MOSSBAUER SPECTROSCOPY AND CRYSTAL CHEMISTRY
OF NATURAL Fe-Ti GARNETS
by
Kenneth Bruce Schwartz
S.B., Massachusetts Institute-of Technology.(1976)
Submitted in Partial Fulfillment
of the Requirements for the
Degree of
Master of Science
. at the
Massachusetts Institute of Technology
May 6, 1977.
Signature of Author
(and date)
Department of Earth and Plane ary Sciences
Certified by
, TJhesis Supervisor
Accepted by.
UIT I
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i
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TABLE OF CONTENTS
page
Abstract
1
I.
Introduction
2
II.
Samples and Experimental Techniques
8
III.
Mbssbauer Spectroscopy of Melanites
14
and Schorlomites
IV.
Chemical Analyses of Titaniferous Garnets
55
V.
Crystal Chemistry of Natural Fe-Ti Garnets
63
VI.
Optical-absorption Properties and Color
68
VII.
Conclusions
74
Acknowledgments
76
References
77
Biographical Sketch
80
Appendix
Al
-
ii
-
LIST OF FIGURES
Figure 1
3
Figure 2
17
Figure 3
23
Figure 4
31
Figure 5
34
Figure 6
39
Figure 7
41
Figure 8
43
Figure 9
45
Figure 10
47
Figure 11
69
LIST OF TABLES
Table 1
9
Table 2
11
Table 3
12
Table 4
15
Table 5
16
Table 6
57
Table 7
58
Table 8
60
Table 9
64
-1Abstract
A suite of thirteen melanite and schorlomite garnets from
a variety of geological and geographical terrains have been
studied by Mssbauer spectroscopy, wet chemistry, and electron
microprobe analysis.
The titanium contents of these garnets
varied from 2.1 to 15.1 wt. % TiO
The M~ssbauer spectra
were much more complex than those for andradite garnets,
indicating mixed valencies and site occupancies for the iron
cations.
A more sophisticated fitting procedure than any
previously attempted was employed in order to accurately
interpret these complex spectra.
The crystal chemistry of natural Fe-Ti garnets is dependent on bulk chemistry and mineral paragenesis.
Any silicon
deficiency is accounted for by substitution of Al and Fe into
the tetrahedral site with Ti occupying octahedral coordination.
Divalent cations are found in 8-fold coordination,
except when an excess of Ca drives them into the octahedral
site.
Significant amounts of Ti3+ have been identified in some
samples due to discrepancies between values of Fe 2+
3+ found
by Massbauer spectroscopy and standard chemical analysis.
The
variation of color from orange-brown to deep red and opaque
This variation seemed to be
3+
responsible for the dark
correlated with Ti content, ,with Ti
was studied in zoned samples.
colors in some Fe-Ti garnets.
-2-
Mssbauer Spectroscopy and Crystal Chemistry
of Natural Fe-Ti Garnets
I. Introduction
Garnet is an orthosilicate having eight formula units,
X 3 Y2
3 01 2 ,
per unit cell.
There are three types of oxygen
coordination polyhedra formed about cations.
The X cations
are coordinated by eight oxygens in the form of a distorted
cube, described by some crystallographers as a triangular
dodecahedron.
The Y cations are octahedrally coordinated
and the Z cations have tetrahedral coordination.
The
structure consists of alternating ZO4 tetrahedra and YO
6
octahedra, which share corners to form a continuous three
dimensional framework (Figure 1).
The tetrahedra are
independent in that they do not share corners with one
another.
The octahedra also have this characteristic.
The
only edge sharing involves the dodecahedra, which share
edges with tetrahedra, octahedra, and other dodecahedra.
The crystal chemistry of garnets is described by Novak and
Gibbs (1971).
The general formulae for simple silicate garnets are
expressed by quadravalent cations (e.g. Si4+ ) in the
tetrahedral site, trivalent cations in the octahedral site
and divalent cations in 8-fold coordination.
Titanium can
occur in the garnet structure in amounts up to 15% by weight
-3-
Figure 1.
Portion of the garnet structure
projected down the c axis showing the
framework of alternating tetrahedrgi and
octahedra (shaded portion) and the 8-fold
triangular dodecahedra coordinating the
X cation.
Large open circles represent
oxygens, smaller ones the Y cations, solid
circles the Z cations, and hatched ones the
X cations.
From Novak and Gibbs (1971).
-Lv
-5-
It is usually restricted in natural garnets to
3+
the andradite species, Ca 3 Fe 3
2 Si 3 0120 It also substitutes,
of TiO
in small amounts, into the grossular component, Ca3Al2S 3012.
The introduction of titanium into silicate garnets has
long been known to wreak havoc on conventional crystal
chemical principles (Howie and Woolley, 1968; Dowty, 1971;
Burns, 1972; Huggins, Virgo and Huckenholz, 1976b).
Chemical analyses of titanium-rich garnets have revealed
that the ratio of divalent to trivalent to quadrivalent
cations deviates from 3:2:3 found in most non-titaniferous
garnets.
Usually there is an excess of divalent and quadri-
valent cations and a dearth of trivalent cations.
In
addition, the inclusion of titanium into the garnet structure
is accompanied by a severe silicon deficiency.
Thus, the
amount of silicon falls short of the theoretical 6.0 cations
in the Z site per 24 oxygens.
Another'problem involving titanium garnets concerns
the oxidation state of iron and the proportions of Fe 2 + and
Fe3+ ions.
It has been observed that Fe +/Fe + ratios for
melanites and schorlomites can be higher from wet chemical
analysis than those from M~ssbauer analysis of the same
sample (Burns, 1972; Whipple, 1973).
Such discrepancies
arise from the assumption that all reduced cations in the
2+
samples are Fe2.
It has been postulated that some of those
cations are, in fact, some other reduced species (e.g. Ti +)
3+
in these garnets has been suspected for
The presence of Ti
-6-
some time (Zedlitz, 1933; Howie and Woolley, 1968).
Recently, optical absorption spectroscopy has been used
to confirm the presence of Ti3+
(Manning and Harris, 1970;
Moore and White, 1971; Burns, 1972). Although a quantitative
analysis of Fe2+/EFe and Ti 3+/Ti values could not be
obtained, certain features of the garnet spectrum were
assigned to interactions including Ti 3+
M5ssbauer spectroscopy has emerged as potentially the
most accurate way of determining the role of Fe and Ti in
the crystal structure of melanites and schorlomites.
The
difference between Fe + determined by M6ssbauer analysis
and the total reducing capacity of the sample determined by
wet chemistry is the only quantitative measure of Ti3 +.
The
earliest Mossbauer study (Dowty, 1971) was inconclusive
because of a lack of wet chemical data and because controversy
exists over its M*ssbauer assignments.
Other attempts have
been made (Burns, 1972; Whipple, 1973; Huggins
et al., 1976b)
with varying degrees of success.
The complexity of M'ssbauer spectra for melanites and
schorlomites have hindered the previous studies.. Prior to
the present work, the most complicated spectra have not been
dealt with adequately.
This is mostly due to the large
number of low intensity peaks which are obscured in the
spectra.
Naturally, the proper solution of these spectra
must be the first step towards determining the crystal
chemistry of Fe-Ti garnets.
Once the coordination and valence
-7-
of Fe has been deduced, it can be combined with wet chemical
and electron microprobe analysis to describe cation distributions in the garnet structure.
This is accomplished with
the help of site preference data for Al, Fe, and Ti in
synthetic garnets (Huggins
et al. , 1976a).
The cation
distributions used in the present study are based on that
information.
But, the key to this procedure is obtaining
accurate information on the chemistry of Fe from a series
of complicated Mossbauer spectra.
M6ssbauer spectra for Fe-Ti garnets also indicate that
there may be a systematic variation in the crystal chemistry
based on thermal and petrologic conditions in the host rocks.
The distribution and valence of Fe varies throughout the
suite of samples, but there is a correlation between the
Mossbauer spectrum of a sample and the history of its host
rock.
-8-
II. Samples and Experimental Techniques
Thirteen titanium-bearing garnets of varying paragenesis
and TiO2 content were investigated in this study.
Some of
the samples were collected by R.A.Howie and used in the
study of Howie and Woolley (1968).
The rest were obtained
from the mineralogical collection at Harvard University.
The samples are described in more detail in Table 1.
Five of the samples were chemically analysed on a MAC
electron microprobe operating with a line current of 30
nanoamps.
Automatic operating procedures and data reduction
techniques based on the Bence-Albee matrix corrections and
standards of known composition were used.
The raw probe
data were then corrected for Fe + and Fe3+ contents based
These
on information obtained from Mossbauer spectroscopy.
corrections are described in detail in Chapter IV.
The raw
probe data are given in Table 2, with all Fe expressed
as
FeO.
The six samples obtained from R.A.Howie were analysed
by wet chemistry and reported in Howie and Woolley (1968).
Those chemical analyses are reproduced in Table 3.
One
sample, a schorlomite from the High Atlas mountains of
Morocco (KS 5), was chemically analysed by both wet chemical
and microprobe techniques.
Consequently, two sets of data
appear in the chemical determinations of this sample.
Differences in the results stem from distinctions between
-9-
TABLE 1.
Description of samples
KS 1
Andradite, alkali syenite, Semarule, Botswana.
Obtained in powder form from R.A. Howie. Previously
Howie #1.
KS 2
Melanite, nepheline syenite, Loch Borolan, Scotland.
Obtained in powder form from R.A. Howie. Previously
Howie #4.
KS 3
Schorlomite, carbonatite complex, Magnet Cove, Arkansas.
Obtained in powder form from R.A. Howie. Previously
Howie #11.
KS 5
Schorlomite, carbonatite complex, Tamazert, Haut Atlas
de Midelt, Morocco. Obtained in hand specimen form
from R.A. Howie. Powder and probe mount made for this
study. Black color in hand sample. Sample from same
rock as Howie #8.
KS 6
Schorlomite, rock type unknown, Kirovsk, Kola Peninsula,
USSR. Obtained from Harvard, sample 103154. Sample
opaque in thin section.
KS 7
Melanite, volcanic rock, Frascati, Italy. Obtained from
Harvard sample 87882. Same locality as HVH (1976b)
sample F. Sample orange-brown through thin section,
unzoned.
KS 8
Melanite, on serpentinite, near Dallas gem mine,
San Benito Co., California. Obtained in hand specimen
form from R.A. Howie. Sample from same rock as Howie
#7.
KS 10
Garnet, volcanic rock, Monte Somma, Vesuvius, Italy.
Obtained from Harvard, sample 85482. Same locality as
Howie #2. Sample zoned in thin section from light
orange-brown to deep red and almost opaque.
