Spectroscopic Characterization of Minerals and Their Surfaces

Chapter 14 57

Fe-Bearing

Oxide, Silicate, a n d A l u m i n o s i l i c a t e Minerals

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 18, 2014 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch014

Crystal Structure Trends in Mossbauer Spectra Roger G. Burns and Teresa C. Solberg Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 Correlations have been sought between Mossbauer spectral parameters and several crystal structure properties of Mg-Al oxide and silicate minerals hosting iron cations as minor constituents. Plots of isomer shift (I.S.) versus various bond length-related parameters, including mean metal-oxygen distance (R) of a coordination site and volume per oxygen in a unit cell (V ), fail to delimit I.S. ranges for Fe ions in tetrahedral, octahedral and five-fold coordination sites. However, I.S. vs polyhedral volume (V ) of a site cluster in narrow ranges specific to each coordination symmetry. Better correlations exist for ferrous I.S. vs ferric I.S. data when coexisting Fe and Fe ions occupy similar or identical coordination polyhedra in the same mineral. The upper limit for tetrahedral Fe I.S. in silicates is shown to be ≤0.25 mm/sec., whereas the lower limit for octahedral Fe is ~0.29 mm/sec. The correlations point to inconsistencies in Mossbauer spectral parameters and cation site occupancy assignments for clintonite, yoderite and sapphirine. New Mossbauer spectral data obtained for these minerals demonstrate that: clintonites from skarn deposits contain tetrahedral Fe and octahedralFe3+and Fe , with relative enrichment of Fe in tetrahedral sites; only octahedral Fe and Fe occur in sapphirines from granulite facies rocks; and five-coordinated Fe predominates over octahedral Fe ions in yoderites from high grade metamorphic rocks. ox

3+

P

2+

3+

3+

3+

3+

2+

3+

2+

3+

3+

3+

Mossbauer spectroscopy has proven to be a useful technique for characterizing the valences, coordination numbers, electronic configurations, and magnetic states of iron cations in rock-forming minerals (1-3). Two diagnostic parameters, isomer shift and quadrupole splitting, usually serve to identify paramagnetic Fe cation species in Mossbauer spectra of silicates measured at ambient temperatures. However, ambiguous crystal chemical assignments have been proposed for iron when it is present as a minor constituent of some minerals, particularly magnesian and aluminosilicate phases containing low concentrations of Fe and Fe substituting for Mg and Al ions in octahedral, tetrahedral and five-fold coordination 2+

3+

3+

0097-6156/90/0415-0262S06.50/0 © 1990 American Chemical Society

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

2+

S7

14. BURNS &SOLBERG

Fe-Bearing Minerals

263

environments (4-9). Correlations have been sought, with varying degrees of consistency, between the Mossbauer s p e c t r a l parameters and a number of c r y s t a l structure parameters (5-7) i n attempts to demonstrate the presence of tetrahedral or five-coordinated f e r r i c iron i n some aluminosilicate minerals. Several of these c o r r e l a t i o n s are examined i n t h i s paper and are extended to a d d i t i o n a l oxides and s i l i c a t e s l i s t e d i n Table I containing F e and F e ions i n a v a r i e t y of oxygen coordination polyhedra. The l i m i t a t i o n s of determining iron cation s i t e occupancies i n rock-forming minerals from Mossbauer s p e c t r a l and c r y s t a l structure parameters are tested f o r three Mg-Als i l i c a t e s , sapphirine, c l i n t o n i t e and yoderite, i n which F e cation assignments are c o n t r o v e r s i a l . Since attention i s focused on i r o n d i l u t e minerals, complications o r i g i n a t i n g from thermal electrond e l o c a l i z a t i o n i n the F e - F e minerals magnetite, deerite, i l v a i t e , cronstedtite, etc. (10 11) and from superparamagnetism i n nanocrystalline Fe(III) oxides magnetite, hematite, maghemite, goethite, etc. (12) are not considered here.


