.
U n d e r long-wave u l t r a - v i o l e t light these tourmalines are inert but a m u s t a r d
y e l l o w g l o w is seen u n d e r short-wave c o n d i t i o n s .
T h e t y p i c a l l i q u i d i n c l u s i o n s often seen i n tourmalines o c c u r i n the three
r o u g h specimens. M o r e o v e r , i n the crystal R G M 107 219 large w e l l - f o r m e d
crystals m a y be observed, h a v i n g m u c h i n c o m m o n w i t h those described b y
W e b s t e r (1961). F i g . 23 shows one of these crystals w h i c h f r o m the habit a n d
interference c o l o u r s , c o u l d be apatite.
Orange and brownish
tourmaline
- T h i s group consists of three
w h i c h an orange c o l o u r is p r e d o m i n a n t . T h e refractive indices a n d
given i n T a b l e 8. It is striking that the v a r i a t i o n i n the densities
lowest is 3.053, the highest 3.057. A p a r t f r o m those of t w o e m e r a l d
these values are higher t h a n any of the r e m a i n i n g samples.
specimens i n
densities are
is s m a l l , the
green stones,
A l l three have a very distinct to strong d i c h r o i s m i n tones of pale y e l l o w
a n d y e l l o w - o r a n g e . T h e i r a b s o r p t i o n spectra are not v e r y characteristic, a general
a b s o r p t i o n of the b l u e a n d violet parts o n l y m a y be observed. U n d e r long-wave
ultra-violet light n o fluorescence c a n be seen, but they have a d u l l y e l l o w g l o w
under short-wave u l t r a - v i o l e t light.
T h e l i q u i d i n c l u s i o n s , n o r m a l l y seen i n tourmalines, o c c u r i n a l l three.
A s stated i n T a b l e 8 t w o of the b r o w n i s h tourmalines are r e d d i s h b r o w n
a n d one is y e l l o w - b r o w n . T h e i r refractive indices are generally slightly higher
t h a n those of the green tourmalines m e n t i o n e d i n this table. T h e i r densities are
between the lowest a n d highest data measured for the green stones, but distinctly
l o w e r t h a n those of the orange stones.
T h e y e l l o w - b r o w n specimen R G M 163 131 is strongly d i c h r o i c , the c o l o u r s
being pale y e l l o w a n d b r o w n i s h orange. T h e t w o r e d d i s h b r o w n cut stones have
32
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 23. Liquid and crystal inclusions in emerald green tourmaline R G M 107 219, 70x.
a very strong d i c h r o i s m i n tones of greenish y e l l o w a n d reddish b r o w n .
R e g a r d i n g their a b s o r p t i o n spectra they have m u c h i n c o m m o n w i t h red
tourmalines. A b r o a d a b s o r p t i o n b a n d i n the green, centred near 5 2 0 0 A , together
w i t h weak lines near 4 6 0 0 a n d 4 5 0 0 A i n the blue a n d a n a r r o w b a n d at 5 3 7 0 A
i n the green part of the spectrum, m a y be seen.
These tourmalines g l o w a d u l l y e l l o w u n d e r short-wave ultra-violet light
but are inert under long-wave i r r a d i a t i o n . T h e y c o n t a i n the l i q u i d inclusions
n o r m a l l y seen i n tourmalines. F i g . 24 illustrates these inclusions i n the y e l l o w b r o w n stone R G M 163 1 3 1 , together w i t h two-phase inclusions o c c u r r i n g o n l y i n
this specimen.
CONCLUSIONS
A l t h o u g h far too little m a t e r i a l was available for e x a m i n a t i o n , the data o b t a i n e d
give sufficient i n d i c a t i o n to c o n c l u d e that the two groups of tourmalines described
differ i n c h e m i c a l c o m p o s i t i o n . T h e e m e r a l d green specimens p o s s i b l y are related
to uvite, w h i l e the orange a n d b r o w n i s h stones c o u l d c o n t a i n a h i g h content of
elbaite.
Other gem minerals
PYROXENE
Orthopyroxene
- ( R G M 107 200, 163 149 to 163 152). A l t h o u g h o n l y a few
specimens were available for investigation it is w o r t h w h i l e to p a y attention to
t h e m o n account of their properties. T h e i r c o l o u r is a deep y e l l o w i s h b r o w n .
W i t h the n a k e d eye n o i n c l u s i o n s m a y be observed. T h e y therefore are certainly
of g e m q u a l i t y a n d interesting as c o l l e c t o r ' s items.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Fig. 24. Liquid and two-phase inclusions in yellow-brown tourmaline R G M 163 131, HOx.