KS 11
Melanite, on serpentinite, Bald Point, Calaverus Co.,
California. Obtained from Harvard, sample 105086.
Sample zoned in thin section from light orange-brown
to deep red.
KS 13
Schorlomite, melanite-nephelinite agglomerate, Rusinga
Island, Kenya. Obtained in powder form from R.A. Howie.
Previously Howie #12.
KS 14
Schorlomite, rock type unknown, Morocco. Obtained from
Harvard, sample 112864. Suspected same locality as
KS 5.
-10-
TABLE 1.
(cont'd)
KS 15
Melanite, on serpentinite, Bald Point, Calaverus Co.,
California. Obtained from Harvard, sample 92295.
KS 16
Melanite, on serpentinite, San Benito Co., California.
Obtained from Harvard, sample 92453. Suspected same
locality as KS 8.
TABLE 2.
Uncorrected Electron Microprobe Chemical Analyses
1
KS 5
3
29.98
30.39
30.61
31.68
9.20
8.80
8.65
1.31
1.24
FeO
22.19
MnO
Avg (9)
3
KS 7
7
Avg (8)
32.11
31.85
36.12
35.06
35.39
8.61
7.98
7.77
1.82
3.04
2.39
1.20
2.38
2.45
2.23
6.30
6.80
6.55
22.58
22.19
21.84
21.71
21.87
19.48
18.82
19.23
0.37
0.39
0.40
0.36
0.39
0.34
0;25
0.25
0.25
MgO
0.89
0.94
0.85
0.45
0.44
0.40
0.51
0.75
0.61
CaO
32.25
32.57
33.39
32.98
32.96
33.02
33.25
33.07
0.16
0.18
0.19
.98.86
98.23
97.62
97.50
97.97
97.50
Avg(9)
2
3
4
Avg (7)
34.63
32.94
31.72
33.39
32.61
Sio 2
TiO
2
Al2 03
-31.79
Avg (6)
4
Na203
96.18
96.14
2
4
96.47
KS 6
5
(wt. % oxides)
KS 10
Sio 2
TiO
35.25
KS 11
5
35.54 * 34.00
1.65
1.66
3.51
2.14
9.52
13.43
8.66
9.97
Al203
10.16
6.96
4.04
6.89
3.48
0.54
3.06
2.00
FeO
14.35
19.40
21.61
18.53
16.05
15.76
16.71
17.02
MnO
0.61
0.28
0.61
0.48
0.07
0.03
0.10
0.07
MgO
0.25
0.14
0.14
0.20
0.30
0.76
0.25
0.41
CaO
34.48
32.79
31.68
33.14
35.34
34.95
34.84
34.84
96.75
96.76
95.59
96.00
97.69
97.20
97.00
96.92
2
Na2 0
-12-
TABLE 3.
Wet Chemical Analyses (wt. % oxides)
KS 1
KS 2
KS 3
KS 5
KS 8
KS 13
Sio2
36.59
33.58
26.76
31.35
34.01
28.52
TiO 2
0.45
3.96
14.46
8.65
8.44
15.10
Al 0
23
4.98
2.61
5.36
1.44
2.69
2.12
Fe2 03
25.59
24.63
16.90
23.73
19.24
18.25
FeO
0.19
1.67
3.37
2.00
2.26
4.15
MnO
0.35
0.69
0.70
0.65
0.03
0.65
MgO
0.87
0.84
1.43
0.69
0.80
0.49
CaO
31.06
30.04
30.91
30.79
32.14
31.03
Na20
0.12
K
0,02
2
0
100.08
98.02
99.89
99.30
99.75
100.31
-13-
wet chemical and probe data.
Some of these distinctions
may be systematic and this problem will be discussed in
Chapter V.
The M*ssbauer spectrometer used was an Austin Science
Instruments model S3,K3, equipped with a 1024 channel
Nuclear Data multi-channel analyzer.
The spectra were one-
half of a mirror image spectrum using 512 channels
maximum velocity of approximately + 4 mm/sec.
Co57 in palladium (25-50 mc.) was used.
and a
A source of
The sample was
.
always approximately six inches from this oscillating source.
The sample was ground under acetone, mixed with sucrose, and
mounted in a round hole in a flexiglass disc of 2.2 cm
diameter.
The sample was within 1 cm of the counter with
a lead shield framing the hole.
relative to Fe foil.
The samples were calibrated
The system was frequently re-standard-
ized to account for any drift in the midpoint of the spectrum
or the gradient of the channels.
The spectra were fitted with a least-squares computer
program, fitting Lorentzian peaks, written by A.J.Stone, et
al. (1969) and modified by F.E.Huggins (1974).
The system-
atics of the fitting procedure will be discussed in Chapter
III.
-14-
III. M5ssbauer Spectroscopy of Melanites and Schorlomites
With the development of M~ssbauer spectroscopy in
mineralogical research during the past ten years (Bancroft,
Maddock and Burns, 1967; Bancroft, 1973), this nuclear gamma
ray resonance technique has emerged as the most powerful
method for solving the crystal chemistry of titaniferous
andradite garnets (Dowty, 1971; Burns, 1972; Huggins
1976b; Amthauer
et al., 1976).
et al.,
However, the complexity of
the M6ssbauer spectra of melanites and schorlomites has
impeded attempts to deal adequately with the subject.
In
the present study, a more sophisticated analysis of the
M*ssbauer spectra was made than in any previous work.
The
peak positions and M6ssbauer parameters for solved spectra
are given in Table 4 and Table 5, respectively.
For
illustrative purposes, the peak positions are also plotted
on Figure 2.- The method and rationale of the fitting
procedure is discussed later in this chapter.
The earliest MO5ssbauer study of titanium garnets was
conducted by Dowty (1971).
The M6ssbauer experiment was
performed on six melanites and schorlomites and the spectra
were fit with up to four doublets.
However, controversy
quickly arose over Dowty's fitting procedure and assignments.
He chose to fit the spectra to two sets of doublets for octahedral ferric iron on the basis of chi-squared values even
TABLE 4.
Fe 3+/oct
Fe 3+/tet
Peak Positions
(mm/sec)
KS 1
KS 2
KS 3
KS 5
KS 7
KS 8
KS 10
KS 11
KS 14
KS 15
KS 16
KS 3
@77 0 K
KS 7
@77 0 K
0.146
0.104
0.086
0.096
0.120
0.136
0.120
0.108
0.043
0.083
0.095
0.163
0.181
0.702
0.711
0.701
0.708
0.714
0.725
0.681
0.710
0.650
0.703
0.704
0.779
0.766
-0.489 -0.404 -0.431 -0.519
0.889
Fe2+ /4
0.823
0.893
0.889
0.831
0.922
1.622
1.983
-0.091 -0.061
3.001
0.819
-0.347 -0.462
0.091
-0.131
2.497
Fe 2+/8
-0.500
-0.027
-0.164
Fe 2+/6
-0.486
2.976
-0.140
2.456
2.420
-0.082
-0.040
3.012
2.975
All positions at room temperature (f%300 0 K) except where noted.
All valves relative to Fe foil.
-0.133 -0.157
2.353
2.408
0.941
-0.130
2.681
0.004 -0.004
3.164
3.199
TABLE 5.
Mossbauer parameters for melanites and schorlomites
Fe
/oct
Fe
I.S.
Q.S.
Wdth
%Area
0.42
0.41
0.39
0.40
0.42
0.43
0.40
0.41
0.35
0.39
0.40
0.56
0.59
0.62
0.61
0.59
0.59
0.56
0.60
0.61
0.62
0.61
0.26
0.29
0.35
0.40
0.29
0.29
0.27
0.31
0.33
0.32
0.32
100
87.2
60.3
83.4
86.7
94.8
89.3
90.7
KS 3
@77 0 K
0.47
0.62
0.35
KS 7
@77 0 K
0.47
0.59
0.32
KS 1
KS
KS
KS
KS
KS
KS
KS
KS
KS
KS
2
3
5
7
8
10
11
14
15
16
Fe
I.S.
Q.S.
Wdth
%Area
0.22
0.21
0.23
0.19
1.36
1.23
1.32
1.41
0.35
0.35
0.39
0.27
8.9
24.5
16.5
8.3
0.17
1.31
0.32
4.9
0.17
1.33
0.29
9.6
56.9
0.29
1.27
0.35
24.4
86.3
0.24
1.40
0.28
8.4
90.4
/4
I.S.
Q.S.
Wdth
%Area
0.80
1.65
0.56
8.3
0.95
2.07
0.47
82.4
81.0
Fe +/6
KS 1
KS 2
XS 3
/tet
Fe
10.6
/8
I.S.
Q.S.
Wdth
%Area
I.S.
Q.S.
Wdth
%Area
1.17
2.66
0.56
4.1
1.45
1.46
3.06
3.04
0.28
0.56
3.9
2.8
1.47
3.09
0.28
5.0
1.16
2.59
0.32
5.2
1.47
3.02
0.39
5.8
1.14
2.56
0.36
9.3
1.11
1.13
2.49
2.57
0.47
0.40
17.6
19.0
1.28
2.81
0.47
1.58
3.16
0.47
4.6
1.60
3.20
0.27
5.3
KS 5
KS
KS
KS
KS
KS
KS
KS
7
8
10
11
14
15
16
KS 3
@77 0 K
KS 7
@77 0 K
3.6
All spectra at room temperature ("%300*K) except where noted.
All values in mm/sec relative to Fe foil.
-17-
Figure 2.
Peak positions for M~ssbauer
spectra at room temperature.
Note the
consistency of the positions, which is a
major criterion in determining the accuracy
of the final fits.
-0t5
10
05
,
1,5
210
215
30
2:0
2:5
3.0
I
3.5
KS 16
KS 15
KS 14
I
0
0
0
0
KS 11
01T
KS 10
II
KS
8
KS
7
KS
5
T
KS
3
T
KS
2
KS
1
I
6 84 0
01T
II
1 II 1
0OT
I I
I
-0-5
0.'5
1.'0
mm/sec
I
3.5
-19-
though the garnet structure contains only one octahedral
site.
By fitting two octahedral ferric doublets, chi-squared
values were lowered in some cases by over 50 percent.
However, no convincing explanation was given of why the
spectra were fit this way.
The decision to include a second
doublet, called the 02 doublet by Dowty (1971, Figure 3), precluIn
ded any possibility of correctly interpreting the spectra.
some samples, the 02 doublet accounted for over 40 percent
of the total absorption.
This feature caused the values
of
isomer shift for tetrahedral ferric iron to be anamalously
low.
Subsequent work (Burns, 1972; Huggins
et al., 1975)
rejected this assignment on the basis that there is only
one six-coordinate site in the garnet structure.