2 +

3+

3 +

2+

3+

r

C o r r e l a t i o n s Between Isomer S h i f t

and

Quadrupole S p l i t t i n g 2+

Parameters

3 +

D i s t i n c t regions are c l e a r l y defined for F e and F e ions i n d i f f e r e n t coordination environments when the two parameters, isomer s h i f t (I.S.) and quadrupole s p l i t t i n g (Q.S.), derived from roomtemperature Mossbauer spectral measurements of a v a r i e t y of i r o n bearing oxide and s i l i c a t e phases l i s t e d i n Table II, are p l o t t e d against one another as i n Figures 1 and 2. Although the Mossbauer parameters summarized i n Table II are taken from a v a r i e t y of sources, the majority of the l i s t e d minerals have been measured independently during the course of the present i n v e s t i g a t i o n . Errors for the isomer s h i f t s are estimated to be ±0.03 mm/sec. Ranges of ±0.1 mm/sec. commonly observed f o r the quadrupole s p l i t t i n g parameter r e f l e c t compositional variations due to atomic s u b s t i t u t i o n i n many of the minerals. Figure 1 shows that octahedrally coordinated ferrous iron i n the vast majority of rock-forming minerals i s c l e a r l y distinguished from F e ions i n eight-coordination (e.g. garnets, vesuvianite), square planar s i t e s (e.g. g i l l e s p i t e , eudialyte), and tetrahedral coordination (e.g. spinel, s t a u r o l i t e , m e l i l i t e ) , although parameters for five-coordinated F e ions (e.g. andalusite, g r a n d i d i e r i t e , vesuvianite) overlap ranges f o r octahedral ferrous i r o n . For f e r r i c iron, however, the separation between ranges of I.S. and Q.S. shown i n Figure 2 i s less d i s t i n c t than f o r ferrous iron when F e ions occur i n octahedral, tetrahedral and f i v e coordination environments. Three problem cases are apparent from the data p l o t t e d i n Figure 2. They involve sapphirine, c l i n t o n i t e and yoderite. The c r y s t a l structures of these minerals each contain A l (octahedral i o n i c radius, r • 0.53 Â) or Mg (r = 0.72 Â) ions i n two or more environments with d i f f e r e n t coordination numbers: octahedral, five-«fold and/or tetrahedral. Ambiguities exist over s i t e occupancies of F e ions ( r = 0.65 Â) deduced from the Mossbauer s p e c t r a l parameters for each mineral. The I.S. data for Mg-rich sapphirine (SAM) appear to be too low for F e ions i n octahedral coordination, leading to suggestions (4.5) that f e r r i c iron substitutes for A l i n smaller tetrahedral s i t e s instead of octahedral s i t e s having larger metal-oxygen distances. For c l i n t o n i t e s (CL), two doublets assigned to tetrahedral and octahedral Fe ions substituting for A l i n the c r y s t a l structure not only l i e outside ranges observed for f e r r i c iron i n f o u r - f o l d and s i x - f o l d coordinations i n other s i l i c a t e minerals including micas, but also indicate r e l a t i v e enrichments of F e ions i n the smaller tetrahedral 2 +

2 +

3+

3 +

2+

o c t

o c t

3+

o c t

3+

3 +

3+

3 +

3+


264

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES


Table I. Mineral Formulae and Legend to Figures Mineral

Ideal Formula

andalusite acmite aenigmatite almandine annite andradite actinolite babingtonite cordierite chloritoid clintonite corundum epidote eudialyte ferriannite ferridiopside ferrifayalite grandidierite gillespite glaucophane grossular hibonite howieite ilmenite kyanite LiA102 (synth) laihunite ludwigite melilite montmorillonite mullite muscovite neptunite olivine orthopyroxene osumilite periclase phlogopite perovskite pyrope riebeckite Mg sapphirine Fe sapphirine schorlomite sanidine sillimanite

Al2Si05 NaFe Si206 Na2Fe 5TiSi6O20 Fe 3Al Si 0i2 KFe 3Si3A10io(OH)2 Ca3Fe 2Si30i2 Ca2(Mg,Fe)5Si8022(OH)2 Ca2Fe Fe Si50i4(OH) Mg2Al4Si50i8 Fe 2Al4Si20iO(OH)4 CaMg2Al4Si30io(OH)2 AI2O3 Ca2Fe Al2Si30i2(OH) Na4Fe 2ZrSi60i7(OH)2 KFe 3Si3Fe Oio(OH)2 Ca(Mg,Fe )(Si,Al ,Fe )2Ο6 2+ 3+ 0 MgAl3BSi0g BaFe Si40io Na2Mg3Al2Sie022(OH)2 Ca3Al2Si30i2 Ca(Al,Fe )i20i9 NaFe ioFe 2Sii2031(OH)13 Fe Ti03