T w o specimens ( R G M 163 149 a n d 163 150) are strongly c o r r o d e d almost
complete crystals w i t h a short p r i s m a t i c habit. T h e first one h a d an o r i g i n a l weight
of 23.56 carats a n d has been p a r t l y used for a c h e m i c a l analysis (see T a b l e 9).
R G M 163 150 weighs 19.09 carats, measuring 16 χ 1 4 x 9 m m .
T h e r e m a i n i n g samples are crystal fragments, the perfect cleavage giving
rise to cleavage planes h a v i n g a b r o n z y lustre.
T h e average density of a l l specimens a v a i l a b l e is 3 . 2 9 1 ; ranging f r o m
3.267 for the crystal R G M 163 150 to 3.310 for sample R G M 163 149.
R e g a r d i n g the refractive indices, a v a r i e d f r o m 1.662 to 1.670, w h i l e γ
v a r i e d f r o m 1.672 to 1.678. S a m p l e R G M 163 152 appeared to have the highest
birefringence (0.012).
U s i n g a U n i v e r s a l Stage a great n u m b e r of measurements were c a r r i e d out
to find the o p t i c a x i a l angle of sample R G M 107 2 0 0 . T h e average value f o u n d
was 2 V = 8 6 ° .
A l l specimens have a strong p l e o c h r o i s m i n tones of r e d - b r o w n a n d y e l l o w i s h
7
33
34
Zwaan, Gem minerals from U mba, Tanzania, Scripta Geol. 20 (1974)
T a b l e 9. C h e m i c a l p r o p e r t i e s
from Umba, T a n z a n i a .
" G e o l o g i s c h en M i n e r a l o g i s c h
Chemical a n a l y s i s
Si0
9
Ti0
2
o f o r t h o p y r o x e n e RGM
163 149
( A n a l y s t s : M i s s H.Bontje and L . B e l f r o i d
I n s t i t u u t " of Leiden
Numbers o f
i o n s on the
57.00
Si
1 .984 "
0.01
AÍ
0 .016 ,
0.70
AÍ
0 -013"
0.53
Fe
+
3
0 .013
FeO
7.69
Fe
+
2
0 .224
MnO
0.20
Mn
0 .006
MgO
33.12
Mg
1 .717
CaO
0.54
Ca
0 .021
University).
b a s i s o f 6 (0)
2.000
A 1
2°3
Fe 0
2
3
>
1 .994
j
Na 0
2
κ ο
2
P
2°5
H 0
+
2
0.03
Mg
86 .7
Fe
12 .3
Ca
1 .0
H 02
Total
99.82
+2
+3
1OOMg/(Mg+Fe +Fe +Mn)
87.6
brown­green. T h e i r a b s o r p t i o n spectrum is characterized b y a strong n a r r o w b a n d
at 5 0 6 0 A i n the green, w h i l e w e a k e r bands are to be seen at 5 4 8 0 , 4 8 3 0 a n d
4 5 0 0 A . V e r y vague bands c a n be seen at 5115 a n d 5045 A , w h i l e the violet part
of the spectrum is generally absorbed. T h i s spectrum is apparently due to ferrous
i r o n a n d is s i m i l a r to the absorption spectrum of b r o w n i s h orthopyroxenes f r o m
India.
T h e orthopyroxenes f r o m U m b a are rather clean. S ome of t h e m have
needle­like i n c l u s i o n s w h i c h c o u l d be e x s o l u t i o n l a m e l l a e , c o m m o n i n o r t h o ­
pyroxenes of p l u t o n i c r o c k s . R e c e n t l y L o r i m e r a n d C h a m p n e s s (1973) described
such l a m e l l a e i n an o r t h o p y r o x e n e f r o m the S tillwater C o m p l e x , M o n t a n a , and
f o u n d t h e m to be augite.
A c h e m i c a l analysis was made of specimen R G M 163 149. T h e results are
given i n T a b l e 9. T h e M g content is i n agreement w i t h what c o u l d be expected
f r o m the o p t i c a l data m e n t i o n e d above. M o r e o v e r a m e t h o d , described b y Z w a a n
(1955), was used to determine the m o l e c u l a r percentage of enstatite i n specimen
R G M 107 2 0 0 . T h e relative distances between the diffraction lines 10 3 1 a n d 0 6 0
on X ­ r a y p o w d e r photographs R G M 201 207 a n d R G M 201 4 9 5 , of this specimen,
were measured a n d f o u n d to be 0.90 m m . T h i s corresponds w i t h a m o l . % of
enstatite of 87.
