THE AMDRICAN MINERALOGIST, VOL. 55, SEPTEMBER_OCTOBER, 1970

CRYSTAL FIELD SPECTRA AND EVIDENCE OFCATION ORDERING IN OLIVINE MINERALSI

Rocnn G. BunNs,z Department oJ Geology and. Mineralogy,Unioersity of Oxford,, England

Assrnecr

Polarized absorption spectral measurements are described of several Mge+-pgr+ m6Fe2+-Mnz+ olivines. The spectra contain three overlapping absorption bands in the nearinfrared originating from electronic transitions within Fd+ ions. The central, more intenseband arises from Fd+ in the M (2) positions, whilst the ,14(1) positions are represented bytwo broad, outer bands. The relative intensities of the three bands are polarization de-pendent, indicating that Fd+ olivines are strongly pleochroic in the infrared. The bandmaxima move to longer wavelengths with increasing iron and manganese contents of theolivine. The compositional variations form determinative curves for Fe2SiO4 contents ofunanalysed olivines of the forsterite-fayalite series.

Fd+ site populations have been estimated from the areas under the absorption bands.In the forsterite-fayalite series Fd+ ions are evenly distributed over the M(l) and M(2)positions, but there are indications of slight relative enrichments in the M(2) positions ofsome specimens. Significant cation ordering occurs in the fayalite-tephroite series, in whichFd+ ions are enriched in the M (1) positions as a result oI the preference of Mn2+ ions for theM(2) positions. The uniform distributions of Fd+ ions in the forsterite-fayalite series areexplained by crystal field theory.

INrnonucrrorr

Olivines remain one group of ferromagnesian silicate minerals inwhich cation ordering has not been demonstrated by conventionalmethods. fn the amphibole and pyroxene crystal structures, for example,the evidence of cation ordering from X-ray measurements (Ghose, 1961,1965) has been substantiated by Miissbauer spectroscopy (Bancroft,Burns and Maddock, 1967a; Bancroft, Burns and Howie, 1967; Ghoseand Hafner, 1967) and site population determinations have been made.However, although the olivine structure contains two distinct cationsites, M(1) and M(2), neither X-ray (Hanke, 1965; Birle, Gibbs, Mooreand Smith, 1968) nor Mdssbauer (Bancroft, Burns and Maddock, 1967b)measurements have been able to detect ordering of Mg2+, Fe2+ and Mnz+ions. The Iack of success by Miissbauer spectroscopy has been due to thedifficulty in resolving contributions to the spectra from Fe2+ ions in theM(l) and M(2) positions. In the room-temperature Miissbauer spectraof olivines (Sprenkel-Segel and Ilanna, 1964; Bancroft et al,, 1967b) the

r Paper presented at the Eighth International Union of Crystallography Congress,Stony Brook, New York, August 1969 [Abstract published in Acta Crystallogr., A25,559,(re6e)1.

2 Present address: Department of Earth and Planetary Sciences, Massachusetts Insti-tute of Technology, Cambridge, Massachusetts 02139.

1608

SPECTRA AND ORDERING IN OLIVINE

parameters for iron in the two sites are almost identical.l Furthermore,since iron and manganese have similar scattering factors it. has beendifficult to distinguish between Fe2+ and Mn2+ ions in silicate structuresby X-ray methods.

In the crystal field spectra of olivines, however, contributions fromFe2+ ions in the two crystallographic positions can be distinguished.This paper describes polarized absorption spectral measurements on arange of specimens from which it has been possible to estimate sitepopulations in olivines of the forsterite-fayalite series, (Mg, Fe)2SiOa,and fayalite-tephroite series, (Fe, Mn)rSiOr.