Key to Figures

3+

2+

2 +

2

3

2+

3+

2+

3+

2+

3+

2+

2+

3+

3+

F e

F e

3+

2 S i 2

3+

8

2+

3+

2+

3+

2+

Al2Si05 LÎA102 2+

F

3+

Fe o.8 e 0.8Si04 (Mg,Fe )2Fe B05 Ca2(Mg,Al) (Si,Al)07 (Na,Cai/2)0.7Al4Si4Oi0(OH)4 Al Si 0i3 KAl2Si3A10io(OH)2 KNa2LiFe 2Ti2Si8024 (Mg,Fe)2Si0 (Mg,Fe )2Si206 KMg2Al3Sii203().H20 MgO KMg3Si3A10io(OH)2 CaTi03 Mg3Al2Si30i2 Na Fe Fe 2Si8022(OH) (Mg,Al) (Si,Al) O20 (Mg,Fe,Al) Si,Al) O20 Ca3(Fe ,Ti)2(Si,Fe )3Ο12 KA1SÎ308 AI2S1O5 2+

6

3+

2

2+

4

2+

2+

2

3+

3

2

8

6

8

3+

6

3+

AA AC AE AL AN AR AT BA CD CH CL CO EP EU FA FD FF GD GI GL GR HI HO IL KY LA LH LU ME MO ML MU NE OL OR OS PE PH PV PY RI SAM SAF SC SD SI

Structure Reference JJLL. 11

21 21 22. 21

2A 22. 23

21 21 21. 28

21 30,113 11

2Z 21 2A 21 21 21 38. 39

AÛ

22+21 Al AA+A1 11+Al Al 41

Al M

hi H 51 54. 55 39^56

51 11 59. 60

24

£1 22. 23

£2 £1 1A £1 M 11+£1


57

14. BURNS & SOLBERG

Fe-Bearing Minerals

ICS


Table!. Continued

Mineral

Ideal

Formula

stilpnomelane spinel staurolite talc taramellite vesuvianite

K ( F e , F e - A l ) l 0 S i l 0 O 3 0 (OH) 12 MgAl204 Fe 2Alg(Si,Al)4Ο22(OH)2 Mg3Si40io(OH)2 Ba4Fe Fe 2TiSie024(OH)4 Ca (Mg,Fe)4AlioSiis070(OH,F)8

voltaite vivianite yoderite

K2Fe sFe 4(SO4)12.I8H2O Fe 3(P04)2.8H20 (Mg,Al)8Si4O20

2 +

3 i

2+

2+

3+

19

2+

3+

2+

Key t o Figures

Structure Reference

SN SP ST TA TM VE

te te IteJA

VO W YO

2Â 21 21

2JU22

21

T a b l e I I . C r y s t a l S t r u c t u r e and Mossbauer S p e c t r a l Parameters f o r F e r r i c I r o n C o o r d i n a t e d t o Oxygen i n Oxide and S i l i c a t e M i n e r a l s a

b

0.,35 0.22 0.,38

1. 83 2. 65 0. 30

1.,935 1.,836 2.,025

17..12 17..12 17..84

9,.54 5,.15 10,.87

32.4 3.2 12.2

21

oct tet oct oct oct oct oct oct oct tet oct

0.,39 0.,23 0.,43 0. 39 0.,39 0. 38 0.,38 0..33 0.,50 0..27 0..35

1. 28 0. 97 0. 99 0. 31 0.,55 0. 65 0. 89 0. 95 1.,10 0. 68 0..48

1.,976 1.,649 2.,121 2.,101 2..024 2.,101 2.,048 1..935 2.,016 1.,730 1..913

18..62 18..62 21..12 21..12 18,.22 19..06 18..56 16,.51 18..90 18..90 14 .04

10,.19 2,.29 12,.53 12,.21 11 .05 12,.02 18,.3 8 .99 10,.88 2 .63 9.06

27.1 0.8 0 0.4 0 6.7

*•

3.8 14.4 0.8 9.0

84. 85

oct oct oct tet tet tet 5CN oct oct tet 5CN oct oct tet oct

0..36 0..39 0..42 0,.19 0,.18 0,.41 0,.33 0 .36 0 .42 0 .18 0 .22 0 .35 0 .38 0 .16 0 .39

2..01 1..01 0..46 0..39 1..34 0,.85 1.20 0,.48 0 .58 0 .52 2 .65 0 .67 0 .99 0 .62 0 .91

2..036 2..106 2..107 1..685 1,.629 2,.077 2 .042 1.930 1 .910 1 .808 1 .978 2 .081 1 .916 1 .761 2 .077