F r o m the above data it m a y be c o n c l u d e d that the orthopyroxenes available
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
for investigation have very s i m i l a r properties, w i t h the M g content between 87 a n d
88. It depends o n the n o m e n c l a t u r e used whether the name enstatite o r bronzite
applies to this o r t h o p y r o x e n e . A s a b r o n z y lustre m a y be observed, especially o n
cleavage planes, the name b r o n z i t e m a y have the preference.
Clinopyroxene
- O n l y one specimen ( R G M 163 139), a crystal fragment w e i g h i n g
3.89 carats, was available for investigation. Its c o l o u r is a l i v e l y light green. U n d e r
the C h e l s e a c o l o u r filter the same c o l o u r m a y be observed. I n the a b s o r p t i o n
spectrum t w o lines of about e q u a l intensity c a n be seen at 5 0 8 0 a n d 5 0 5 0 A i n
the green, w h i l e a very w e a k line is present i n the r e d at about 6 9 0 0 A .
T h e density is 3.255. I n o r d e r to a v o i d damage to the specimen no accurate
measurements of the refractive indices have been c a r r i e d out. U s i n g the distant
v i s i o n m e t h o d the m e a n refractive i n d e x of 1.68 was observed o n a refractometer.
T h e birefringence is rather strong because d o u b l i n g of i n t e r n a l cracks c a n distinctly
be seen.
T h e pattern of X - r a y p o w d e r p h o t o g r a p h R G M 201 748 of this specimen is
very s i m i l a r to that of a d i o p s i d e f r o m Z i l l e r t a l , A u s t r i a .
It is very l i k e l y , b y virtue of its properties, that this c l i n o p y r o x e n e is a
m e m b e r of the diopside group. A s n o i n c l u s i o n s c a n be observed, apart f r o m some
i n t e r n a l cracks, the m a t e r i a l is of g e m quality. Its c o l o u r resembles that of the
n e w green variety of grossular garnet r e c o r d e d f r o m the same area ( B a n k et a l . ,
1970). It w o u l d be wise to be alert to the appearance of this c l i n o p y r o x e n e o n
the market.
SCAPOLITE
A s the author described this m a t e r i a l i n detail i n a special issue of the J o u r n a l of
G e m m o l o g y o n the o c c a s i o n of the 7 0 t h anniversary of M r B . W . A n d e r s o n
( Z w a a n , 1971), it w i l l suffice here to give a s u m m a r y .
T h e r e were o n l y t w o specimens a v a i l a b l e for investigation, a r o u g h one
( R G M 107 194) a n d a stone cut f r o m a r o u g h crystal fragment ( R G M 151 2 6 4 ) .
R G M 107 194 has a y e l l o w c o l o u r a n d weighs 7.23 carats. Its density is
2.659, co is 1.562, e is 1.543. A very distinct d i c h r o i s m i n tones of s t r a w - y e l l o w
a n d almost colourless m a y be observed, w h i l e n o characteristic a b s o r p t i o n spectrum
has been detected. It has a weak y e l l o w i s h orange fluorescence u n d e r l o n g - w a v e
ultra-violet light a n d glows a rather strong s i m i l a r c o l o u r u n d e r short-wave
i r r a d i a t i o n . N a t u r a l etching figures are observable o n one of the n a t u r a l faces of
the stone. F r o m the centre of these triangular depressions needle-like i n c l u s i o n s
emanate.
T h e X - r a y p o w d e r p h o t o g r a p h ( R G M 201 714) of this stone appeared to be
characteristic for scapolite. T h e observed X - r a y p o w d e r data are c o m p a r e d w i t h
those of three other scapolites, that is, one f r o m M o z a m b i q u e , one f r o m M a d a g a s car a n d a m a r i a l i t e f r o m an u n k n o w n l o c a l i t y . These data agree best w i t h those of
the m a r i a l i t e . Its o p t i c a l properties, however, indicate a scapolite i n the d i p y r e
field. A n y w a y , this scapolite seems to have a c h e m i c a l c o m p o s i t i o n i n w h i c h the
m o l e c u l a r percentage of m a r i a l i t e is p r e d o m i n a n t .
R G M 151 264 is an oval-shaped faceted stone of 7.51 carats, of light y e l l o w
c o l o u r a n d c o n t a i n i n g needle-like inclusions, w h i c h are i n fact h o l l o w tubes w i t h a
prismatic habit.
35
36
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
T h e density is 2 . 6 7 1 , ω is 1.567, e is 1.548. Its d i c h r o i s m is distinct i n tones
of pale y e l l o w a n d almost colourless. N o characteristic absorption bands c o u l d be
observed. U n d e r l o n g - w a v e ultra-violet light the stone has a w e a k r e d d i s h orange
fluorescence, w h i l e under short-wave i r r a d i a t i o n the g l o w is m u c h stronger i n the
same c o l o u r .
