Clays and Clay Minerals,
Vol.44, No. 4, 540-545, 1996.
R E F I N E M E N T OF THE S T R U C T U R E OF
NATURAL FERRIPHLOGOPITE
MARIA FRANCA BRIGATFI, LUCA MEDICI, AND LUCIANO POPPI
Dept. of Earth Sciences, University of Modena-Largo S. Eufemia, 19-41100 Modena, Italy
A b s t r a c t - - T w o ferriphlogopite-lM crystals with a composition (K0.99Na0.01)l~_l.0o(Mg2.73Fe2+0.17Fe3+0.08Ti0.01)~_2.99[(Fe3+0.95Si3.05)~=4.o0Olo.17](OH)l.79F0.04 (sample S1) and (K1.02)~_l.o2(Mg2.68Fe2+0.2oFe3+0.11Mno.ol)~_3.00[(Fe3+o.gsSi3.05)~ 4.00Olo.18](OH)l.75Fo.07(sample $2) occur within an alkali-carbonatic complex
near Tapira, Belo Horizonte, Minas Gerais, Brazil. Each crystal was studied by single-crystal X-ray
diffraction. The least-squares refinements of space group C 2 / m resulted in R values of 0.031 for S1 and
0.025 for $2. Results showed that Fe 3§ substitutes for Si within the tetrahedral sites and that the Fe
distribution is fully disordered. The octahedral sites are preferentially occupied by Mg. The presence of
Fe 3+ within the tetrahedral sheet produces increased cell edge lengths. For sample S1, a = 5.362 A, b =
9.288 ,~, c = 10.321 ,~ and the monoclinic [3 angle was: [3 = 99.99 ~ For sample $2, a = 5.3649 A, b
= 9.2924 it, c = 10.3255 ,~ and the monoclinic [3 angle was: 13 = 99.988 ~ The tetrahedral rotation angle
of et = 11.5 ~ is necessary for tetrahedral and octahedral sheet congruency. The enlarged tetrahedral sites
are regular, with cations close to their geometric center. Ferriphlogopites have identical mean bond lengths
for M1 and M2 sites within standard deviation. The M1-O3 and M2-O3 bond lengths are longer than the
mean so that 0 3 may articulate with the tetrahedra.
Key Words----Crystal structure refinement, Ferriphlogopite.
INTRODUCTION
This p a p e r reports the detailed c r y s t a l c h e m i s t r y o f
a n e a r e n d - m e m b e r f e r r i p h l o g o p i t e - l M . T h e crystals
were o b t a i n e d f r o m a n a l k a l i - c a r b o n a t i c c o m p l e x outc r o p p i n g n e a r Tapira ( B e l o Horizonte, M i n a s Gerals,
Brazil). A l t h o u g h e v i d e n c e o f Fe 3+ tetrahedral substitution has b e e n r e p o r t e d b y several authors for trioct a h e d r a l true m i c a s u s i n g c h e m i c a l ( J a k o b 1925; F a r m er a n d B o e t t c h e r 1981; N e a l a n d Taylor 1989; G u i d o t t i
a n d D y a r 1991) a n d spectroscopic t e c h n i q u e s ( D y a r
1990; R a n c o u r t et al. 1992, 1994), t h e r e are f e w structure r e f i n e m e n t s o f crystals w i t h Fe 3§ substituting for
Si 4+ (Steinfink 1962; D o n n a y et al. 1964; S e m e n o v a
et al. 1977, 1983; H a z e n et al. 1981; C r u c i a n i a n d
Z a n a z z i 1994; Brigatti et al. 1995). F u r t h e r m o r e , there
are n o r e c e n t reports o f natural trioctahedral m i c a s
w i t h o u t A13+ a n d w i t h o c t a h e d r a l a n d interlayer content close to e n d - m e m b e r phlogopite.