While no investigation has been published of the compositional varia-tions of the crystal fi,eld spectra of olivines, several papers have describedabsorption spectra of various individual specimens. Thus, room-tem-perature measurements in the visible and near infrared regions have beenmade on various forsteritic olivines in unpolarized (Clark, 1957; Grum-Grzhimailo, 1958, 1960; White and Keester,1966) and polarized (Farrelland Newnham, 1965; Burns, 1965,1969; Shankland, 1969) l ight, and ofa fayalite in polarized l ight (Burns,1965, 1966, 1969). The spectra ofolivines at elevated temperatures (Burns, 1965, 1969; Fukao, Mizutaniand Uyeda, 1968) and pressures (Balchan and Drickamer, 1959) havealso been reported. fn addition, the difiuse reflectance spectra of Mnz+in tephroite and glaucochroite have been described (Keester and White,1968).

Compositional variations of vibrational and rotational bands in theinfrared spectra of several olivines of the forsterite-fayalite and fayalite-tephroite series have been measured (Lehmann, Dutz and Kolter*nann,1961; Tarte, 1962; Duke and Stephens, 1964). However, no system-matic studies of the electronic spectra of intermediate Mgz+-pe2+ andFe2+-Mn2+ olivines have been reported so far. This paper, therefore,also describes the compositional variations of the crystal field spectra ofvarious olivine minerals of the forsterite-favalite and fayalite-tephroiteseries.

Expnnrlmrrnl Mnrnons

Measurements were made on eleven minerals of the forsterite-fayalite series and fiveminerals of the fayalite-tephroite series. Many of the specimens, which were obtained as

1 In the Mijssbauer spectra of fayalite at elevated temperatures (above 639"K) theouter quadrupole doublet is resolved into two peaks of equal intensity (Eibschtitz andGaniel, 1967). In the low temperature spectra (below 55oK) the hyperfne splitting hasbeen resolved into two sets of lines originating from difierent Fd+ ions (Kundig, et aJ.1967). However, these measurements have not been extended to olivines of intermediatecompositions and no site population data are yet available.

1610 ROGER G. BURNS

petrographic thin sections from rock collections at Berkeley, Cambridge and Oxford, hadbeen analyzed previously. The compositions of the remainder were determined by electronmicroprobe analysis, using Geoscan (analyst, Burns at Cambridge) and MAC 400 (anatyst,Dr. D. C. Harris at Ottawa) instruments. The compositions and sources of the olivines aresummarized in Table 1. Concentrations are expressed as mole percentages of fayalite(FezSiOq), tephroite (MnrSiOa) and larnite (CazSiOe) components. All but two of theMgz+-p"z+ olivines contain less than three percent Mn2SiO4. The compositions of theFe2+-Mnz+ olivines are more variable and the specimens include a picrotephroite with23.4 percent Mg2SiOr and a roepperite with 8.6 percent ZnxSiOa and 8.4 percent Mg:SiOr.

Spectra were measured over the range 4000-22000 A in polarized light by a microscopetechnique (Burns, 1966). A thin section (0.003-0.05 cm) cut from rock containing theolivine or from a single crystal and mounted in Canada balsam, was placed on a three-axisuniversal stage mounted on a polarizing microscope with calcite Nicol polarizers. A singlecrystal of the olivine was brought into the field of view of the microscope and a suitablevibration or crystallographic axis was oriented E-W to coincide with the polarization ofthe lower Nicol. The microscope was then mounted in the sample beam of a Cary model 14recording spectrophotometer. An identical microscope system holding a glass slide andCanada balsam was placed in the reference beam.

All three polarized spectra corresponding to light vibrating along the a, p and 7 axeswere measured. Since olivine belongs to the orthorhombic system (space group Pbnm),vibration and crystallographic axes coincide with a :b, B: c and,7: 6,r, The 7 spectra con-tain intense absorption bands and cou'ld be measured conveniently on conventional petro-graphic thin sections. However, absorption bands in the a and B spectra are weak, especiallyin forsteritic olivines, and thick sections or mounted plates had to be used to record thesespectra.

The Cary spectrophotometer plots the spectrum on a chart recorder, which registersthe optical density (log Io/I) at each wavelength.2 In order to resolve overlapping bandsit is necessary first to replot the spectra on a wavenumber (cm.-l) scale.