These properties indicate a scapolite somewhere between d i p y r e a n d m i z zonite.
ZIRCON
T h e z i r c o n s available for investigation m a y be d i v i d e d into t w o groups, one of
four distinct crystals o r parts of crystals a n d one of 26 s m a l l pebbles.
I n the first group, R G M 107 195 is a part of a crystal w i t h a fine r e d d i s h
b r o w n c o l o u r , its weight is 7.89 carats, its size a p p r o x i m a t e l y 9 χ 8 χ iy
mm. The
habit is p r i s m a t i c , w h i c h is due to the fact that the f o r m { 1 1 0 } is the best developed
one. M o r e o v e r the forms { 1 1 1 } , { 3 3 1 } a n d { 3 1 1 } m a y be observed, together
w i t h c o n c h o i d a l fractures at several places.
2
R G M 163 192 is also a reddish b r o w n crystal, h a v i n g a p y r a m i d a l habit.
T h e best developed f o r m is { 3 1 1 } . O t h e r forms observed are { 1 1 1 } a n d { 1 0 1 } .
T h e weight of this crystal is 2.73 carats, its size is 8 χ 7 / x 5 m m . T h e fracture is
conchoidal.
2
R G M 163 193 is a complete reddish b r o w n crystal w i t h the same habit as
R G M 163 192. Its weight is 2.79 carats, the size is 7 y x 6 χ 6 m m .
R G M 163 194 is a r e d d i s h b r o w n crystal fragment o n w h i c h o n l y one
crystal face c a n be seen. C o n c h o i d a l fractures are w e l l developed. Its weight is
3.12 carats, the size 1 0 / x 6 χ 3 / mrn.
2
2
2
I n the second group, R G M 163 195 consists of 19 s m a l l pebbles, a l l reddish
to y e l l o w i s h b r o w n . T h e total weight is 10.33 carats, the smallest w e i g h i n g 0.42
carats a n d the largest 0.75 carats.
R G M 163 196 consists of seven s m a l l pebbles, a l l pale y e l l o w - b r o w n to
colourless. T h e i r total weight is 3.88 carats, the smallest pebble being 0.40 carats,
the largest one w e i g h i n g 0.65 carats.
T h e lowest density f o u n d is 4.667 for the crystal fragment R G M 163 194,
w h i l e sample R G M 163 193 appeared to have the highest specific gravity of 4.684.
F o r b o t h lots of s m a l l pebbles the density of the w h o l e lot was measured, giving
4.675 for R G M 163 195 a n d 4.683 f o r R G M 163 196. A l l z i r c o n s described here
have refractive indices far above that of methylene i o d i d e a n d a h i g h birefringence.
T h e last property c o u l d be d e r i v e d f r o m the fact that, especially w i t h the four
larger specimens, distinct d o u b l i n g of crystal face edges o r i n t e r n a l cracks was
observed w h e n l o o k i n g t h r o u g h the stone.
A l l zircons have a characteristic absorption spectrum. Strong n a r r o w bands
(lines) c a n be seen at 6 9 1 0 , 6 6 2 5 , 6 5 9 0 a n d 6535 A i n the red, 5 8 9 5 A i n the
y e l l o w , 5 6 2 5 , 5375 a n d 5 1 5 0 A i n the green, 4 8 4 0 A i n the blue a n d 4 3 2 5 A i n
the v i o l e t part of the spectrum. W e a k e r lines are observable at 6 8 3 0 A i n the red,
6 2 1 0 a n d 6 1 5 0 A i n the orange a n d 4 6 0 0 A i n the violet. It must be n o t e d that
the four larger specimens have a m o r e distinct spectrum than the s m a l l pebbles.
A s usual w i t h z i r c o n of g e m quality, these specimens are free or almost free
from inclusions.
T h e properties indicate that these z i r c o n s are of the s o - c a l l e d h i g h type. T h e
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
T a b l e 10. C h e m i c a l a n a l y s i s
(Analyst:
L.Belfroid)
and X - r a y
37
powder
d i f f r a c t i o n d a t a f o r t u r q u o i s e RGM 107 191 f r o m Umba, T a n z a n i a .
d(obs.)
I
d(obs.)