Steinfink (1962) d e t e r m i n e d the crystal structure o f
a trioctahedral m i c a u s i n g p r e c e s s i o n a n d W e i s s e n b e r g
p h o t o g r a p h s . Its c o m p o s i t i o n was: (Ko.9Mno.l)Mg3Si 3
( F e , M n ) l O l o ( O H ) 2. F r o m t h e s t r u c t u r e r e f i n e m e n t ,
Steinfink located M n into the interlayer site a n d
s h o w e d that iron p r o d u c e d tetrahedral ring distortion
f r o m h e x a g o n a l s y m m e t r y to o n e that i n v o l v e s tetrah e d r a l rotations o f a p p r o x i m a t e l y 12 ~ D o n n a y et al.
(1964) d e r i v e d f e r r i p h l o g o p i t e structural p a r a m e t e r s
theoretically u s i n g unit-cell data a n d the crystal c h e m ical c o m p o s i t i o n r e p o r t e d b y Steinfink (1962). T h e calculated values for s o m e octahedral p a r a m e t e r s disagree w i t h those d e t e r m i n e d e x p e r i m e n t a l l y , b e c a u s e
M 1 - O 4 a n d M 2 - O 3 d i s t a n c e s are m u c h greater t h a n
t h o s e f o u n d experimentally. F u r t h e r m o r e , D o n n a y et
Copyright 9 1996, The Clay Minerals Society
al. (1964), u s i n g the W e i s s e n b e r g film technique, were
the first to refine a synthetic f e r r i a n n i t e crystal. C o m plete r e f i n e m e n t was not p o s s i b l e (final R = 9 . 3 % )
b e c a u s e o f t w i n n i n g . T h e ferriannite structure differed
m a r k e d l y f r o m that o f a crystal studied b y Steinfink
(1962). T h e ferriannite structure o f D o n n a y h a d different c o o r d i n a t e s for several a t o m s a n d it h a d a tetr a h e d r a l r o t a t i o n a n g l e o f 6.3 ~ w h e r e a s that o f Steinrink was 12 ~.
S e m e n o v a et al. ( 1 9 7 7 ) , s t u d i e d a f e r r i p h l o g o p i t e
o f c o m p o s i t i o n : (Kl.03Na0.09Ca0.04)z~=x.16(Mg2.89Fe02~6"4+ 9
M n 0.01)~=3.06 [ ( A1 0.08F e 3+
0.85T1 0.03S1 2.98) E=3.94O 10] ( O H ) 2
Site o c c u p a n c i e s are c o m p l e x a n d are o f n o n - i n t e g e r
values. T h e y n o t e d that, unlike other m i c a structures,
tetrahedral Fe is located at the g e o m e t r i c c e n t e r o f the
t e t r a h e d r o n a n d that, as o b s e r v e d b y Steinfink (1962),
the tetrahedral rings deviate f r o m h e x a g o n a l . T h e discovery of well-crystallized ferriphlogopite with a
c h e m i c a l c o m p o s i t i o n close to that o f e n d - m e m b e r
p h l o g o p i t e p r o v i d e d a n e x c e l l e n t o p p o r t u n i t y to study
the c r y s t a l c h e m i s t r y o f ferriphlogopite in detail. T h i s
Table 1. Selected crystal data and unit cell parameters.
a (•)
b (A)
c (]k)
[3 (o)
V (A 3)
Nob~
Rsym X 100
Robs • 100
540
Sl
$2
5.362(1)
9.288(1)
10.321(2)
99.99(1)
506.2(1)
991
1.97
3.13
5.3649(4)
9.2924(6)
10.3255(9)
99.988(8)
506.95(7)
894
2.28
2.47
Note: R~ym = (~hk, E~_,llh~ -- Ihd)/(:~, ~ - , IhkO.