17,.78 21,.62 21 .62 21 .62 17 .32 18 .12 18 .17 18 .16 16 .78 16 .78 16 .78 17 .45 14 .68 20 .96 18 .12

10 .86

24.3 1.3 1.2 0 0.1 10.6 2.0 15.8 1.6 0.4 74.3 8.4 5.8 0.1 10.6

te

corundum (sapphire epidote ferriannite

ferridiopside ferrifayalite grandidierite glaucophane hibonite

ilmenite kyanite LiA102(syn) laihunite

P

5 .17 9.44 9.40 3 .30 7 .94 12 .56 9 .16 2 .79

A

f

oct 5CN oct

o x

V

e

andalusite (viridine) acmite (aegerine) aenigmatite

andradite actinolite babingtonite chloritoid clintonite

V

d

S i t e I . S . Q.S. Symm.

annite

R

c

Mineral

Mossbauer Reference

te

12

21a. te te al

60. 87

12

te te n

te tea. te te te te 90. 98

Continued on next page


266

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

Table IL Continued a


Mineral

b

S i t e I.S. Q.S. Symm.

montmorillonite

mullite muscovite orthopyroxene osumilite periclase perovskite phlogopite riebeckite (crocidolite) Mg sapphirine or Fe sapphirine schorlomite sanidine sillimanite taramellite vesuvianite yoderite or

R

c

V

d o x

oct oct tet oct oct oct tet tet oct tet oct tet oct

0..35 0,.37 0..15 0,.40 0,.37 0,.45 0,.18 0,.25 0,.36 0,.18 0,.35 0,.17 0 .38

0 .56 0 .96 0 .11 1 .04 0 .86 0.68 1 .34 1 .71 0 .54 1 .33 0 0 .50 0 .43

1,.955 1,.940 2,.106 1..629 1..772 2..106 1..824 1..926 1,.649 2,.070

18.56 18.56 18.61 21.27 18.88

oct oct tet tet oct oct oct tet tet oct tet oct 5CN oct 5CN

0,.29 0..30 0,.29 0,.30 0..33 0..35 0,,42 0,.18 0..21 0..37 0..16 0,.47 0..33 0..36 0,.36

1.23 0 .76 1 .23 0 .76 0 .85 1 .49 0 .62 1 .30 0 .48 1 .11 0 .53 0 .44 0 .56 1 .00 1 .00

2..002 2,.080 1,.757 1.,758 2..022 2.,087 2.,024 1..643 1.,649 1..912 1..764 2..021 2,.107 1..963 1.,933

16.4 16.4 16.4 16.4 16.86 16.86 18.22 18.22 22.47 16.62 16.62 22.71 18.74 16.38 16.38

16.76 19.54 17.32 17.32

V

e P

9.52 11.83 2.24 2.59 12.44 0 9.49 2.30

11.63 9.47 2.75 2.78 10.78 12.72 11.0 2.30 9.18 2.79 6.60 9.82 6.07

A

f

Mossbauer Reference

6.4 0.4 0.1 0

0 15.7 5.0 5.0 5.6 2.0 8.3 3.3 0 0 0.3 3.4 4.0 6.5 1.8 1.6 3.8

a

Isomer s h i f t r e l a t i v e to Fe metal standard, mm/sec, at 298°K b Quadrupole s p l i t t i n g , mm/sec. Mean metal-oxygen distance i n coordination s i t e , Â. Volume per oxygen i n a unit c e l l , Â . Polyhedral volume of a coordination s i t e , A3. Bond length deviation (16) ** This paper. c

d

100 101 102

3

e

f


103 60,104 *• 17

**

106 107.108 109. ** 75,110


—

CO

CO

u φ

^

h

0.2

0.71

0.8

0.91

1.0

1.11

1.2

1.31

AVE

1.2

2.2

OST

• AT

Q.S. (mm/sec)

1.7

• CL

• AT

• SAF

•r*

|SAM #CN Κ

ν

ιΜ

^

·

• SAF #SA« Μ

#

r

°

• GL

2.7

Ο • A • Ο

r

φΟΙ

24

Ο

2

24

OK

2 +

&2

2

2

8-coordination Fe octahedral Fe 5-coordination Fe * tetrahedral Fe * square planar Fe *

·'* · ·Αε

•AH

φ

Figure 1. Isomer s h i f t versus quadrupole s p l i t t i n g data f o r F e ions i n a v a r i e t y of coordination environments i n s i l i c a t e and oxide minerals. C a l i b r a t i o n of each 295K Mossbauer spectrum from which the parameters are derived i s based on reference zero v e l o c i t y at the midpoint of the α-Fe spectrum. The legend to mineral symbols i s contained i n Table I.