I
9 1 1
1
2
2 .13
1
35 08
6 73
6
2 .06
1
3 26
6 17
8
2 .02
2
FeO
0 .78
4 77
3
1 .902
2
MgO
0 51
4 06
1
2
1 .842
2
CaO
0 .06
3 67
10
1 .814
1
31 .69
3 43
5
1 .782
1
19 .66
3 29
4
1 .673
1
0 10
3 08
4
1 .608
1
3 16
2 89
9
1 .545
1
98 90
2 51
3
1 491
3
2 40
1
1 476
2 30
2
1 391
1
2
1
2
2 23
2
CuO
A 1
4 60
2°3
Fe 0
2
P
3
2°5
H 0
2
co
<
2
insol.
Total
r e d d i s h b r o w n variety, h a v i n g m u c h i n c o m m o n w i t h the c o l o u r of certain tourmalines, w i l l certainly be w e l l received as a gemstone. T h e pebbles, however, are
too s m a l l for cutting. O n l y w h e n larger samples w i t h this c o l o u r are p r o d u c e d b y
the m i n e w i l l it be w o r t h w h i l e to pay attention to them.
TURQUOISE
I n 1967 some prospecting for turquoise was done i n a l i m o n i t e bearing clay,
alternated b y sandy layers. These sediments are apparently d e r i v e d f r o m a l u m i n o u s
r o c k s . V e i n s of turquoise w i t h a w a x y lustre, up to one centimetre i n thickness,
are e m b e d d e d i n the sediments. T h e c o l o u r of the m a t e r i a l varies f r o m y e l l o w i s h
green to b l u i s h green. G o o d quality turquoise was not f o u n d , except for m i n o r
quantities. T h e m a t e r i a l is generally massive, although crusts w i t h concretionary
shapes o c c u r .
I n T a b l e 10 a c h e m i c a l analysis f r o m m a t e r i a l ( R G M 107 191) c o l l e c t e d b y
the author is given. W h e n c o m p a r e d w i t h analyses of turquoise f r o m other
sources, the relatively l o w C u content is striking.
It was i m p o s s i b l e to measure the refractive indices accurately, due to the
c r y p t o c r y s t a l l i n e to fine-granular state, but b y means of o p t i c a l e x a m i n a t i o n i n
i m m e r s i o n m e d i a a m e a n value of 1.64 was f o u n d i n g r a i n p r é p a r â t e s . M o r e o v e r ,
a w e a k p l e o c h r o i s m i n tones of b l u i s h green a n d y e l l o w i s h green to almost
colourless c o u l d be observed. T h e density, too, c o u l d not be measured easily, the
38
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
m a t e r i a l being too i m p u r e . D a t a indicate a m e a n value not far above 2.63. I n
T a b l e 10, X - r a y p o w d e r data, c a l c u l a t e d f r o m X - r a y p o w d e r p h o t o g r a p h R G M
201 276, are given.
Associated minerals
AMPHIBOLE
A b r o w n i s h b l a c k , l o n g p r i s m a t i c m i n e r a l is i n t e r g r o w n w i t h p i n k i s h c o r u n d u m
R G M 107 206. Crystals up to 20 m m i n length w i t h o u t t e r m i n a l faces c a n be
observed. I n t h i n section it has a weak p l e o c h r o i s m w i t h a y e l l o w i s h , β y e l l o w i s h
b r o w n a n d γ b l u i s h green. T h e m i n e r a l has a p a r a l l e l e x t i n c t i o n . V a r i o u s measure­
ments of the optic a x i a l angle give a m e a n value of 2 V y = 8 4 ° . T h e refractive indices
are a 1.645, β 1.652 a n d γ 1.662. X - r a y p o w d e r photographs R G M 201 496 a n d
201 853 have a pattern s i m i l a r to that of the a m p h i b o l e s of the a n t h o p h y l l i t e
group. F r o m these data a n d its occurrence w i t h c o r u n d u m , it m a y be c o n c l u d e d
that this m i n e r a l is a m e m b e r of the o r t h o a m p h i b o l e s . It is very l i k e l y that it w i l l
be a gedrite, rich i n i r o n .
M o r e o v e r , m i n o r quantities of another a m p h i b o l e c a n be observed i n this
sample, h a v i n g a greenish b l a c k c o l o u r a n d properties i n d i c a t i n g a m e m b e r of the
c o m m o n h o r n b l e n d e group ( X - r a y p o w d e r p h o t o g r a p h R G M 201 852).