In addition, Dowty (1971) failed to recognize the
presence of different types of ferrous species in his samples.
His T 2 doublet, which was assigned to tetrahedral ferrous
iron, had an unreasonably large width (up to 0.87 mm/sec. in
some samples).
groups (Huggins
Subsequent work by this author and by other
et al., 1976b) showed that absorption at
high velocity is explained by the presence of two or three
ferrous iron peaks of reasonable width, representing 8, 6,
and 4-coordinated Fe
ions.
The low velocity counterparts
of these ferrous peaks are masked in Dowty's fit by his 02
peak.
Burns' (1972) analysis of two natural and one synthetic
titaniferous garnet samples was significant in that it
-20-
dispelled the notion of two octahedral ferric doublets.
It was also the first study which identified 8-coordinate
In addition, it was the first
ferrous iron in a melanite.
attempt to combine wet chemistry and MOssbauer spectroscopy
on a single sample for the purpose of quantitatively determining the cation distribution of Fe and Ti and the valence
of Ti.
Unfortunately, the study was limited to but a few
natural samples, none of which are as complex as the
Mbssbauer spectra of certain schorlomites measured in the
present study.
It became apparent that a combination of wet chemistry
and electron microprobe analyses and Mbssbauer spectroscopy
was the only way to deduce the crystal chemistry of melanites
and schorlomites.
The electron microprobe had been used in
earlier studies (Howie and Woolley, 1968; Manning and Harris,
1970) and its ability to accurately determine chemical
composition was apparent.
However, neither of these studies
included M6ssbauer spectroscopy.
The first major attempt to describe the crystal chemistry
of a suite of natural titanium-bearing garnets using all
three analytical methods was reported by Huggins
(1976b)1 .
et al.
Several specimiLns of varying titanium content
and paragenesis were studied.
HVH obtained chemical data
by electron microprobe analysis and Fe +/Fe
peak areas in the M*ssbauer spectra.
1. Hereafter
referred to as HVH.
ratios from
To reduce the difference
-21-
between the ferrous and ferric recoil-free fractions, which
are significantly different in the garnet structure at room
temperature (Whipple, 1973; Amthauer
et al., 1976), and to
get better resolution of ferrous peaks, the experiments were
performed at 77
K.
The presence or absence of Ti + was
estimated in some samples by comparing the MOssbauer-determined ferrous iron content with the total number of reduced
cations found either by calculating the electron microprobe
data according to garnet stoichiometry or by wet chemical
analysis.' Using more information on the distribution of
Fe + and Fe3+ found by M6ssbauer spectroscopy and the relative
preference of Al, Fe and Ti for the tetrahedral site (Huggins
et al., 1976a), the cation distribution in melanites and
schorlomites was deduced (HVH, Table 7).
Their rationale
is essentially correct and this method for determining crystal
chemistry is'adopted in the present study.
The main criterion for evaluating the merit of HVH work
is the validity of their Mbssbauer fits.
Unfortunately, the
fitting procedure which they used irreparably compromised
that accuracy.
HVH decided to fit the spectra without using
constraints on peak parameters and positions.
By choosing
this method, they were compelled to fit only peaks which
were apparent visually since convergence could not be achieved
on more complicated fits without using constraints.
No
attempt was made to resolve the low velocity Fe + peaks which
-22-
These
are hidden beneath the low velocity ferric peaks.
Fe + peaks cannot be seen visually due to low Fe 2+/Fe3+
ratios.
HVH did not attempt to resolve the high velocity
ferric peaks either.
Instead, the tetrahedral and octahedral
Fe3+ peaks were fitted as a composite peak, Ac.
The most
complicated spectrum of HVH was "solved" with a six peak
2+
2+
fit representing five species of Fe: Fe /8-fold, Fe /6fold, Fe 2+/4-fold, Fe +/oct , and Fe +/tet.
An example of
their "fitted" spectrum is reproduced in Figure 3.
In lieu of low velocity ferrous peaks, HVH assumed these
peaks were primarily located under peak Aiv, the low velocity
Fe 3+/tet peak (Figure 3), at 770 K.
While admitting the
possibility that this unconstrained fitting procedure might
cause "small but significant systematic errors in the values
of isomer shift and quadrupole splitting," HVH insisted that
"constrained fits with additional peaks are probably no more
correct or informative than the unconstrained fits" they used.
This was based on the assumption that their fitting procedure
would not "significantly affect the ratios for the different
kinds of iron cations in the garnets," which was.the main
interest of their study.
The decision of HVH to perform the M6ssbauer experiment
at 770 K was made to decrease the difference in recoil-free
fractions for Fe + and Fe 3+.
However, even at 77 K the
recoil-free fractions could differ sufficiently to cause
errors of up to 10% in the Fe 2+/EFe ratios.
Therefore, a
-23-
Figure 3.
M~ssbauer spectrum of schorlomite
from Ardnamurchan, Scotland; sample Ar of
Huggins et al. (1976b) . Spectrum fitted to
six peaks..
-
Also shown is
the labelling
scheme far the peaks and the definitions of
the doublets for the five distinct types of
iron cations.
From Huggins et al.
(1976b) .
z
at
0
2
4
z
Ai, Ayi
Peaks :
I
I
A
Bi,
I
I
I I
2*
Fvi IFeviii
FeF4
~Fe~
Doublets :
Biyiii
jWniv+Lt!;;4
,
6I-
-A62
-2.1
0
VELOCITY
2.1
mm/sec
42
different approach was adopted in the present study.
By
measuring the Mossbauer spectra at room temperature (300 K),
the high velocity Fe 3+/tet peak is better resolved than at
77 0 K.
This eliminates the necessity to combine the high
velocity peaks for ferric iron and results in a more
realistic fit and more accurate values of isomer shift and
quadrupole splitting of Fe +/tet.
The crux of the present study is to demonstrate that
the fitting procedure of HVH does not do justice to the
complexity of the M5ssbauer spectra of titaniferous garnets,
so that crystal chemical data deduced by HVH for iron in
these titanium-bearing andradites are inaccurate.
There
are three basic reasons for rejecting the fitting procedure
and spectral analyses of HVH.
The first is the acceptance
by HVH of unreasonably large variations of isomer shift
and certain peak positions from sample to sample.
For
example, at 770K, the values for isomer shift for Fe +/tet
in their samples vary from 0.16 mm/sec to 0.28 mm/sec, a
span of 0.12 mm/sec.
is
Additionally, the variation for Fe 2+/4
0.18 mm/sec.
Since isomer shift is not significantly affected by
site distortion or nearest neighbor interactions, but is
primarily a function of the valence and coordination number
of the ion, such a large variation is disturbing.
The
difference is much greater than that which can be attributed
to experimental error.
The amount of variation in isomer
shift which can be tolerated is demonstrated by performing
-26-
the MZ5ssbauer experiment on the same sample at two
This was
different times and comparing the results.
done for two samples in the present study (KS 5 and KS 11).
The isomer shift variation ( AS ) was less than 0.025 mm/sec
for KS 5
mm/sec.
and peak positions varied by a maximum of 0.02
For KS 1, AS was less than 0.002 mm/sec and peak
positions did not change by more than 0.006 mm/sec.
Granted,
However, such
these fluctuations are for the same sample.
large variations in the spectra of HVH (almost an order of
magnitude larger in some cases) either
points to some odd
structural variation in their garnets or suggests a faulty
fitting procedure in their study.
In the present study,
AS was less than 0.06 mm/sec for all iron species, which
is a reasonable maximum for AS in garnets.
Any larger
fluctuation is indicative of a faulty fitting procedure.
Another problem with the fitting procedure used by
HVH concern/s changes in the position of the high velocity
Fe 2+/4 peak (B
) over the suite of samples.
position varies from a low of 1.74 mm/sec
2.06 mm/sec
The Biv peak
to a high of
and is responsible for the unreasonable varia-
tion of isomer shifts for Fe +/4 mentioned above.
The
position of a given peak should not change by more than
*
All peak positions from HVH quoted in this study are 0.18
mm/sec higher tham those given in that work, due to different
calibration standards (Fe/Pd in HVH compared to metallic Fe
in this study).
-27-
0.1 mm/sec.
The acceptance of such a large variation in
this case raises skepticism over the validity of the HVH
fitting scheme.
The consistency of peak positions in the
present study is demonstrated in Figure 2.
The second reason to doubt the accuracy of the Mossbauer solutions of HVH concerns handling of peak half-widths.
By leaving all parameters unconstrained, no attempt was made
to control the widths of the various peaks.
The decision
ignored the best criteria for assessing peak half-widths,
common sense.
A case in point concernes the ferrous peaks
of sample Ar from the HVH study, which had widths for ferrous
iron in 8-fold, 6-fold, and 4-fold coordination of 0.30 mm/sec,
0.45 mm/sec, and 0.53 mm/sec, respectively.
Unless the
assumption is made that all iron species with the same valence
state have the same peak half-widths,
the site with the most
nearest neighbor interactions would be expected to have the
broadest peaks.
This is because chemical variability of the
environment of a site is a major cause for line broadening.
In garnet, the eight fold site shares more edges (10) with
other coordination polyhedra than either the octahedral (6)
or tetrahedral (2) sites (see Table 1, Novak and Gibbs, 1971).
Cations in the 8-fold site are therefore more likely to be
affected by neighboring cations through next nearest neighbor
interactions.
The trend of sample Ar is opposite to what
would be expected if next nearest neighbor interactions
occurred.
-28-
Another example of inconsistency of peak width concernes
the Ac peak, in general, for each of the HVH specimens.
This peak is defined as a composite of the two high velocity
ferric iron peaks, Fe +/tet plus Fe +/oct.
In Mossbauer
spectra, the two peaks of a quadrupole doublet generally
have the same width for iron cations in the same oxidation
state.
Therefore, even if the two high velocity ferric
iron peaks had exactly the same position, the width of A
C
should always be at least equal to the width of A. and A ..
lv
vl
If there is not exact overlap, the width of Ac should be
significantly larger.
However, in every case, the half-
width of the Ac peak in the HVH samples is either equal to
or smaller than the half-width of A.iv or A vi..
Dependence
on the computer rather than knowledge of the physical
constraints of the system to determine the values of
Mossbauer parameters does not inspire confidence in the HVH
fitting procedure.
Additionally, controversy concerning
peak half-widths would reasonably cast doubt on the accuracy
of area information (and hence Fe 2+/Fe3+ ratios) derived
from their spectra.
Another inconsistency regarding the physical constraints
on the system involves the sample from San Benito, California
(SB).
In that sample, HVH reported the presence of Fe 3 +/oct,
Fe +/6 and Fe
/4.
However, it seems highly unlikely that
only ferrous iron would substitute into the tetrahedral
-29-
site of a garnet in which ferric iron predominates.