0.7

ftSAM •SAF


3.7

3


C O

Ε

CO

Φ

υ

0.1

0.2

0.3

h

0.2

0.41

0.5h

S

AN AM

0.7

-^S

··»-

SC

φα

AM

-L 1.2

SC^jOP

-4s A M

1.7

Q.S. (mm/sec)

§ML #LM # F A φβΑ · Κ Υ |MS #SI

IFF

AN

2.2

Figure 2. Isomer s h i f t versus quadrupole s p l i t t i n g data f o r F e ions i n several coordination environments i n d i f f e r e n t s i l i c a t e and oxide minerals. The 295K Mossbauer spectra are c a l i b r a t e d against α-Fe standard.

•co

φ Ft I

• FA

φ

3+

3

3 +

3

2.7

• octahedral Fe Δ 5-coordination Fe * • tetrahedral Fe '


14. BURNS &SOLBERG

Fe-BearingMinerals

5Î

269

s i t e s r a t h e r t h a n t h e l a r g e r o c t a h e d r a l s i t e s (6.7) . Y o d e r i t e (YO) has A l and Mg i n o c t a h e d r a l and f i v e - f o l d c o o r d i n a t i o n s i t e s , b u t i t i s i m p o s s i b l e t o d e t e r m i n e unambiguously t h e s i t e occupancy o f F e i o n s i n y o d e r i t e from i t s Mossbauer s p e c t r a l p a r a m e t e r s a l o n e CJEL> . In a t t e m p t s t o v e r i f y t h e c r y s t a l c h e m i s t r y o f F e ions i n these m i n e r a l s , o t h e r c o r r e l a t i o n s have been sought between t h e two Mossbauer s p e c t r a l p a r a m e t e r s , I . S . o r Q . S . , and c r y s t a l s t r u c t u r e p a r a m e t e r s b a s e d on bond d i s t a n c e s and s i t e d i s t o r t i o n p r o p e r t i e s (2.5-9). These c o r r e l a t i o n s a r e now c r i t i c a l l y e x a m i n e d . 3 +


3 +

Correlations

Between Isomer S h i f t

and C r y s t a l

Structure

Parameters

The i s o m e r s h i f t p a r a m e t e r i s s e n s i t i v e t o t h e e l e c t r o n d e n s i t y around the n u c l e u s . Values of I.S. f o r F e are p r e d i c t e d to decrease w i t h i n c r e a s i n g s - e l e c t r o n d e n s i t y a t t h e n u c l e u s ( 2 ) , and t o depend on i r o n o x i d a t i o n s t a t e as w e l l as t h e t y p e and b o n d - l e n g t h s o f l i g a n d s c o o r d i n a t e d t o i r o n (12) . Thus, h i g h o x i d a t i o n s t a t e s , i n c r e a s e d c o v a l e n t b o n d - c h a r a c t e r and s h o r t e n e d bond d i s t a n c e s a r e each p r e d i c t e d t o lower t h e I . S . o f F e phases. In oxygen c o o r d i n a t i o n environments, t h e r e f o r e , increased metal-oxygen d i s t a n c e s i n c o o r d i n a t i o n polyhedra are expected to r e s u l t i n higher I . S . v a l u e s f o r Fe c a t i o n s i n such s i t e s . Crystal structure p a r a m e t e r s t h a t r e f l e c t i n t e r a t o m i c d i s t a n c e s i n c l u d e : (1) t h e a v e r a g e m e t a l - o x y g e n d i s t a n c e i n a c o o r d i n a t i o n s i t e , R; (2) t h e volume p e r oxygen i n a u n i t c e l l , V ; and (3) t h e p o l y h e d r a l volume, 5 7