Vol. 44, No. 4, 1996
Refinement of the structure of natural ferriphlogopite
541
Table 2. Chemical data (a = weight %; b = unit cell content#) and mean atomic number ( e ) of octahedral (M) and interlayer
(K) sites (c) for the refined ferriphlogopite crystals.
a
SiO2
TiO2
Fe203
FeO
MgO
MnO
BaO
Na20
K20
H20
F
Sum
b
S1
$2
S1
40.40
0.18
18.06
2.74
24.31
0.07
0.09
0.10
10.32
3.55
0.18
100.00
40.13
0.09
18.55
3.15
23.71
0.09
0.04
0.00
10.48
3.45
0.31
100.00
Si
Fe 3+
[4]Sum
Ti
Fe 3§
Fe 2+
Mg
Mn
[~
Na
K
[~2]Sum
OH
F
O
Sum
c
$2
3.05
0.95
4.00
0.01
0.08
0.17
2.73
3.05
0.95
4.00
0.11
0.20
2.68
0.01
3.00
2.99
0.01
0.99
1.00
1.79
0.04
10.17
12.00
M1 Xref
M2 Xref
M I + M 2 Xreft
M1 + M 2 EPMA~:
K Xref
K EPMA
SI
$2
13.48(2)
13.12(3)
39.7
39.5
19.0(1)
18.8
13.78(5)
13.53(3)
40.8
40.5
19.46(4)
19.4
1.02
1.02
1.75
0.07
10.18
12.00
Key: # calculated on the basis of O]2_x_y(OH)xFy. Xref = X-ray refinement; EPMA = electron microprobe; t (2M2+M1);
= sum of octahedral cation electrons. Standard deviations are given in parentheses.
study a n d the r e f i n e m e n t o f the c r y s t a l s y m m e t r y is
significant b e c a u s e p r e v i o u s l y refined crystal structures are o f p o o r quality a n d i n v o l v e crystals w i t h
c o m p l e x c h e m i c a l substitutions.
EXPERIMENTAL
Data Collection and Refinement
The two Al-free phlogopite crystals e x a m i n e d (sample
T a s 2 2 - 1 : S 1 and T p q l 6 - 6 B : $2) occur within ultramafics
from near Tapira, 300 k m West o f Belo Horizonte, Minas
Gerais, Brazil. Locality description, petrogenesis and
data for host rock crystallization are given by Beurlen
and Cassadanne (1981) and Brigatti et al. (1995). Crys-
tals for the X-ray study were c h o s e n from crushed rock.
Only Al-free crystals w h o s e precession photographs displayed sharp reflections and minimal spot streaking for
k ~ 3n reflections were selected. The absence o f h + k
2n reflections confirmed the C-centered space group.
T h e intensity distribution along rows (13/) and (021) indicated the 1M polytype (Bailey 1988). Two crystals
were selected; their dimensions were: sample S1 = 0.20
• 0.16 • 0.08 m m and sample $ 2 = 0.25 • 0.19 x
0.09 m m . E a c h crystal was m o u n t e d onto a Siemens P4
rotating-anode single-crystal diffractometer. T h e operating conditions were: M o - K a graphite-monochromatized
radiation, operated at 40kV, 2 0 3 m A and equipped with
Table 3. Final atomic fractional coordinates and equivalent isotropic (~) and anisotropic (_~2 • 104) thermal factors.