The spectra of olivines were found to consist of overlapping absorption bands. Thesewere resolved into component Gaussian curves in the optical density/wavenumber spectrabv means of a Dupont rnodel 310 curve resolved This instrument also computes the arearatios of the component Gaussian curves.

TUB OrrvruE CRysrAL Srnucrunp

The structure of olivine has been described by several crystallog-raphers (Bragg and Brown, 1926; Belov, Belova, Andrianova and Smir-nova, 1961; Hanke and Zeerrrar', 1963; Gibbs, Moore and Smith, 1963;Hanke, 1965;Birle, Gibbs, Moore and Smith, 1968). The crystal struc-ture of fayalite (Birle el al 1968) consists of independent SiO+ tetrahedraIinked by divalent cations in six-fold coordination. There are two non-equivalent six-coordinate positions in the structure, which are designated

I This orientation difiers from that of Farrell and Newnham (1965) who referred theirspectra to the alternative space group setting Pnma.In this setting, d: a, P: b and y: 5.

2 log I nf I : ect: optical density (O.D.) where 1o and 1 are the intensities of the incidentand transmitted beams, respectively; c is the concentration of the absorbing species (molesper liter or gm. ions per liter);J is the thickness of the sample (cm.); and e is the molarextinction coefficient (liter mole -1 cm.-l).

o

-dr

l a 3 -

F N fi +1 4 \ON OC C\ C) : Ol e dr A \o

SPECTR-A AND ORDERING IN OLIVINE

O N r N 1=i <. -; -i

: < ' r r o €

O o + ' : ON N

$ : N

T o * c i oN NN

O \ O N N ,a + r \ o o\ O N

r o : o

: D O l f l

6 d - i oa

i O O N

- i - j - j d o@ i

€ o r NN N

a a ? a Io c o : oN N

r Q N N l

i €

o O N 4. lN r o o Ii €

: o € o \ |

o \ o

n +1 O.+ \oi 6 i C ) m

A q a a Ir 4 \ o o I4 0

q : a ? |N \ O O O

- x A - *

4B?84i >>ds

+ 1 n H o I6 i O O

N

co

rr

N\o

ra

O D \ O B :d o i d - j d C c I

H N N : OO \ O \ O N N . . I

o i J ' ; d c i E ' q I

O \ 4 D O. O \ o r : . I' ? d ; d - i ! 6 i

\ o H o b 4 . Io ' d r d c i q q IN N $ t r t r

q n e O

t A 1 1 ' l : a ta O o \ o O Y r IN 6 N A

S O H n e

c ; 6 i - j ; o o o o

q { N N N Ov \ o . H c i . . r

c ; * q d J q ! - is4 tr tr tr

S O \ O r :- j < . - j x j c j ; o le r D :

N + l o N $$ e N N . n

o i - ; d ; o < ? r ? o€ D C t r

N F o \ O : l' . . ' + . io n + \ o o : - . '

o \ m e o N4 i d r o o C ) . 4

; 6 d € j d q q c ;O O N t r t r

o a o o

c O s - O O O O ON \C)

I O C ) H N N

o i $ 6 i o o ; + |N €

< t r 6 N o \ d r

a : $ c t r

4 : O. O i i \ O N O

C = r o - i o d Pq H * 1 t r

r D < t -. N i H O < r , I

e . o c ; c ; o c j q I

d ^ v V A ^ A oc) Y. t r .box: : : .qL 6 r i > > u Z S E

zFO

a

2F

ts

a

O

oa

v

zots

o

o(,

|]

F

r6t2 ROGER G. BURNS

b = l o . 4 9 A -( a ) ' o = 4 ' 8 3 A

Tc= 6 .104

(p)

Io(2)

o(3)

Ftc. 1. The olivine crystal structure. This (100) projection of the fayalite structureshows oxygen coordination polyhedra about cation positions at r:0.5. The figure showsa chain of linked octahedra extending along the c axis. Metal-oxygen distances in eachcoordination site are indicated. @ }i[O) ;A M(2) ; O oxygen; . silicon.