5 7

o x

Vp. Isomer S h i f t v e r s u s Average M e t a l - O x y g e n . Attempts t o e s t a b l i s h the presence of t e t r a h e d r a l F e i o n s i n s i l i c a t e s have sought c o r r e l a t i o n s between I . S . and average bond l e n g t h s o f (Si,Al)04 t e t r a h e d r a (2 5-7) but were not e x t e n d e d t o o t h e r c o o r d i n a t i o n environments. T h e r e f o r e , d a t a f o r a wide range o f f e r r i c - b e a r i n g m i n e r a l s a r e shown i n F i g u r e 3 , i n which a v e r a g e m e t a l - o x y g e n d i s t a n c e s , R, f o r oxygen c o o r d i n a t i o n p o l y h e d r a i n s e v e r a l m i n e r a l s t r u c t u r e s c o n t a i n i n g t e t r a h e d r a l , o c t a h e d r a l and f i v e - c o o r d i n a t e d s i t e s a r e p l o t t e d a g a i n s t t h e I . S . d a t a summarized i n T a b l e I I . The I . S . d a t a f o r c l i n t o n i t e , magnesian s a p p h i r i n e and y o d e r i t e a r e e a c h p l o t t e d i n two s e p a r a t e r e g i o n s b e c a u s e , as n o t e d e a r l i e r , F e site o c c u p a n c i e s f o r t h e s e m i n e r a l s a r e ambiguous. F i g u r e 3 shows t h a t , although smaller metal-oxygen distances i n t e t r a h e d r a l s i t e s are a s s o c i a t e d w i t h low F e I . S . v a l u e s and h i g h e r I . S . p a r a m e t e r s a r e found f o r F e i o n s i n o c t a h e d r a l s i t e s w i t h l a r g e r R, t h e p l o t t e d d a t a f a i l t o d e l i m i t t h e upper range f o r t e t r a h e d r a l F e i o n s and t h e l o w e r range f o r o c t a h e d r a l F e ions. N e v e r t h e l e s s , t h e r e i s an i n d i c a t i o n that f e r r i c i r o n occurs i n octahedral coordination i n s a p p h i r i n e and not i n t e t r a h e d r a l s i t e s , but t h e d a t a f o r c l i n t o n i t e are l e s s c o n c l u s i v e . For y o d e r i t e , although the data f o r f i v e coordinated F e i o n s l i e between the ranges f o r t e t r a h e d r a l and o c t a h e d r a l f e r r i c i r o n , the value of the I.S. i s a l s o c o n s i s t e n t with Fe occupancy o f t h e s i x - c o o r d i n a t e d s i t e s i n y o d e r i t e . 3 +

r

3 +

3 +

3 +

3 +

3 +

3 +

3 +

Isomer S h i f t V e r s u s Volume P e r Oxygen. The a v e r a g e m e t a l - o x y g e n d i s t a n c e p a r a m e t e r p l o t t e d i n F i g u r e 3 i s u n s a t i s f a c t o r y i n many ways b e c a u s e i t o b s c u r e s d i f f e r e n c e s between oxygen l i g a n d - t y p e s (e.g. f r e e 0 " and OH" i o n s , b r i d g i n g S i - O - S i and n o n - b r i d g i n g S i - O " , e t c . ) and between ranges o f m e t a l - o x y g e n d i s t a n c e s i n d i s t o r t e d coordination polyhedra. S i m i l a r c r i t i c i s m s may be l e v e l e d a t c o r r e l a t i o n s between isomer s h i f t and volume p e r oxygen, V , c a l c u l a t e d f r o m u n i t c e l l p a r a m e t e r s and t h e number o f f o r m u l a u n i t s 2

o x



1-6

1-8 +

SC OP

I

• HI

0-2

•os

I.S.

0·3

SAM

PSAM

SAM

· "

•HI

•

•os AI

.3+

0·4

0*5

3 +

• t e t r a h e d r a l Fe" Δ 5 - c o o r d i n a t i o n Fe° • octahedral F e

•co •s]•κν

·ΑΤ

• AE •LH

c

_ AR

·ΑΤ

•m

^ • • A

•

• *t

# E r

AN

(mm/sec)

• CH

A

4c,

3 +

F i g u r e 3. Average m e t a l - o x y g e n d i s t a n c e p l o t t e d a g a i n s t i s o m e r shift for Fe ions i n d i f f e r e n t c o o r d i n a t i o n environments i n m i n e r a l s . The d a t a f o r c l i n t o n i t e ( C L ) , y o d e r i t e (Y0) and Mgs a p p h i r i n e (SAM), w h i c h a r e h i g h l i g h t e d b y s p i k e s , a r e each p l o t t e d i n two r e g i o n s because d u a l f e r r i c s i t e o c c u p a n c i e s have been s u g g e s t e d f o r t h e s e m i n e r a l s .

OC

o

Spectroscopic Characterization of Minerals and Their Surfaces

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