Atom
x/a
y/b
z/c
B~q
13,,t
[32zt
~333t"
[312t
Ol
02
03
04
T
M1
M2
K
S1
-0.0024(5)
0.3360(3)
0.1305(2)
0.1335(3)
0.07557(7)
0.0
0.0
0.0
0.0
0.2206(2)
0.1669(1)
0.5
0.16667(4)
0.0
0.33281(8)
0.5
0.1705(2)
0.1704(2)
0.3918(1)
0.3990(2)
0.22663(4)
0.5
0.5
0.0
2.12(5)
2.14(4)
0.61(2)
0.60(3)
0.64(1)
0.54(2)
0.48(1)
2.16(2)
212(8)
210(6)
40(3)
41(4)
45(1)
30(3)
23(2)
206(3)
69(3)
78(2)
14(1)
18(2)
17(1)
12(1)
11(1)
73(1)
36(2)
35(1)
23(1)
19(1)
20(1)
22(1)
20(1)
40(1)
0
-9(3)
-1(1)
0
0(1)
0
0
0
-1(3)
25(2)
10(1)
8(2)
9(1)
9(1)
9(1)
16(1)
0
-10(1)
0(1)
0
0(1)
0
0
0
O1
02
03
04
T
M1
M2
K
$2
-0.0020(4)
0.3357(3)
0.1304(2)
0.1331(3)
0.07552(7)
0.0
0.0
0.0
0.0
0.2203(2)
0.1671(1)
0.5
0.16664(4)
0.0
0.33280(8)
0.5
0.1705(2)
0.1706(1)
0.3914(1)
0.3988(2)
0.22666(3)
0.5
0.5
0.0
2.04(5)
2.02(4)
0.62(2)
0.61(3)
0.70(1)
0.61(2)
0.60(1)
2.11(2)
194(8)
215(6)
53(3)
54(4)
62(1)
48(3)
47(2)
216(3)
70(3)
69(2)
14(1)
16(2)
19(1)
14(1)
14(1)
72(1)
33(2)
32(1)
19(1)
17(1)
19(1)
20(1)
20(1)
34(1)
0
-6(3)
2(1)
0
0(1)
0
0
0
-11(3)
19(2)
3(1)
6(2)
6(1)
6(1)
7(1)
10(1)
0
-10(1)
0(1)
0
0(1)
0
0
0
t exp[-(hZ[3~ + . . . + 2hk[312 + . . .)]. Standard deviations are given in parentheses.
~,3t
[323t
Clays and Clay Minerals
Brigatti, Medici, and Poppi
542
Table 4. Bond lengths (/~) and other important structural parameters.
T-O1
T-O2
T-O2'
T-O3
(T-O)
M1-O3 (•
M1-O4 (•
$1
$2
Tetrahedron
1.680(1)
1.679(2)
1.681(2)
1.678(1)
1.680
1.6804(9)
1.677(2)
1.684(2)
1.675(1)
1.679
K-O1 (•
K-OI' (•
K-O2 (•
K-O2' (•
(K-O)~....
(K-O) .....
A(K-O)
2.105(1)
2.062(2)
2.091
M2-O3 (•
M2-O3' (•
M2-O4 (•
(M2-O)
Octahedron M1
2.100(1)
2.058(2)
(M1-O)
2.086
S1
$2
Interlayer
Parameter
Tetrahedral rotation ct (~
Basal oxygen Az (,~)
Tetrahedral quadratic elongation TQE 1
Tetrabedral angle variance TAV 1
Tetrahedral bond length distorsion BLDT2 (%)
Tetrahedral angular distorsion ADT2 (%)
Tetrahedral edge length distorsion ELDr 2 (%)
Tetrahedral basal edges mean bond length (O-O)basal ('~)
Tetrahedral volume (/~3)
Octahedral bond length distorsion BLDM2 (%)
3.464(3)
2.930(2)
3.461(2)
2.932(2)
2.931
3.462
0.531
3.464(2)
2.933(2)
3.464(2)
2.934(2)
2.934
3.464
0.530
Octahedron M2
2.094(1)
2.099(1)
2.066(1)
2.086
2.096(1)
2.102(1)
2.068(1)
2.089
11.5
0.001
1.0000
0.20
0.06
0.37
0.23
2.736
2.431
0.89
0.66
11.907
11.904
3.0967
2.7967
3.0967
2.7967
1.1073
1.1073
11.5
0.001
1.0000
0.19
0.18
0.35
0.15
2.738
2.429
0.91
0.65
11.983
11.950
3.1013
2.8047
3.0960
2.8030
1.1058
1.1045
Sheet thickness (/~)
Tetrahedral
Octahedral
2.277
2.151
2.272
2.159
Interlayer separation (,~,)
Dimensional misfit Aa,M3 (A)
3.459
0.629
3.466
0.618
Octahedral volume (,~)
Octahedral unshared edges mean bond length eu3 (lk)
Octahedral shared edges mean bond length e~3 (A)
eu/e~3
M1
M2
M1
M2
M1
M2
MI
M2
M1
M2
Note: 1 Robinson et al. 1971; 2 Renner and Lehmann 1986; 3 Toraya 1981. Standard deviations are given in parentheses.