The previous discussion has focussed on internal
inconsistencies in the MO*ssbauer fitting procedure of HVH.
That is, unreasonable variation of parameters and unlikely
values for peak half-widths are valid reasons for rejecting
a fitting procedure regardless of other criteria.
A third
reason for doubting the MOssbauer data obtained by HVH is
based on its
systemmatic disagreement with parameters
obtained during the present study.
The value
for the isomer shift parameter of Fe 2+/4
is indicative of such discrepancies.
HVH reported isomer
shifts which, though variable, had a maximum of 0.87 mm/sec
( at 770 K).
This value was noted to be anomalously low for
Fe 2+/4 in silicates.
Amthauer
et al. (1976) also obtained
unusually low values using the same fitting procedure as HVH.
Both works
ited electron hopping between Fe 2+/4 and adjacent
cations as the cause of the low values.
However, the
isolated position of the tetrahedral site makes such a
transition highly unlikely.
The only edges shared by the
tetrahedral site are those with the 8-fold site, which
contains no cations which could receive the electron.
The
4-coordinate site shares only corners, bounded by 02- ligands,
with the 6-fold site and is totally independent of other
tetrahedral sites.
Electron hopping in these cases, although
often proposed, should not be considered if an alternative
-30-
solution is available.
An alternative explanation is that
the isomer shift for Fe 2+/4 at 770K is 0.95mm/sec, as found
in the present study, which is in agreement with other
minerals containing tetrahedral ferrous iron (see reference
in HVH).
Other discrepencies in the HVH data pertain mostly to
peak positions. These contrarities are demonstrated in
Figure 4, which compares the peak positions of the present
study with comparable samples of HVH and Amthauer, et al.,
(1976) at 770 K.
The most serious disagreement involves the
HVH assumption that the low velocity ferrous iron peaks are
located underneath Aiv
peak.
the low velocity tetrahedral ferric
As demonstrated later, the present study has estab-
lished
that the ferrous peaks are located between 0.09
mm/sec and -0.13 mm/sec, while the low velocity Fe +/tet
peak is at approximately -0.4 mm/sec.
The discrepency in ferrous iron peak positions is
particularly significant because it relates to the fundamental
assumption made by HVH.
They were willing to admit that the
values of their M6ssbauer parameters are probably in error
due to their simplified fitting procedure.
However, they
insisted that this would not affect peak area ratios and,
therefore, the relative proportions of different iron cations
in the garnets.
This presupposes that the areas of different
peaks would not be appreciably different from the true values.
But, in order to determine relative proportions of tetrahedral
-31-
Figure 4.
Bar diagram comparing peak positions
from the present M6ssbauer study (KS 3, KS 7)
with those of Huggins et al. (1976b) (Ar, F)
and Amthauer et al. (1976) (1).
The notation
used for peak positions is consistent with
that used in the earlier publications. Note
the discrepancies between the low velocity
ferrous iron peaks of KS 7 and KS 3 and those
of peak AIV or (1) of Huggins et al. (1976b)
and Amthauer et al. (1976), respectively.
Note also the intermediate position of peak
A
or 3 (corresponding to high velocity
3+
ferric iron) between the high velocity Fe /
C
tet and Fe +/oct peaks, which were resolved
in the present study.
All spectra at 77*K.
-0t5
1.0
1~o
05
1.
.5
210
20
3,0
3I
215
2~
2
6
Avi
Aiv
Aiv
I
Ar,
B1v
A1
8 0
KS 7
1 1.
O T
8
0OT
8
11I
1 1 11
-0.5
|*'
I I
6 840
KS 3
3.5
0.5
1.0
1.5
mm/sec
2.0
2'5
3'.0
3.5
-33-
ferric iron, HVH subtracted B.
+ B . + B ...
areas from
the Aiv peak area to compensate for the presumed enhance--ment of the Aiv peak by the low velocity components of the
ferrous iron doublets.
If, in fact, the low velocity
ferrous iron peaks are not actually located beneath the A.
lv
3+
3+
peak, then the computed (Fe /tet)/(Fe /oct) values of HVH
would be incorrect.
In fact, the low velocity ferrous iron
peaks lie under the Avi peak and actually enhance the Fe +
oct peak as much as or more than the Fe +/tet peak (see
Figure 5).
This error shows up in four of the HVH samples
(SB, OL, ML, and Ar), all of which contain more Fe
Fe
/tet.
/4 than
As mentioned above, this cation distribution is
highly unlikely, and certainly is curious enough to warrant
an explanation.
Ferrous iron should have a lower preference
for the tetrahedral site (if only on valence and ionic radii
arguments); no sample processed in the present study shows
such a tendency.
This is a clear indication that the
assumption that a simplified fitting procedure will not
compromise the crystal chemical determinations of these
garnets is incorrect.
The situation demands a more sophisti-
cated fitting procedure capable of producing more accurate
area determinations.
The present study offers such a
procedure.
It is obvious that the only way to truly solve Mossbauer for melanites and schorlomites is by fitting a doublet
for each species of iron.
In order to accomplish this goal,
-34-
Figure 5.
MOssbauer spectrum of schorlomite from
Magnet Cove, Arkansas.
Sample KS 3.
Spectrum
fitted to five doublets, assigned to Fe
Fe
/tet, Fe
/4, Fe
/6, and Fe
/8.
/oct,
AV~
/4)
I
-S
t
c-i
-1
2.1
Z
0
0
4.0
-2.0
0
-
mm/sec
2.0
4.0
heavy parameter constraints must be used, at least in the
beginning of the fitting process.
In addition, a criterion
must be established for evaluating the physical credibility
of the fitted spectra.
The two main parameter constraints used in the present
study were peak width and area.
In all fits, the areas for
a doublet of a given iron species are always made equal.
The half-widths of the two peaks in a doublet are also
always constrained to be equal.
In the initial fitting
procedure, it was sometimes necessary to constrain all
ferrous peak widths to be equal and all ferric peak widths
This constraint could be lifted for most
to be equal.
samples if peak positions were apparent, so as to not
unduly bias area determinations.
containing
However, for samples
ferrous iron in more than one site, the spectra
were too complicated to ever lift this constraint.
The Mossbauer experiments were performed at room
temperature ( 3000K).
Although the recoil-free fractions
for ferric and ferrous iron are unequal at this temperature,
an alternative method was used for converting area information into Fe +/Fe + ratios.
The relationship between
2+
RM
oss
_ % area Fe
2+
peaks
% area Fe + peaks
and R
chem
-
# atoms Fe 3
# atoms Fe +
is
known
to be related by f3 /f 2 , the ratio of recoil-free fractions
of Fe + and Fe
as follows:
-37-
Rchem
3
Ro ss
f2
For garnets, f3 /f 2 is known to be a constant with a value
of 1.30 for Fe +/8-fold and Fe +/oct (Whipple, 1973).
If
we assume that the recoil-free fraction for ferrous and
ferric iron in other coordinations in the garnet structure
is the -same, this constant can be used to determine Fe +/Fe +
ratios at 3000K.
The ideal way to solve complicated Mossbauer spectra
(e.g. KS 3) with overlapping peaks is to obtain approximate
values for isomer shift and quadrupole splitting for each
species of iron suspected in the sample.
Once peak positions
are located, two additional constraints can be employed to
resolve low intensity peaks which in melanites and schorlomites are the low velocity Fe + peaks and the high velocity
Fe 3+/tet peak.
One of these constraints is to fix the
position of a peak.
The other is to fix the value of isomer
shift for any doublet.
The first step in solving the spectra
of KS 3 (Figure 5) was to obtain fairly accurate parameters
for as many iron species as possible.
Fortunately, nature
supplies a method for obtaining these values in Fe-Ti garnets.
Due to differences in paragenesis, many different
combinations of iron site occupancies are found in the
crystal structures of melanites and schorlomites.
These
variations are indicated by the different types of Mossbauer
spectra seen in samples from different localities (see Figures
-38-
5,6,7,8,9).
Starting with the simplest sample, an andradite
containing almost no titanium (KS 1, Figure 6),
the isomer
shift and quadrupole splitting for Fe +/oct can be obtained.
2+ 3+
Andradite garnets, Ca3 Fe2 (SiO4 )3 , contain only one
species of iron and their spectra can easily be solved using
only the two basic constraints (peak areas and peak widths
equal in the quadrupole doublet).
The parameters obtained
are in excellent agreement with those reported by Lyubutin
et al.,
(1970) and Bancroft et al.,
(1967).
The next most complicated spectrum (KS 5, Figure 7)
gives the location of the Fe +/tet peaks.
The Fe 3+/oct
and the low velocity Fe +/tet peak are visually apparent
(Burns, 1972).
Constraining widths and areas of each doublet
to be equal, the computer easily fitted the high velocity
Fe 3+/tet
peak at 0.893 mm/sec.
With the high velocity
Fe +/oct peak at 0.708 mm/sec, it is obvious that the
simplification of HVH to fit these two peaks as a combined
peak even at 770K was ill conceived.
The high velocity
Fe +/tet peak can, in fact, also be visually distinguished
if it is compared with the spectrum of KS 1, which contains
only Fe +/oct, as demonstrated in Figure 10.
It should also
be noted that the peak half-widths for the two doublets are
almost equal (0.393 mm/sec for Fe 3+/tet; 0.397 mm/sec for
Fe 3+/oct), giving credence to the assumption that peaks for
Fe cations with the same valence can, if necessary, be
constrained to have equal width.
-39-
Figure 6.
M*ssbauer spectrum of andradite from
Semarule,'Botswana.
Sample KS 1.
Spectrum
fitted to one doublet, assigned to Fe 3+/oct.
-
qb-
z0
0
On
-2.0
0
mm/sec
2.0
4.0
-41-
Figure 7. M~ssbauer spectrum of schorlomite
from Morocco. Sample KS 5. Spectrum fitted
to two doublets, assigned to Fe /oct and
Fe
/tet.
c(
2-
0
z0
1.0
0
V)
2.0
-2.0
0
mm/sec
2.0
4.0
-43-
Figure 8.
M*ssbauer spectrum of melanite
from San Benito County, California. Sample
KS 8. Spectrum fitted to two doublets,
assigned to Fe 3+/oct and Fe 2+/6.
F
0
2.0
z
0
0
(j)
4.01
6.0
-2.0
0
mm/sec
2.0
4.0
-45-
Figure 9.
M~ssbauer spectrum of melanite
from Frascatti, Italy.
Sample KS 7.
Spectrum fitted to three doublets, assigned
3+
3+
2+
to Fe loot, Fe /tet, and Fe /8.
z
0
li-
0
V)
5.0
tg
10.0
-2.0
0
mm/sec
2.0
4.0
-47-
Figure 10.