N o t o c c u r r i n g w i t h one of the g e m minerals, but still w o r t h y of m e n t i o n is
a h a n d s p e c i m e n consisting of a m p h i b o l e i n a lateritic r o c k ( R G M 107 189). M a n y
almost b l a c k , l o n g p r i s m a t i c to needle-like crystals, up to 20 m m i n length, are
visible. I n t h i n section the m i n e r a l is b l u i s h green, a n d has a distinct p l e o c h r o i s m
i n tones of a y e l l o w i s h green, β pale y e l l o w - g r e e n a n d γ b l u i s h green. T h e mean
e x t i n c t i o n angle γ Λ c is 1 8 ° , the m e a n optic a x i a l angle 2 V a = 8 2 ° . T h e refrac­
tive indices are a 1.631, β 1.647 a n d γ 1.653. T h e average value o b t a i n e d f r o m
density measurements is 3.099. T h e patterns of the X - r a y p o w d e r photographs
R G M 201 828 a n d 201 829, t a k e n of this a m p h i b o l e , have m u c h i n c o m m o n w i t h
that of the members of the actinolite group. T h e m i n e r a l therefore m a y be c o n ­
sidered an a m p h i b o l e b e l o n g i n g to this group.
KYANITE
K y a n i t e appears to be a c o m m o n m i n e r a l i n the U m b a m i n e , o c c u r r i n g abundantly,
i n t e r g r o w n w i t h c o r u n d u m a n d garnet. T h e author was given b o t h loose crystals
a n d handspecimens. R G M 107 188 consists of a great n u m b e r of l o n g b l a d e d
crystal fragments up to 45 m m . These have a light blue c o l o u r but i n parts are
colourless. I n t h i n section the m i n e r a l is colourless. O p t i c a x i a l angle measurements
gave a n average of 2 V a = 8 1 ° . T h e refractive indices are a 1.713, β 1.722 and
γ 1.728. D e n s i t y measurements gave a m e a n value of 3.639
T h e r e m a i n i n g specimens ( R G M 107 2 2 2 to 107 224. 107 228), are m a i n l y
c o m p o s e d of kyanite together w i t h a l m a n d i n e garnet, pale orange-pink c o r u n d u m
a n d v e r m i c u l i t e . R G M 107 2 2 8 , the biggest sample, is about 50 χ 35 m m . It has a
greenish blue c o l o u r , resembling that of aquamarine. Parts of the m i n e r a l are of
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
g e m quality. I n t h i n section it is almost colourless, its p l e o c h r o i s m extremely weak
i n tones of p a l e b l u e a n d colourless. T h e m e a n v a l u e of the o p t i c a x i a l angle 2 V a
was f o u n d to be 8 2 ° , its refractive indices are a 1.714, β 1.723 a n d γ 1.730. X - r a y
p o w d e r p h o t o g r a p h R G M 201 765 (specimen R G M 107 222) has a pattern char­
acteristic for kyanite.
VERMICULITE
A l t h o u g h most of the specimens described i n this paper are coated w i t h v e r m i c u l i t e ,
there are few samples i n w h i c h the m i n e r a l occurs i n i m p o r t a n t quantities ( R G M
107 216, 107 2 2 2 , 107 2 2 3 , 107 225 a n d 107 228). It has been f o u n d together
w i t h c o r u n d u m , especially w i t h r u b y as i n sample R G M 107 2 2 5 , a n d w i t h kyanite.
T h e size of the l a m i n a e m a y v a r y up to a diameter of about 35 m m , as i n specimen
R G M 107 2 1 6 .
T h e m i n e r a l has a g o l d e n b r o w n c o l o u r . I n t h i n section it is almost c o l o u r ­
less, its p l e o c h r o i s m therefore being weak. T h e o p t i c sign is negative, 2 V being
very s m a l l . T h e refractive indices are a 1.526, β 1.546 a n d γ 1.548. B y r a p i d
heating the m i n e r a l exfoliates w i t h m u c h swelling. X - r a y p o w d e r p h o t o g r a p h
R G M 201 3 3 8 , taken of specimen R G M 107 2 2 5 , was f o u n d to be a pattern
characteristic of v e r m i c u l i t e w i t h strongest diffraction lines 14.32(10)-4.55(7)3.58(6)-2.85(7)-2.62(5)-2.55(4)-2.39(7) a n d 1.530(7).
Summary
A d e s c r i p t i o n is given of various g e m minerals f r o m the U m b a area i n T a n z a n i a .