X S C A N S software (Siemens 1993). Cell parameters
were d e t e r m i n e d using least-squares refinement o f 120
m e d i u m - h i g h angle reflections. T h e s e values are g i v e n
in Table 1 together with other i n f o r m a t i o n relative to
data collection and refinement. T h e limiting spheres
were s a m p l e d for the r a n g e 3 -< 2 0 --< 70 ~ ( - 1 --< h -<
8; - 1 4 --< k - < 14; - 1 6 --< 1 -< 16) using the co scan
w i n d o w width o f 2.6 ~ for S 1 a n d 1.5 ~ for $ 2 and variable s c a n n i n g speeds f r o m 1 to 30~
T h r e e standard
reflections were c h e c k e d e v e r y 100 reflections to m o n itor crystal a n d electronic stability. N o instability was
found. Lorentz-polarization corrections were m a d e and
absorption effects corrected using a c o m p l e t e t~ scan
f r o m 0 - 3 6 0 ~ at 10 ~ ~J intervals. T h e r e were ten selected
reflections for S 1 a n d fourteen for $ 2 (• > 83~ Intensity data of symmetrically-equivalent reflections were
averaged a n d the resulting discrepancy factor (RsynO calculated (Table 1).
The structure refinements were m a d e b y a full-matrix
least-squares procedure. Reflections were selected with
I --> 5 ~ (I) and using O R F L S (Busing et al. 1962).
A t o m i c parameters f r o m Brigatti a n d Davoli (1990) for
space group C2/m were used as initial values for e a c h
refinement. Fully ionized scattering factors were used
for octahedral M1 and M 2 a n d ditrigonal K + sites.
M i x e d scattering factors ( 0 / 0 2 - ) were a s s u m e d for anion sites, A composite 7 5 % Si and 2 5 % Fe versus 7 5 %
Si 4+ and 2 5 % Fe 3+ was used for tetrahedral sites. For
the final steps o f anisotropic refinement, scattering
c u r v e s appropriate to c o m p o s i t i o n were applied.
A f t e r r e f i n e m e n t , the scattering c o n t r i b u t i o n s for H
were e x a m i n e d for difference e l e c t r o n - d e n s i t y m a p s
Vol. 44, No. 4, 1996
Refinement of the structure of natural ferriphlogopite
I
%7
v
I
1
543
I
I
9~,z
3.10
g3.4
A
O
I
1
v
04
3.08
%7
V3.2
%7
II)
~3.0
II1
(~0
O@ 0
~7~
04
C~
A
O
302
i
0
I
V
2.8
2.66
U'
I
2.68
,
I
~
t
2.70
2.72
<0--0>,,...,
~
~
3.06
O
I
2.74.
(i)
Figure 1. Variation of longer K-O mean bond length (triangles= <K-O>oo,~) and shorter K-O mean bond length (circles= < K - O > ~ , ) vs. mean length of tetrahedral basal edges
(<O-O>b~). Filled symbols= {4]Fe3+-rich phlogopites, sampies: 1 (S1) and 2 ($2) from this study, 3 from Semenova et
al. (1977), and 4 from Brigatti et al. (1995). Open symbols=
Al-rich phlogopites, phlogopites and biotites from literature
(Alietti et al. 1995; Bigi and Brigatti 1994; Brigatti and Davoli 1990; Hazen and Burnham, 1973; Joswig 1972; Ohta et
al. 1982; Takeda and Ross 1975).