Comparison of the M*ssbauer spectra
of andradite and schorlomite.
The schorlomite
contains Fe 3+/tet and shows the presence of a
distinct high velocity tetrahedral ferric peak.
The two spectra were run consecutively with the
same spectrometer settings.
-
The difference in
the slope of the spectra in the vicinity of
1 mm/sec is due to the extra, non-coincidental
Fe +/tet peak in the schorlomite.
-Yi-
0
to0
2.0 -
0
2.0-
z
o
60-
oI
I I
ig i i
8.0 -I,
I
0
mm/sec
4.0
-49-
The next sample in our development of a consistent fitting
procedure is for a melanite from a serpentinized peridotite in
San Benito County, California, which also gives a four peak
spectrum (KS 8, Figure 8).
Due to the low grade metamorphic
paragenesis at this locality, the garnet contains only octahedrally coordinated iron (Huggins et al., 1976a).
However,
state. The high
some of the iron in the sample is in the Fe
2+
velocity Fe /6 peak is visually apparent at ^v2.5 mm/sec.
Adopting the same set of constraints used to fit specimen KS 5,
the low velocity Fe +/6 peak of KS 8 is fitted by the computer
at -0.131'mm/sec.
When this peak position is compared with
that of the low velocity Fe +/tet peak (at -0.431 mm/sec) found
in KS 5, another assumption of HVH is seen to be in error.
The low velocity Fe +/6 peak is no where near the Fe 3+/tet
peak and this fact is demonstrated with relatively simple
spectra using few constraints.
Although the amount of Fe + in this sample is low, it in
no way compr6mises the position of the ferrous iron peaks.
Comparing these positions with the location of Fe 2+/6 peaks in
2+
from nearby
other samples containing substantially more Fe
areas with identical paragenesis (see KS 11, KS 15, and KS 16
peak positions in Figure 2),
it is seen that peak positions
do not vary from sample to sample by more than the 0.1 mm/sec
allowed by this fitting procedure.
The parameters for Fe 2+/8 can be determined from the
spectra of samples such as KS 7 (Figure 9).
The spectrum in
Figure 9 obviously represents at least three species of iron
-50-
with four peaks visually apparent.
The approximate peak
positions for the ferric iron doublets can be obtained from
previous samples and the location of the high velocity ferrous
can be visually estimated to be at
peak
With
3.0 mm/sec.
five peaks accurately known, there is little difficulty
locating by computer the low velocity ferrous iron peak in two
steps.
First, all ferric peak widths are constrained to be
equal; and second, widths and areas of component doublets are
constrained to be equal.
The ferrous iron isomer shift,
1.47 mm/sec, definitely correlates with 8-fold coordinated
2+
Fe
Since KS 7 is a fairly complicated spectrum, containing
two visually obscured peaks, a more complicated fitting scenario
was pursued in an attempt to lower the chi-square values.
Peak positions and isomer shift were constrained to have
accepted values in the early stages so as to allow the computer
to fit the spectrum without fear of divergence.
In this case,
the resulting fit was exactly the same as the first one,
corroborating the peak positions for sample KS 7.
At this stage, the location of the doublets for Fe
Fe
/oct, Fe +/6, and Fe 2+/8 were accurately known.
/tet,
The peak
positions were confirmed by the solution of spectra similar to
the ones described above (see Figure 2).
For all samples,
isomer shifts of each iron species varies by <0.06 mm/sec.
The consistency of this important parameter is the justification for the highly constrained fitting procedure needed to
solve the most complex spectra.
Even with all this help, it
-51-
is a long and difficult task to fit very complicated spectra
The best
and satisfactory fits are not always obtained.
example illustrating this procedure is demonstrated by the
Mbssbauer spectrum of KS 3 (Figure 5).
From a visual inspection of Figure 5,
it
is
obvious that
KS 3 is a ten peak spectrum containing Fe 2+/4 as well as the
The position of the
other four iron species described above.
2+
high velocity Fe /4 peak can be readily estimated. For the
first fitting sequence
a value of 0.9 mm/sec was selected for
its isomer shift, thereby assigning a position to the low
velocity Fe 2+/4 peak.
The positions of the ferric iron peaks
and the high velocity ferrous iron peaks were also initially
constrained to prevent the fitting process from diverging.
Isomer shifts for ferrous iron species were also constrained,
along with the usual width and area constraints.
first
During the
fitting sequence the process diverged after the first
stage, with a chi-squared value of 7122.92 (the value on the
final fit was 850.31).
After three more fitting sequences, each involving careful
re-adjustment of peak positions,
chi-squared was reduced to
1000 and the envelope fitted the spectrum fairly well.
The
fitting process did not diverge provided the widths and areas
were constrained (all ferric widths equal; all ferrous widths
equal; each doublet with equal peak areas) and ferrous iron
isomer shift constraints were not lifted.
To confirm that
these isomer shift constraints were not biasing the data,
they
-52-
they were lifted one step at a time.
In every case, as soon as
the second isomer shift constraint was lifted the process
diverged.
The result was three very similar fits, each with
two ferrous iron isomer shifts constrained.
The fit finally
selected as being the most accurate was the one in which the
isomer shift for Fe +/4 was left unconstrained.
for this was two-fold.
The reason
First and foremost, isomer shifts for
Fe +/6 and Fe +/8 were already known from other samples,
leaving these parameters as the two least likely to bias the
data by being constrained.
Second, this fit gave a chi-squared
value substantially lower than either of the alternative fits.
By this method, a highly complex ten peak M*ssbauer spectrum
was successfully fitted without assumptions which comprised
the credibility of the fitting procedure or unduly biased
the data.
Other samples with equally complex spectra were fitted by
the same method.
However, for various reasons confidence in
these solutions is not as high.
Suitable parameters cannot be
obtained for KS 6 (Kola Peninsula, USSR), even without lifting
any ferrous iron isomer shift constraints (see Appendix).
Even
though the fit is internally consistent with a chi-squared
value of 711.60, it should be rejected because it gives
extremely large peak widths and questionable locations for the
Fe +/6 and Fe
/8 peak positions.
Quadrupole splittings for
these iron species are in such poor agreement with the other
samples that this spectrum must be regarded as unsolved.
reason to reject the spectral solution is that the crystal
Another
-53-
chemical data obtained from the peak areas is not reasonable,
as it places more ferrous iron in tetrahedral coordination
than ferric iron.
The sample from Rusinga Island, Kenya (KS 13) presents a
different problem.
From a visual inspection of its spectrum
(see Appendix), it is apparent that the highest velocity peak
is situated in an intermediate position (2.67 mm/sec) between
the normal Fe 2+/6 and Fe +/8 locales (2.45 mm/sec and 3.0 mm/
sec, respectively).
The spectrum can be fit to an envelope
using the standard parameters for Fe
/oct, Fe
/tet, Fe
and the estimated position of the high velocity peak.
/4,
The
two ferric and tetrahedral ferrous parameters are normal and
the outermost peak can be matched with a peak at -0.125 mm/sec.
This solution is considered correct, but the implication of the
unusual doublet is unclear.
The high velocity peak is located
in the same area as peaks for many samples in the study of
HVH.
Apparently, this intermediate peak is a real feature in
this sample.
HVH attributed this quadrupole doublet to
composite Fe +/6 and Fe 2+/8 absorption.
tion can be offered here.
No alternative explana-
However, this phenomena does not
seem to be as widespread as one might assume from the HVH
study.
Only two samples out of an original fourteen (including
the totally unfit spectrum of KS 4 - see Appendix) show this
effect.
It should be noted, however, that the chemical information
derived from the spectrum of KS 13 is unacceptable, and its
-54-
crystal chemistry is rejected in this study.
The sample was
previously studied (Howie and Wooley, 1968) and chemically
analyzed by wet chemical methods.
The M*ssbauer spectrum
indicates more ferrous iron in the sample than the total
"reduced cations" found by wet chemical analysis (Fe2+/
2+
3+
3+
= 0.287).
This
= 0.254 versus Fe /Fe
Fe
Moss
chem
difference exceeds that attributable to experimental error and
it must be concluded that the true meaning of the M6ssbauer
spectrum for KS 13 is still unknown.
If, as is likely, it is
due to some electron transfer between iron in the 6-fold and
8-fold site (which occurs faster than the MOssbauer transition),
it is noteworthy that it does not affect the Fe
/4 parameters.
This lends further evidence to the conclusion that Fe +/4 takes
no part in whatever electron hopping phenomena may occur in
the garnet structure.
The fitting procedures described above are considered
superior to previous attempts to resolve the M*ssbauer spectra
of Fe-Ti garnets.
The fundamental assumption-using width and
area constraints, rejecting otherwise reasonable fits for their
incompatability with other data or physical reality, and
resorting to parameter constraints where necessary- should be
seen as suitable for any comparable suite of samples undergoing
M8ssbauer analysis.
-55-
IV.
Chemical Analyses of Titaniferous Garnets
Analytical data on the chemistry of titaniferous garnets
used in the present study are summarized in Tables 2 and 3,
which give electron microprobe and wet chemical analyses,
respectively.
(Fe and Ti),
Due to the presence of multi-valent cations
the raw chemical data are not sufficient to
adequately describe the chemistry of the samples.
With
additional information obtained from M6ssbauer spectroscopy,
a more precise crystal chemical assignment can be determined.
As previously stated, the wet chemical data are those
reported by Howie and Woolley (1968).
The determination of
ferrous iron in these garnets was reported to be difficult due
to their resistance to attack in a boiling mixture of HF and
H 2SO
and to the dark color of the resulting solution, which
made the titrimetric end-point hard to detect.
Howie and
Woolley (1968), however, did obtain reproducible results and
determined ferrous iron contents by titration against potassium
dichromate.
This method only measured the reducing capacity
in the sample, which is usually assumed to be due to the
3+
3+
2+
was
to Fe . The possible presence of Ti
oxidation of Fe
noted, but not quantitatively determined.
The present study indicates conclusively that some of the
reducing capacity is due to Ti3+ .
The amount of Ti 3 + can be
measured by taking the difference between the value of Fe
/
EFe found by M~ssbauer analysis and the total reducing
capacity of the sample obtained from chemical analysis.
This
approach was previously used by Burns (1972) and Whipple (1973).
-56-
A comparison of Fe
/EFe values determined by MOssbauer and wet
chemistry, and the ensuing Ti +/ETi values for these samples,
are given on Table 6a.
A check was made on the accuracy of the proportions of
mixed valencies for Fe and Ti cations by calculating the total
positive charge of the cations in the samples.
Ideally, this
value should be 48.0, corresponding to the 24 oxygen anions
upon which the garnet formula was computed.