T h e refractive indices a n d densities of the r h o d o l i t e garnets v a r y w i d e l y , but a l l
lie between the properties of p y r o p e a n d a l m a n d i n e towards the p y r o p e e n d member. T h e size of the unit cell varies m a i n l y i n r e l a t i o n to b o t h the C a a n d M n
contents, as c a n be d e r i v e d f r o m m i c r o p r o b e analyses. T h e X - r a y p o w d e r patterns
are s i m i l a r to that of p y r o p e , whereas their absorption spectra show bands
characteristic of a l m a n d i n e . T h e results indicate that these rhodolites have n o
special properties b y w h i c h they c a n be distinguished f r o m either p y r o p e o r a l ­
m a n d i n e , except their rose-red c o l o u r . It is very u n l i k e l y that one o r another trace
element w i l l o n l y o c c u r i n this type of garnet a n d w i l l be responsible for its special
c o l o u r . Therefore it w o u l d be l o g i c a l to d i s c a r d the name. B o t h needles a n d short
p r i s m a t i c crystals of rutile are the most frequent m i n e r a l i n c l u s i o n s i n r h o d o l i t e .
T h e s e are not distinctive for the locality.
T h e a l m a n d i n e garnets generally have higher refractive indices a n d densities
t h a n the rhodolites. F r o m b o t h m i c r o p r o b e a n d wet analyses it is o b v i o u s that
their unit c e l l dimensions v a r y m a i n l y w i t h the C a a n d M n contents. B o t h apatite
a n d short p r i s m a t i c rutile are c o m m o n inclusions i n these garnets, a n d pyrrhotite
a n d z i r c o n are present o c c a s i o n a l l y . T h e s e i n c l u s i o n s are not diagnostic for the
locality.
T h e diversity of the colours of the c o r u n d u m s is fascinating. N o t o n l y r u b y
a n d sapphire, but also green, p i n k , deep violet, g o l d e n y e l l o w a n d p a r t i - c o l o u r e d
specimens o c c u r . M o s t of the properties are n o r m a l for c o r u n d u m . T h e absorption
39
40
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
spectra, however, are either r u b y o r sapphire spectra o r a c o m b i n a t i o n o f the t w o ,
depending o n the c o l o u r o f the specimen. E l e c t r o n m i c r o p r o b e determinations o f
green c o r u n d u m indicate a h i g h F e content w h i c h m a y be the cause o f its c o l o u r .
B l u e c o r u n d u m m a y o w e its c o l o u r t o T i , whereas the r e d variety has a h i g h C r
a n d a l o w T i content. R u t i l e a n d apatite o c c u r frequently as inclusions w h i l e ,
especially i n deep violet c o r u n d u m s , graphite is f o u n d . P y r r h o t i t e is rather
c o m m o n i n ruby. A p a r t f r o m the above m e n t i o n e d graphite, w h i c h u p to n o w
has o n l y been f o u n d i n U m b a c o r u n d u m s , these i n c l u s i o n s are n o t distinctive f o r
the l o c a l i t y .
T o u r m a l i n e s , h a v i n g a very attractive e m e r a l d green c o l o u r , show a deep
r e d under the C h e l s e a c o l o u r filter. T h e a b s o r p t i o n spectrum c a n be ascribed t o
c h r o m i u m a n d v a n a d i u m . E l e c t r o n m i c r o p r o b e determinations c o n f i r m that b o t h
C r a n d V cause the e m e r a l d green c o l o u r . Besides l i q u i d i n c l u s i o n s , as n o r m a l l y
seen i n t o u r m a l i n e , crystals are enclosed w h i c h c o u l d be apatite. T h e properties
of the green t o u r m a l i n e have m u c h i n c o m m o n w i t h those o f uvite.
O r a n g e a n d b r o w n i s h tourmalines have properties s i m i l a r to those o f elbaite.
T h e y o n l y c o n t a i n the tell-tale l i q u i d i n c l u s i o n s .
O r t h o p y r o x e n e s w i t h a deep y e l l o w i s h b r o w n c o l o u r appeared to be M g
r i c h , their m o l e c u l a r percentage of enstatite being about 8 8 . T h e absorption
spectrum c a n be ascribed t o ferrous i r o n . A light green c l i n o p y r o x e n e o f g e m
q u a l i t y was f o u n d to have m u c h i n c o m m o n w i t h the members o f the d i o p s i d e
group.
Y e l l o w scapolites f r o m the U m b a area have somewhat v a r y i n g properties.
B o t h m a r i a l i t e - r i c h a n d d i p y r e to m i z z o n i t e members were f o u n d .
Z i r c o n s w i t h r e d d i s h b r o w n to y e l l o w i s h c o l o u r s o c c u r . F r o m the g e m m o l o g i c a l p o i n t of v i e w the r e d d i s h b r o w n specimens are very attractive. A l l samples
e x a m i n e d are h i g h type z i r c o n s .
O n a s m a l l scale turquoise occurs. A c h e m i c a l analysis indicates a relatively
l o w C u content. M o s t of the m a t e r i a l is o f p o o r to moderate quality.