(DED). Small positive anomalies were found near the
0 4 atom (atomic coordinates = 0 . l l , 0.5, 0.33) suggesting that the O H vector is almost normal to plane
(001). Nevertheless, O - H distances o f 0.69 ,& were
shorter than expected one (0.95 ,~) and their D E D intensity was less than 4 (r above background. Thus, the
H positions were not reliable and were not refined.
A v e r a g e electron counts o f cation sites are reported
in Table 2. A t o m i c coordinates and temperature parameters are listed in Table 3. Relevant bond distances,
selected tetrahedral, octahedral and interlayer parameters are reported in Table 4. O b s e r v e d and calculated
structure factors are available f r o m the authors upon
request.
C h e m i c a l Analyses
Electron-probe microanalysis ( E P M A ) were performed with an A R L - S E M Q instrument using wavelength-dispersive techniques. Operating conditions
were 15-kV accelerating voltage, 15-nA sample current and a defocused electron b e a m (spot size o f about
3 ixm). The same crystals were used for both the refinement and the chemical analyses. Each analyzed
crystal was chemically homogeneous. Semiquantitative scanning for all elements with Z > 8 did not reveal additional elements within detection limits. Ti and
Ba contents were corrected for overlap of Tik~ and
BaL~ peaks. The F - was obtained following the m e t h o d
2.66
2.68
2.70
2.72
<0--0>bo,,,
2.74
(i)
Figure 2. Length of unshared octahedral edges (e~M2) vs.
mean length of tetralaedral basal edges (<O-O>bas~). Symbols
and samples as in Figure 1.
of F o l e y (1989). Chlorine was b e l o w the detection limit (X < 0.001 wt%). Crystals used for ( O H ) - and Fe 2+
determination were obtained from the same samples
that yielded the crystals used for the structure refinement. The weight loss was determined by thermogravimetric analysis in A r gas to prevent Fe oxidation
using a Seiko 5200 thermal analyzer. Heating rate was
I0 ~
and the flow rate was 200 ml/min. The Fe 2+
was measured by the s e m i - m i c r o v o l u m e t r i c method
( M e y r o w i t z 1970). T h e oxide percentages and the
structural formulae based on O12_x_y(On)xFy are reported in Table 2.
DISCUSSION
The phlogopites studied lack A1 and are deficient in
Si, which accounts for Fe 3+ tetrahedral substitution.
Moreover, the phlogopites have a large quantity of M g
and a small quantity o f Ti and Mn. Therefore, their
octahedral composition is near to e n d - m e m b e r phlogopite.
For phlogopites and biotites, the tetrahedral basal
bond lengths increase as t4]A1 substitutes for [4]Si and
for Al-rich phlogopites, the increased substitution produces a m o r e regular tetrahedra (Alietti et al. 1995).
Fe 3+, a cation with a radius greater than A13+, emphasizes those effects (Table 4). Crystals with tetrahedral
substitutions up to 24% are characterized by m o r e regular ( T Q E = 1.0000; ELDT -< 0.24; BLDT --< 0.0074;
TAV --< 0.20; ADT --< 0.37; symbols as in Table 4) and
larger tetrahedra ( < T - O > -> 1.679/~) with increasing
substitution. These features are also consonant with
basal o x y g e n ' s low deviation f r o m coplanarity (Az --<
0.001 A). Moreover, the position o f the tetrahedral cat-
544
Brigatti, Medici, and Poppi
I
I
I
o f the central atom from the geometrical center o f the
polyhedron is toward O4 and the bond length distortion parameter BLDM (Table 4) is slightly greater for
the M1 site.