The total charges
of the cations for each wet chemically analyzed sample are
given in Table 7a.
The data indicate that valence determina-
tions in the present study are adequate within experimental
limits.
The only sample obtained from R.A. Howie showing
substantial deviation was KS 13, confirming the suspicion that
its crystal chemistry was not correctly described.
The electron microprobe data in Table 2 expresses the iron
contents in terms of weight percent FeO.
This value is,
therefore, imprecise since most of the iron in these garnets is
in the ferric state.
Ferric iron is further suggested by the
fact that the oxide totals are consistently less than 100%.
Such discrepancies can be corrected by expressing the percent
of iron actually present as Fe203 and multiplying the FeO
content by the factor 1.11.
This factor reflects the ratio of
formula weights between Fe +01.5 and Fe +0.
The amount of
iron present as ferric can be determined directly from peak
area data in the M5ssbauer spectra which has been converted to
Fe
/Fe + ratios, as discussed in Chapter III.
For example,
the correct weight percentages of FeO and Fe 203 are estimated
-57-
TABLE 6.
Fe
/EFe and Ti +/ETi for melanites and schorlomites.
Fe +/EFe
Ti /ZTi
Moss-wet
Moss
Wet
KS 1
0.00
0.01
0.47
KS 2
0.05
0.07
0.13
KS 3
0.19
0.18
0.00
KS 5
0.00
0.09
0.26
KS 8
0.07
0.12
0.12
Ti +/ETi
Moss-probe
KS 5
0.09
KS 6
0.03
KS 7
0.04
KS 10
0.10
KS 11
0.00
-58-
TABLE 7.
Cation distribution by valence
+4
+3
+2
Total
KS 1
6.020
4.166
5.711
48.000
KS 2
6.158
3.817
5.969
48.021
KS 3
6.341
3.188
6.524
47.976
KS 5
6.146
3.893
5.870
48.003
KS 8,
6.557
3.176
6.129
48.014
+4
+3
KS 5
6.252
3.508
+2
6.234
KS 7
6.098
3.750
6.179
KS 10
6.036
3.786
6.249
KS 11
6.767
2.514
6.691
-59-
as follows (for point 3 of KS 7):
Probe FeO = 19.48 wt.
% FeO
Corrected MOssbauer data finds
Fe 3 + / Fe = 0.9362
Fe2+ f Fe = 0.0638
(19.48)
(0.063()
=
18.237
The value 18.237 represents the amount of iron in the ferric
state.
However, this value is low because the probe assumes
this Fe Ls FeO when it is really Fe 2 0 3 .
The adjustment is made
using thB factor 1.11:
(18.237)(1.11) = 20.243 wt. % Fe 0
2 3
The value 20.24 is the actual weight percent Fe203 in the
sample.
The true value of FeO is the percentage of the probe
FeO value which is truly Fe
:
(19.48) (0.0638) = 1.24 wt. % FeO
The corrected electron microprobe analyses for the
garnets in this study are given in Table 8.
Various point
analyses are given for each sample, along with the average
value.
The number of point analyses comprising the average
value is also given.
Although three of the samples (KS 5, KS 6,
Ks 7) are chemically homogeneous, two others (KS 10, KS 11) are
strongly zoned for certain elements.
zoning as color variations.
Thin sections show such
In both sections, the color
varies from red to dark blue and opaque.
The dark areas are
always associated with high titanium content (to be discussed
in Chapter VI).
The locations of the point analyses for these
TABLE 8.
Corrected Electron Microprobe Chemical Analyses
KS 5
1
3
(wt. % oxides)
KS 6
Avg (6)
4
5
KS 7
3
Avg (9)
7
Avg (8)
SiO
29.98
TiO
9.20
8.80
8.65
8.61
7.98
7.77
1.82
3.04
2.39
Al C
Fe2 03
1.31
1.24
1.20
2.38
2.45
2.23
6.30
6.80
6.55
24.63
25.06
24.63
17.58
17.47
17.60
20.24
19.56
19.98
FeO
0.00
0.00
0.00
6.00
5.97
6.01
1.24
1.20
1.23
MnO
0.37
0.39
0.40
0.36
0.39
0.34
0.25
0.25
0.25
MgO
0.89
0.94
0.85
0.45
0.44
0.40
0.51
0.75
0.61
CaO
32.25
31.79
32.57
33.39
32.98
32.96
33.02
33.25
33.07
0.16
0.18
0.19
100.61
99.97
99.35
99.50
99.91
99.47
2
2
3.12
Na C
98.63
98.61
98.91
KS 10
.
KS
2
4
5
Avg (9)
2
3
4
Avg (7)
35.35
35.54
34.00
34.63
32.94
31.72
33.39
32.61
1.65
1.66
3.51
2.14
9.52
13.43
8.66
9.97
Al2 03
10.16
6.96
4.04
6.89
3.48
0.54
3.06
2.00
Fe 203
14.76
19.95
22.23
19.06
15.72
15.43
16.36
16.67
FeO
1.05
1.42
1,59
1.36
1.89
1.86
1.97
2.01
MnO
0.61
0.28
0.61
0.48
0.07
0.03
0.10
0.07
MgO
0.25
0.14
0.14
0.20
0.30
0.76
0.25
0.41
CaO
34.48
32.79
31.68
33.14
35.34
34.95
34.84
34.84
98.21
98.74
97.80
97.90
99.26
98.72
98.63
98.58
Si0
2
TiC 2
Na 20
TABLE 8 (cont'd).
Numbers of metal ions per 24 oxygens
KS 5
KS 6
KS 7
1
3
Avg (6)
4
5
Avg (9)
3
7
Avg (8)
Si
5.136
5.196
5.224
5.341
5.437
5.436
5.922
.734
5.814
Ti
1.184
1.131
1.112
1.095
1.017
0.995
0.227
0.373
0.296
Al
0.268
0.247
0.239
0.473
0.488
0.451
1.215
1.310
1.270
Fe
3.177
3.228
3.165
2.230
2.224
2.263
2.496
2.404
2.468
Fe 2+
0.000
0.000
0.000
0.851
0.844
0.862,
0.168
0.167
0.168
Mn
0.051
0.051
0.062
0.051
0.051
0.051
0.039
0.039
0.039
Mg
0.226
0.236
0.215
0.111
0.112
0.103
0.128
0.187
0.148
Ca
5.918
5.829
5.957
6.030
5.982
6.031
5.804
5.827
5.824
0.061
0.061
0.062
16.243
16.217
16.254
16.000
16.041
16.027
Na
15.961
15.918
15.974
KS 10
KS 11
2
4
5
Avg (9)
2
3
4
Avg (7)
Si
5.781
5.876
5.768
5.793
5.356
5.486
5.598
5.500
Ti
0.202
0.209
0.448
0.271
1.704
1.191
1.093
1.267
1.965
1.358
0.808
1.360
0.108
0.681
0.605
0.399
1.820
2.484
2.840
2.398
1.961
1.969
2.062
2.115
0.148
0.199
0.224
0.191
0.264
0.260
0.272
0.284
Mn
0.089
0.040
0.092
0.070
0.004
0.010
0.010
0.010
Mg
0.059
0.030
0.031
0.050
0.193
0.070
0.060
0.101
Ca
6.062
5.812
5.758
5.938
6.319
6.307
6.258
6.296
16.125
16.006
15.969
16.072
15.909
15.975
15.958
15.973
Al
Fe
Fe
3+
2+
-62-
zoned samples were designed to give a true reflection of the
average value of the chemical composition.
The Ti +/ETi values for the electron microprobed samples
can be estimated by assuming a total positive charge of 48.
Since the valence of all other cations is already known, any
3+
additional deviation in total charge should be due to Ti .
Only one sample, KS 5, needed a significant amount of Ti + to
bring the total positive charge of the cations within the
error limits set by the wet chemically analyzed samples. This
3+
re-affirms the presence of Ti
in that sample, as indicated
by wet chemical and M4ssbauer analysis (see Table 6).
probed samples showed trace Ti
3+
.
Other
It must be emphasized,
however, that the Ti +/ETi determinations on the probed samples
are only estimates.
The presence of any trace elements may
increase the total positive charge and could mask the presence
3+
3+
may in
of Ti . Samples which show the presence of no Ti
fact contain small, but significant amounts of this cation.
The cation distributions, by valence, are given for each
of the probed samples in Table 7b.
The deviation from the
ratio 6:4:6 (per 24 oxygens) for quadravalent, trivalent, and
divalent cations, respectively, is quite apparent both for
samples analyzed by microprobe and by wet chemical methods
(Table 7a).
The ramifications of this situation will be
discussed in Chapter V.
-63-
V.
Crystal Chemistry of Natural Fe-Ti Garnets
The cation distributions for samples used in this study
are given in Table 9.
The criteria used for determining
metal site occupancy include M6ssbauer analysis for iron,
.traditional crystal chemical considerations (e.g. ionic
radii, valence), and recent experimental work.
All Si and the appropriate amount of Fe determined by
Messbauer spectroscopy are placed in the tetrahedral site.
The
remaining deficiency is accounted for by Al, based on the
relative preference of Al over Ti for the tetrahedral site
found experimentally for synthetic Fe-Ti garnets (Huggins et
al., 1976a).
In every case except one, there is sufficient
Al to fill the remaining tetrahedral sites.
There appears to
be no reason to appeal to tetrahedrally coordinated Ti.
The
one sample where Ti is required to fill the tetrahedral site
(KS 11) has an extremely high Ca content which renders its
chemical analysis suspect.
There is no convincing evidence
available to substantiate tetrahedral Ti.
After the Ti, remaining Al and appropriate Fe are placed
in the octahedral site, the remaining deficiency is accounted
2+
for first by Mg , on the basis of ionic radii considerations
(Huggins et al., 1976b).
The assignment of Mn + in
octahedral coordination is questionable.
show indications of Mn
The samples which
/oct (KS 6, KS 10, KS 11) were
analyzed by the electron microprobe, raising the possibility
that this site occupancy is an artifact of a fallaciously
-64-
TABLE 9.
Si
Cation distribution in natural Fe-Ti garnets.