A m o n g s t the associated minerals kyanite attracts attention. B o t h light b l u e
a n d sea green varieties are f o u n d together w i t h c o r u n d u m a n d garnet. P a r t o f the
m a t e r i a l might be used for cutting. A d e s c r i p t i o n is g i v e n o f v a r i o u s amphiboles
a n d of v e r m i c u l i t e . T h e last n a m e d minerals o c c u r w i t h c o r u n d u m , the v e r m i c u l i t e
especially w i t h ruby.
References
Anderson, B. W., 1959. Properties and classification of individual garnets. - Jour. Gemmology,
7, 1: 1-7.
, 1971. Gem. Testing. - Butterworth & Co Ltd., London: 1-384.
Bank, H., 1970. Hochlichtbrechender orangefarbiger Korund aus Tansania. - Zt. Dt. Gemmol.
Ges., 19, 1: 1-3.
, 1972. Violette Edelkorunde aus dem Umba-Gebiet von Tansania. - Ibidem, 21,
2: 126-127.
Bank, H . & W. Berdesinski, 1967. Chromhaltiger Turmalin. - Z. Dt. Ges. Edelsteink.,
61: 30-32.
Bank, H., W. Berdesinski & J. Ottemann, 1970. Durchsichtiger smaragdgrüner Grossular aus
Tansania. - Z. Dt. Gemmol. Ges., 19, 1: 4-7.
Bank, H. & B. Nuber, 1969. Rosarote Granate aus Tansania. - Ibidem, 18, 2: 69-72.
Zwaan, Gem minerals from Umba, Tanzania, Scripta Geol. 20 (1974)
Campbell, I. C. C , 1972. A comparative study of Rhodesian rhodolite garnet in relation to
other known data and a discussion in relation to a more acceptable name. - Jour.
Gemmology, 13, 2: 53-64.
Crowningshield, R., 1967-68. Developments and Highlights at the Gem Trade Lab in N ew
York. - Gems Gemology, 12, 8: 242-243.
Deer, W. Α., R. A. Howie & J. Zussman, 1962. Rock-forming Minerals. - Longmans, London:
1: 1-333 and 5: 1-371.
Eppler, W. F., 1972. N eedles in corrundum other than rutile. - Jour. Gemmology, 13,
2: 41-44.
Harder, H., 1969. Farbgebende Spurenelemente in den natürlichen Korunden. - Ν. Jb. Miner.
Abh., 110, 2: 128-141.
Hidden, W. Ε. & J. Η. Pratt, 1898. On rhodolite, a new variety of garnet. - Am. Jour. Sei.,
4, 5: 294-296.
Hiemstra, S. Α., 1956. An easy method to obtain X-ray diffraction patterns of small amounts
of material. - Amer. Mineral., 41: 519-521.
Lorimer, G. W. & P. E. Champness, 1973. Combined Electron Microscopy and Analysis of an
Orthopyroxene. - Ibidem, 58: 243-248.
Martin, B. F., 1970. A study of rhodolite garnet. - Jour. Gemmology, 12, 2: 29-36.
Schubnel, H. J., 1972. Pierres précieuses dans le monde. - Horizons de France, Paris: 1-192.
Solesbury, F. W., 1967. Gem corundum pegmatites in Ν. Ε. Tanganyika. - Economic Geology,
62: 983-991.
Stockwell, C. H., 1927. An X-ray study of the garnet group. - Amer. Mineral., 12: 327-344.
Trumper, L. C , 1952. Rhodolite and the pyrope almandine series. - Gemmologist, 21,
246: 26-30.
Webster, R., 1961. Tanganyika Tourmaline. - Ibidem, 30, 356: 41-45.
, 1972. Gems, Their Sources, Descriptions and Identification. - Butterworth & Co Ltd,
London: 1-838.
Zwaan, P. C , 1955. On the determination of pyroxenes by X-ray powder diagrams. - Leidse
Geol. Meded., 19: 167-276.
, 1961. Some notes on the identification of the pyrope-almandine garnets. - Proc. Kon.
Ned. Acad. Wet. B, 64, 2: 305-312.
, 1965. Apatite crystals in a Ceylon spinel. - Jour. Gemmology, 9, 12: 434-440.
, 1967. Solid inclusions in corundum and almandine garnet from Ceylon, identified by
X-ray powder photographs. - Ibidem, 10, 7: 224-234.
, 1971. Yellow scapolite, another gem-mineral from Umba, Tanzania. - Ibidem, 12,
7: 304-309.
M a n u s c r i p t received 9 N o v e m b e r 1 9 7 3 .
41