The large cell v o l u m e s relative to phlogopite are
due to the lengthening o f a, b and c cell parameters
(Table 1). The [~ angles are a m o n g the lowest found
for p h l o g o p i t e s - l M . Thus, we infer that the [3 angle is
related to tetrahedral sheet composition. Figure 3 illustrates several relationships:
2
1
03
505
04
500
V
0
~
W
495
0
0
0
0
V
9~
O
490
'V
V
0.9
1.0
1.1
1.2
1.3
1.4
Clays and Clay Minerals
1.5
4-t4]si (apfu)
Figure 3. Unit-cell volume vs. the amount of tetrahedral
substitution. Symbols and samples as in Figure 1.
ions, as indicated by individual T-O bond lengths and
by BLDT values, is near the middle o f the polyhedron.
A n i n c r e a s e o f tetrahedral e d g e lengths ( < O O > b a s a l ) by Fe 3+ substitution produces an increase o f
the tetrahedral sheet dimensions. This is also reflected
within the g e o m e t r y o f the tetrahedral ring, with the
reduction o f the shorter K - O m e a n bond lengths ( < K O>i,,r
(Figure 1). The increase o f the longer K - O
bond lengths (<K-O>o~t~) can be produced not only
by the presence in 0 4 position of ( O H ) - groups, as
o b s e r v e d for muscovite ( G u g g e n h e i m et al. 1987), but
also occurs to match the lateral dimensions of the tetrahedral and octahedral sheets. The increase of A K
(<K-O>outer-<K-O>inner) produces an increase in the
distortion o f the tetrahedral ring, which has angle values (ct = 11.5 ~ a m o n g the highest o f those previously
reported for trioctahedral 1M true micas.
M1 and M 2 octahedral sites are approximately regular and their dimensions are similar, within experimental error. The M1 and, in particular, the M 2 sites
o f the crystals studied are m u c h larger and less distorted than those of other phlogopites.
The high values of the dimensional misfit (ARM; ArM
> 0.61 /~) between tetrahedral and octahedral sheets
(Table 4) is mostly compensated for, as in all other
micas, by an increased tetrahedral ring distortion. Other than an individual tetrahedron increase, there is a
concomitant increase in the size o f the octahedra. This
is mostly reflected by the increased unshared edges of
the M 2 polyhedron (Figure 2).
As in phlogopite, the M - O 3 distances are considerably larger than the M - O 4 bond lengths for both M1
and M 2 sites. Similar results are expected w h e n m o n o valent F - anions and/or (OH) groups occupy the 0 4
position ( G u g g e n h e i m et al. 1987). The displacement
- - f o r phlogopite-biotite crystals, the increase o f A1
is mostly related to exchange vectors balancing the
octahedral charge increase. The most important exchange vector for Al-rich phlogopites is [41Ala+[6]A13+[4]Si4_~[6]Mg2_~ and the unit-cell v o l u m e remains
nearly constant. Whereas for biotite, the cell v o l u m e
varies considerably, perhaps due to a c o m p l e x set o f
exchange vectors affecting m a n y cation sites.
- - f o r ferriphlogopite crystals showing the main exchange vector ta]Fe3+t41A13Ji, we observed a large increase of the unit-cell dimensions e v e n for low tetrahedral substitution values. Cell volume, in accordance with other structural parameters, is mainly influenced by [4]Fe3§ content.
ACKNOWLEDGMENTS
The authors would like to acknowledge L. Beccaluva, E
Siena and C. Vaccaro for making rock specimens available
for investigation, and E. Galli for their support and helpful
discussions. The clarity of the manuscript was improved by
critical readings by D. L. Bish, J. E. Mauk and an anonymous
reviewer. This research was supported by MURST and CNR
of Italy. The diffraction facilities were supported by Modena
University (Centro Interdipartimentale Grandi Strumenti).
The CNR is also acknowledged for financing the Electron
Microprobe Laboratory at Modena University.
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(Received 24 February 1995; accepted 13 November 1995;
Ms. 2629)