KS 1
KS 2
KS 3
KS 5
KS 8
KS 5
KS 7
KS 10
KS 11
5.991
5.719
4.509
5.325
5.635
5.224
5.814
5.793
5.500
Q.329
0.613
0.658
0.627
0.203
0.125
3+
Fe
Fe2+
Al 3
Ti
0.270
0.009
0.608
0.017
0.365
0.149
0.082
4
0.399
0.101
Z
6.000
6.048
6.000
6.000
6.000
6.000
6.017
6.000
6.000
Ti 4
0.029
0.439
1.832
0.821
0.922
1.008
0.284
0.243
1.166
Ti 3
0.026
0.067
0.286
0.128
0.104
0.012
0.028
3
0.952
0.525
0.458
0.269
0.159
0.090
1.270
1.278
3.179
2.896
1.509
2.663
2.524
2.538
2.265
2.273
Al
Fe'3+
Fe
0.133
Mg2+
0.068
0.181
0.284
0.215
0.050
0.101
0.070
0.010
3.979
3.942
3.676
0.168
0.191
0.148
Mn2+
EY
4.186
3.927
4.000
0.168
0.089
4.039
3.914
Fe2+
2+
Mn
0.049
0.102
0.101
0.092
0.004
Mg2+
0.212
0.215
0.286
0.174
0.197
5.450
5.484
5.577
5.604
5.705
2
Ca +
NaNa1+
2.115
3.955
0.062
0.039
5.957
5.824
5.938
6.296
0.038
0.004
EX
5.711
5.969
6.053
5.870
5.948
6.019
6.031
6.129
6.296
EX+Y
9.897
9.896
10.053
9.909
9.862
9.974
10.010
10.071
9.972
-65-
high Ca content.
As noted earlier, the crystal chemistry
deduced from the MOssbauer spectrum of KS 6 has already been
rejected (see Chapter III).
However, KS 10 might indeed have
It is interesting to note that none of the samples
Mn 2+/oct.
analyzed by wet chemistry requires this curious site
occupancy.
The assumed presence of Ca2+ in octahedral coordination is
based solely on its anomalously high concentration in some of
2+
in samples which
the garnets.
As with Mn
, it arises only
were analyzed by the microprobe, leading to the suspicion
that the high Ca values are caused by instrumentation
problems.
One sample, KS 5, was analyzed both by wet chemis-
try and by the microprobe.
for Ca.
The analyses agree well except
The lack of garnet standards for probe analysis might
be responsible for excess Ca and this possibility should be
examined before Ca 2+/oct can be substantiated.
Consequently,
any conclusions drawn concerning the crystal chemistry of KS 11,
2+
which had the highest Ca content, the most need for Ca /oct,
and Ti 4+/tet, must be considered tentative due to its anomalous, and unconvincing, value for Ca content.
The various substitutions which could be responsible to
account for the crystal chemistry of melanites and schorlomites
Divalent cations in octahedral coordina-
are assessed by HVH.
tion are explained by
M
.4+
2+
/oct + Ti /oct
4.
-
2M
3+
/oct
and this substitution is implied by the deficiency of trivalent
cations in these garnets (see Table 7a and 7b).
-66-
The presence of Fe + in tetrahedral coordination is not
adequately explained by the electron hopping process proposed
Their evidence (unusually low isomer shift for Fe
by HVH.
is not substantiated by the present study.
/4)
In addition, an
examination of the garnet structure (Figure 1),
with its
independent octahedral and tetrahedral sites, shows that
electron hopping between cations in these two sites requires
passage of the electron through an oxygen anion at the corner
of the coordination polyhedra.
Such a process seems unlikely
to account for the significant amounts of Fe +/4 found in some
samples.
The substitution originally suggested by Dowty
(1971)
2M +/oct + Si 4+/tet 2 2Ti 4+/oct + Fe
/tet
seems more reasonable as it also removes trivalent cations and
4+
in octahedral coordination.
places Ti
The introduction of Ti into the structure of garnet can
be accounted for by two of the coupled substitutions of HVH:
M +/oct + Ti
Ti
/oct ' 2M
/oct
/oct
M3 /oct
However, the substitution
+.
Si.4+
Ti.4+
4
.4+
should not be invoked, since there is no proof that Ti /tet
exists in the structure.
In addition to the evidence mentioned
above, attempts to synthesize Ca3
2(TiFe2)O2 failed (Ito and
-67-
Frondel, 1967) indicating reluctance on the part of Ti
enter the tetrahedral site.
to
-68-
VI.
Optical-absorption Properties and Color
With variations in the crystal chemistry of titaniumbearing andradites now understood and catagorized, it is
possible to resolve the controversy existing over the opticalabsorption spectra of melanites and schorlomites.
Many
studies (Manning and Harris, 1970; Moore and White, 1971;
Dowty, 1971; Burns, 1972; Huggins et al., 1976b) have attempted
to interpret the color and absorption spectra of titaniferous
garnets.
This study clarifies some of the confusion.
There are four features in the optical and near infrared
spectra of titanium garnets:
(i) an intense broad band at
5000 cm~.
(ii) A rapid rise in absorption intensity above
1
which is responsible for the yellow-red to black
8j000 cm
colors.
This absorption in the blue end of the visible
spectrum extends into the ultraviolet and has other features
superimposed-on it.
(iii) A broad shoulder at 19,000 cm
and (iv) a sharp band at 22,700 cm~
strongly superimposed on
the intense absorption edge (Figure 11).
The 19,000 cm~
and 22,700 cm-
features were assigned by
Manning and Harris (1970) to crystal field transitions
.
3+
4
4
6
2T
+ 2E in Ti 3+/oct and A + A , E in Fe /oct,
lg
g
2g
g
lg
respectively. These assignments are essentially correct,
although the features may also be enhanced by crystal field
3+
transitions in Fe /tet. Samples used in the Manning and
Harris (1970) study were from Magnet Cove and San Benito County.
It should be noted that the 19,000 cm~1 feature attributed to
-69-
Figure 11.
Diffuse reflectance spectra of
titanium-andradite garnets from San Benito
County (An(Ti)-84*) and Magnet Cove
(An-Ti 15.45) showing absorption features
described in text.
(1971).
From Moore and White
An(T)-84*
- An-Ti 15.20
1.0
0.9
'A
,
An-Ti 15.45
T
E
0~
o
E
0
0
(%J
In
0.7
0.6
mmmm Su
0.5
0.4
E
0.3
0.2
u
0
0
LO
1C
0.1
0.0
500
1000
1500
WAVEL ENGTH
2000
(nonometers)
2500
-71-
Ti +/oct is not present in the spectrum of the Magnet Cove
sample.
This observation may be correlated with the finding
of the present study, which shows the sample from Magnet Cove
3+
(KS 3) to be totally lacking in Ti .Only
the samples from
San Benito County (e.g. KS 8),
which contain significant Ti
,
show this feature.
The 5,000 cm~1 band was attributed to the 5E -- 5 T2 crystal
2+2
field transition in Fe /tet by Manning and Harris (1970).
Moore and White (1971) speculated that this feature might be
due to a charge transfer.
They offered Fe +/8-fold
as the most likely candidate.
+
Ti 4+/oct
This charge transfer is accept-
able in garnet, as each eight-fold site shares edges with two
octahedrally coordinated sites (see Figure 1).
the 5,000 cm~
HVH concluded
band must be related to ferrous iron since the
feature disappears in a sample (from San Benito) which had all
its iron oxidized in a ZrO 2 electrochemical cell.
In fact,
each one of these explanations may be partly correct.
The
samples used by Moore and White (1971) and by Manning and
Harris (1970) from San Benito County and Magnet Cove both
showed the 5,000 cm~
Magnet Cove specimen.
feature, but it was more intense in the
For the Magnet Cove sample, the
mechanisms of Manning and Harris (1970) and Moore and White
(1971) are both possible, since the schorlomites of Magnet
Cove contain both Fe +/tet and Fe +/8-fold.
However, neither
explanation is adequate for the San Benito County garnets
(e.g. KS 8),
since they do not contain ferrous iron in either
of those sites.
A charge transfer involving Fe
/oct, which
-72-
is present in the San Benito sample cannot be responsible for
this low energy features, since oxygen-metal charge transfers
involving Fe2+ occur in the UV (Loeffler et al., 1974) and
2+
3+
2+
.4+
Fe /oct -+ Fe /oct or Fe /oct + Ti /oct charge transfers
would also occur at much higher energies, due to the extreme
distance between these octahedrally coordinated cations.
For the San Benito sample, the 5,000 cm 1 band might be
due to a split crystal field absorption in Fe
/oct, with the
higher energy component obscured under the general absorption
above 8,000 cm~.
Orthopyroxenes show Fe
bands at about 5,400 and 11,000 cm
1
crystal field
(Burns, 1970).
However,
the six-cooordinate M2 site in pyroxene responsible for these
absorption bands is much more distorted than the regular
octahedral site in garnet.
Such a large splitting of the e
g
levels in garnet would be unexpected, unless the unusual
crystal chemistry (e.g. Ti 4+/oct, Fe
/oct) of these garnets
results in local site distortion.
The characteristic red-yellow colors of melanites and
schorlomites are caused by the strong absorption edge in the
blue end of the visible spectra.
This feature is most likely
due to an intense oxygen-metal charge transfer absorption in
the ultraviolet, which is not present when Ti is absent (Moore
and White, 1971).
It was assumed by Moore and White (1971) that
Ti played a role in causing this absorption and their assignment was an 0-2 T3+ charge transfer, with 0
being excluded.
-Fe 3+/tet not
-73-
Howie and Woolley (1968) also correlated the variation of
color with Ti content.
However, HVH attributed the dark colors
(schorlomites can grade in color from red-yellow to opaque) to
2+
Fe /oct due to an apparent "more exact" correlation between
color and Fe
/oct content.
However, during the present study,
it was found that opaque areas existed in a thin section of
a garnet from Italy (KS 10) which contained no Fe
/oct.
Additionally, microprobe studies on KS 11, which is zoned and
contains opaque bands in thin section, revealed that the opaque
areas could be correlated with a 35% increase in TiO2 content.
There was no noticeable increase in FeO.
Therefore, it is more likely that the opaque areas in
these garnets are caused by either a Fe 2+ Ti4+ charge transfer
or a Ti3+ crystal field transition.
in the green, at 19,000 cm~
Ti3+ absorption is centered
(Burns and Huggins, 1973), and a
substantial increase in Ti content will broaden the absorption
peak enough to absorb the red end of the spectrum, causing
opaqueness.
HVH reported a color change for almost opaque to
pale yellow in their San Benito sample when they heated the
specimen. This color change can be explained by the oxidation
3+
of Ti
in the melanite, which would remove this absorption
feature and give the sample its yellow color.
-74-
Conclusions
VII.
The presence of five different types of Fe in natural
Fe-Ti garnets is now confirmed with the resolution and identification of up to ten peaks in the MOssbauer spectra of some
schorlomites.
The need for a sophisticated fitting procedure
to solve such complex spectra has been demonstrated, as it is
seen that any simplified process can lead to significant errors
in the determination of both Fe 2+/Fe + ratios and values for
isomer shift.