by M(l) and M(2).1 Both coordination sites are distorted from octa-hedral symmetry (Fig. 1.). The configuration of the six nearest oxygenatoms surrounding the M(l) position is that of an axially elongatedoctahedron with a rhomboid median plane, in which the cation iscentrally located. Although the point symmetry of the M(1) position is| (C ) , the local symmetry of the M(1) coordination site is approximately4/mmm(D+n) with the tetrad axis through O(3)-M(1)-O(3). Theoxygen coordination polyhedron about the M(2) position is irregularand the point symmetry is /n(C"). However, the local symmetry of theM(2) coordination site may be regarded as approximately 3m (Cg,) withthe triad axis parallel to a. A cation in the M(2) position does not lie atthe center of symmetry of the coordination site, and is offset along 6.A significant feature of the olivine structure is the irrfi.nite bands of

1 The symbolism M (l) and M (2) follows that originally proposed by Bragg and Brown(1926), who used Mg(I) and Mg(II). Hanke (1965) used the alternative symbols,4l andA^Ior the M (1) and M (2) sites, respectively, to distinguish between the two positions withinversion (T) and mirror plane (m) point symmetries.

\ € r.,o-,

o(2) o(1)

SPECTRA AND ORDERING IN OLIVINE 1613

cations which extend along the c axis. The structure of a forsterite with10 percent FezSiO+ is similar (Birle el al, 1968). Each metal-oxygen dis-tance is shorter and the mean distances are reduced from 2.16 A (M(1)

s i te) and 2. lg f \ (M(2) s i te) in fayal i te to2. lO A ( ,41( t ) s i te) and 2.144(M(2) site) in forsterite (Hanke, 1965; Birle, et al,.1968).

No crystal structure determination of a tephroite has been reported. Itis generally inferred from X-ray powder and rotation photographs thattephroite and fayalite are isostructural (O'Daniel and Tscheischwilli,1944; Deer, Howie and Zussman, 1963, vol. 1,35). Neutron diffractionexperiments on FezSiOr and MnzSiO+ have shown that the magnetic andchemical unit cells are identical (Santoro, Newnham and Nomura , 1966).No information is available on the configuration of the oxygen atomsabout the M(l) and M(2) positions or extent of ordering of Mnz+and Fe2+ ions in the structure. Cell parameters indicate that the metal-oxygen distances are larger in the structures of manganiferous olivines.Although Mn2+ ions are predicted to favor the larger M(2) coordinationsite (Ghose, 1962), the similar scattering factors of iron and manganesemake it difficult to detect Mn2+-Fe2+ ordering in the olivine structureby X-ray methods. However, the detection of Mn2+-Fez+ ordering isnow possible usrng counter data and available site refinement programs(Brown and Gibbs, personal communication).

RBSur-rs: SpBcrne oF TrrE FonsrBnr:rB-Fever-rrB SBnrBs

The polarized spectra of olivines of the forsterite-fayalite series havebeen illustrated several times previously (Burns, 1965'; Farrell andNewnham, 1965; Burns 1969, 1970). However, a typical set of spectraof fayalite (specimen 10) taken from the chart recorder of a Cary spec-trophotometer are shown in Figure 2.'fhe unresolved polarized spectraof Mgz+-p"z+ olivines display the following features:

1. Each polarized spectrum of a given olivine is distinctive. Thus,olivines of the forsterite-fayalite series are strongly pleochroic, althoughthe pleochroism is not seen except in fayalite (c:7:pale yellow, B: orange yellow) because the principal absorption bands are Iocatedoutside the visible region (,1000-7000 A).