With.the true value of isomer shift for Fe +/4 now obtained,
it is no longer necessary to invoke electron hopping mechanisms
Any Fe 2 +
involving tetrahedrally coordinated ferrous iron.
which finds its way into tetrahedral coordination does so to
compensate for Ti 4+/oct, and not to partake in some intricate
Additionally, this study provides no
cation exchange process.
substantial reason to assume that any Ti is present in the
tetrahedral site.
This implies that the mechanisms controlling
site occupancy in natural Fe-Ti garnets are not as complicated
as previously thought.
The silicon deficiency is accounted for
by Al and then Fe, dependent on their relative abundances
during mineral paragenesis.
Thermal considerations may be
especially important if the mineral forms under metamorphic
conditions.
The presence of octahedrally coordinated divalent
cations is controlled by the amount of Ca present.
content will force other divalent cations (Mg2+,
into the octahedral site.
A high Ca
Mn 2+,
Fe 2+
-75-
The deficiency of trivalent cations can be explained in
terms of the substitutions needed to place Ti in the crystal
structure.
Titanium, which is primarily in the quadravalent
state, must substitute for trivalent cations in octahedral
coordination.
The accompanying charge imbalance can only be
corrected by placing divalent cations in the 6-fold site
(common) and Fe + in tetrahedral coordination (rare).
Consequently, the presence of large amounts of Ti in the crystal
structure of garnet is accomplished totally at the expense of
trivalent cations.
The controversy which existed over discrepancies between
2+
3+
2+
3+
ratios points out the danger
3
Fe Fe/Fe
/eMoss
Fewet chem and Fe /Fe
of using wet chemical methods alone to determine ferrous iron
content in minerals.
M6ssbauer spectroscopy should be
recognized as a useful tool for identifying the presence of
reduced cations in mineral species. In the present case, the
.3+.. melanites and
2+
in
presence of Fe , and by deduction, Ti
schorlomites indicate that these garnets were formed under
low oxygen fugacities
(Huggins et al., 1976b).
Finally, the possibility exists that the distribution of
Fe in Fe-Ti garnets can be used to infer something about the
petrology and the thermal history of the host rock.
However,
the results of this study indicate that iron distribution is
dependent on bulk chemistry (e.g. Si and Ca content) as well,
making a quantitative handling of the subject very difficult.
-76-
Acknowledgments
The successful completion of this thesis was a joint
effort to which many people contributed.
The author wishes to
thank everyone of his friends who stayed with him throughout
the trying periods.
Many useful and relevant conversations on
M5ssbauer spectroscopy and mineral color were conducted with
Special thamks to Alan Parkes
Bruce Loeffler and Kay Parkin.
for his help on the microprobe.
The author also thanks R.A.
Howie and David Cook of the Harvard mineralogical collection
for supplying the samples.
Special thanks also to Professor
William B. Watson of the MIT History Department for teaching
the author how to write research papers.
On a more personal level, the author gives many thanks
(but not enough) and much love to Margaret Johannets for
drafting the figures and living with him for the past four
years.
The author also wishes to thank the Grateful Dead for
supplying the music
when he got confused ((and knowing what
a long strange trip it's been).
But most important, the
author wishes to thank his thesis adviser, Professor Roger G.
Burns, for introducing him to M*ssbauer spectroscopy and the
mystery of titanium garnets.
This thesis is dedicated to
Roger with appreciation for being able to work with him as a
scientist and associate with him as a fried.
-77-
References
Amthauer, G., Annersten, H., and Hafner, S.S. (1976) The
Mossbauer spectra of 57Fe in titanium-bearing andradites.
Physics and Chemistry of Minerals, in press.
Bancroft, G.M., Maddock, A.G., and Burns, R.G. (1967) Applications of the M*ssbauer effect to silicate mineralogy - I.
Iron silicates of known crystal structure.
Geochim.
Cosmochim. Acta, 31, 2219-2246.
Bancroft, G.M. (1973) M6ssbauer Spectroscopy: An Introduction
for Inorganic Chemists and Geochemists.
McGraw-Hill,
Maidenhead, England.
Burns, R.G. (1970) Mineralogical Applications of Crystal Field
Theory.
University Press, Cambridge, England.
Burns, R.G. (1972) Mixed valencies and site occupancies of
iron in silicate minerals from M*ssbauer spectroscopy.
Canad. J. Spectrosc., 17, 51-59.
Burns, R.G. and Huggins, F.E. (1973) Visible-region absorption
.3+
in fassaite from the Allende meteorite:
spectra of Ti
a discussion.
Amer. Mineral., 58, 955-961.
Dowty, E. (1971) Crystal chemistry of titanium and zirconian
garnet: I. Review and spectral studies.
Amer. Mineral.,
56, 1983-2009.
Howie, R.A. and Wooley, A.R. (1968) The role of titanium and
the effect of TiO2 on the cell size, refractive index
and specific gravity in the andradite-melandite-schorlomite
series.
Mineral. Mag., 36, 775-790.
-78-
Huggins, F.E. (1974) MOssbauer Studies of Iron Minerals Under
Pressure of up to 200 Kilobars.
Ph.D. Thesis, M.I.T.,
Cambridge, Massachusetts.
Huggins, F.E., Virgo, D., Holzl, E., and Huckenholz, H.G. (1975)
4+
3+
between the octahedral
Distribution of Al, Fe , and Ti
and tetrahedral sites in garnets between Ca3 Al 2Si 012
and Ca 3Fe 2Ti .416Si
.5840 12.
Annual Report of the
Director of the Geophysical Laboratory, 1974-1975,
pp. 579-585.
Huggins, F.E., Virgo, D., and Huckenholz, H.G. (1976a)
Titanium-containing silicate garnets. I. The distribution
of Al, Fe +, and Ti4+ between octahedral and tetrahedral
sites.
Amer. Mineral., in press.
Huggins, F.E., Virgo, D., and Huckenholz, H.G. (1976b) Titaniumcontaining silicate garnets. II. The crystal chemistry of
melanites and schorlomites.
Amer. Mineral., in press.
Ito, J. and Frondel, C. (1967) Synthetic zirconian and
titanium garnets.
Amer. Mineral., 52, 773-781.
Loeffler, B.M., Burns, R.G., Tossell, J.A., Vaughan, D.J., and
Johnson, K.H. (1974) Charge transfer in lunar materials:
interpretation of ultraviolet-visible spectral properties
of the moon.
Proc. 5th Lunar Sci. Conf., Geochim. Cosmo-
chim. Acta, Suppl. 5, vol. 3, pp. 3007-3016.
Loeffler, B.M. and Burns, R.G. (1976) Shedding light on the
color of gems and minerals.
Amer. Sci.,
64, 636-647.
-79-
Lyubutin, I.S., Dodokin, A.P., and Belyaev, L.M. (1970) Temperature dependence of the Mossbauer effect for octahedral
iron atoms in garnets.
Soviety Physics - Solid State,
12, 1100.
Manning, P.G. and Harris, P.C.
(1970) Optical-absorption and
electron-microprobe studies of some high-Ti andradites.
Canad. Mineral.,
10, 260-271.
Moore, R.K. and White, W.B. (1971) Intervalence electron
transfer effects in the spectra of the melanite garnets.
Amer. Mineral., 56, 826-840.
Novak, G.A. and Gibbs, G.V. (1971) The crystal chemistry of
the silicate garnets.
Amer. Mineral., 56, 791-825.
Stone, A.J., Augard, H.J., and Fenger, J. (1969) General
constrained non-linear regression for M6ssbauer spectra.
Publ. Danish Atomic Energy Comm. RlSO-M-1348.
Whipple, E.R. (1973) Quantitative M6ssbauer Spectra and Chemistry
of Iron.
Ph.D. thesis, M.I.T., Cambridge, Massachusetts.
Zedlitz, 0. (1933) Uber titanmichen kalkeisen granat.
Zentralbl. Mineral. Geol., Paleontol., Abt. A, 225-239.
-80-
Biographical Sketch
The author was born in Newburgh, New York on November 14,
1954.
He was educated in Valley Stream, New York, graduating
Upon arrival
from Valley Stream South High School in 1972.
at MIT, the author became a member of the Beta Theta Pi
Fraternity.
During his undergraduate years, he studied
geology and geochemistry, in addition to receiving a varsity
letter in wrestling during the 1973-74 season.
from MIT was received in June, 1976.
An S.B. degree
In addition to his work
on MOssbauer spectroscopy, the author spent time working on
the mineralogy and microchemistry of manganese 'nodules.
The author has accepted an offer to enter the Ph.D. program
at the University of California, Santa Barbara, where he will
engage in research in isotope geochemistry.
In addition, the
author plans on pursuing his lifelong hobbies of playing
softball, lying on the beach, and listening to the Grateful
Dead in the benevolent climate of southern California.
- Al -
KS 6
Position
Line
Doublet
Width
%Area
2.99
-
0.010
-
0.028
0.651
0.651
-
0.026
0.651
4.58
-
0.432
0.336
4.99
0.118
0.336
33.72
0.715
0.336
33.72
0.868
0.336
4.99
1.578
2.343
0.651
4.58
0.651
3.72
2.907
0.651
2.99
3.72
1-10
6 = 1.449
= 2.917
Fe2+ /8
2-9
6 = 1.158
= 2.371
Fe +/6
3-8
6 = 0.776
= 1.604
Fe2+ /4
4-7
6 = 0.218
= 1.300
Fe +/tet
5-6
6 = 0.417
=
Fe 3+/oct
All values in mm/sec.
0.597
- A2-
M~ssbauer spectrum of schorlomite from Kola
Peninsula, U.S.S.R.
z
0
O.
O
0
V)
6.0
-2.0
0
mm/sec
2.0
4.0
- A4 -
KS 13
Line
Doublet
Position
Width
%Area
-
0.125
0.598
4.56
-
0.005
0.598
-
0.418
0.359
4.48
9.48
0.092
0.359
31.49
0.707
0.359
0.875
0.359
31.49
9.48
1.591
0.598
4.48
2.667
0.598
4.56
1-8
6 = 1.271
A = 2.792
2-7
6 = 0.793
A = 1.596
Fe 2+tet
3-6
6 = 0.229
A = 1.293
Fe +/tet
4-5
6 = 0.400
A = 0.615
Fe
All values in mm/sec.
/oct
- A5 -
M*ssbauer spectrum of schorlomite from
Rusinga Island, Kenya.
2.0
z0
co
0
V)
6.0
-2.0
0
mm/sec
2.0
4.0
- A7-
Mbssbauer spectrum of melanite from Rusinga
Island.
0
U
I
2.0
z0
0
4.0
'I
SI
(0
SI
I
Sg
SI
II
II
8.0
-2.0
0
mm/sec
2.0
4.0