2. The B spectra consist of tw^o broad bands in the near infrared at8450-9050 A and 11000-12400 A.

3. The a spectra are similar to the B spectra but the maxima centered at8550-9150 A occur at slightly longer wavelengths in each spectrum. Themaxima at about 12800 A in the d spectrum of fayalite is not as welldeveloped as the corresponding band in the 0 spectra, and appears onlyas a prominent inflexion in the forsterite spectra.

25,Un 20,000 15,000

wavenumb€r (€m-l)

10,m0

Ii

ol

t& -

| ^ r FI l :\ r I. \ - ;

r6t4 ROGER G. BURNS

s,(XXr 10,000

wavelength (A)

I 5,000

Fre. 2. Polarized absorption spectra of fayalite (specimen 10) . . . d spectrum;--- B spectrum; - 7 spectrum. (opt ic or ientat ion: d:b ' P, :c;7:a.

4. The ? spectra are three to five times more intense than the d and Pspectra, and appear to consist of three overlapping absorption bands, themost intense of which has an absorption maximum at 10400-10800 A.

5. The absorption edge of an intense charge transfer band located in theultraviolet region appears below about 5000 A. It is most conspicuous

SPECTRA AND ORDERING IN OLIVINE

Teera 2. B.qr.ro Posr:rroNs rN TrrE Por,enrzro AlsotprroN Spocrneot Oltvrxrs rN rnn Nnen fr.rlnenro

B spectrumBand III

1615

7 spectrumBand II

d spectrumBand ISpecimen

Numher

Nfole 16FezSiO{

A cm-r

8560 116808560 116808625 116008770 114008950 112308930 112009010 111009010 111009090 110009150 109309150 109308620 116008740 114509210 11390

8480 117908480 117908510 117508660 115508810 113508850 113008940 112008890 112509010 111009040 110609050 110508510 117508640 115709100 10990

10870 920011000 909011050 905011490 870011860 843011980 8350r2r20 825011900 840012120 825012400 806512390 807011900 840012050 830012180 8210

10460 956010450 957010460 956010520 951010670 937010590 944010640 940010640 940010670 937010780 928010750 930010910 916011050 905011200 8930

c m '

12J

5o

789

101 112I J

1 A

8 412.31 6 . l4 2 . 9J / . J

70.31 1 n

7 8 . 08 5 . 196.19 7 . 969 .04 1 . 53 1 I

in the spectra of fayalites and extends further into the visible region rn

the B spectra than it does in the a and ? spectra. This absorption edge

produces the optical pleochroism of fayalite.

6. Several weak inflexions and4850-4970 A and 6100-6300 Atransfer band in each polarized

sharp peaks, notablY at 4500-4590 A,

are located on the flanks of the charge

spectrum.

7. The absorption maxima of all bands and peaks, together with the

absorption edge, migrate to longer waveiengths (smaller energies) with

increasing FezSio+ component (Table 2.). These shifts show linear trends

when absorption maxima expressed in energy units (wavenumbers) are

plotted against mole percent FerSiO+ in the olivine (Figure 3'). The de-

gree of scatter of points abotit straight lines is most pronounced {or

olivines with appreciable MnzSiO4 contents (specimens 5 and 7). The

plots in Figure 3 could be used as semiquantitative determinative curves

for FezSiOa contents of unanalysed Mgz+-U.2+ olivines.

In order to resolve the overlapping absorption bands, the three

polarized spectra of each olivine were plotted on a wavenumber abscissa

icale and component Gaussian curves were located with a Dupont curve

resolver. The resolved spectra of three olivines (specimens 2,5 and 11)

are shown in Fisure 4.

t6L6 ROGER G, BURNS

0 l0 20 30 40 50 60 70 80 90 loo

mole "1" Fe2SiO4

Frc. 3. Compositional variations of absorption maxima in the polarized spectra ofMgz+-psz+ oliv^ines. E B spectra (s450-9050 A); o o spectra (8550-9150 A); x z specrra(10400-10800 A); A 0 spectra (11000-t2+00 A); o published data for other forsteriricolivines. C Clark (1957);N Farrell and Newnham (1965); W WhiteandKeesrer (1966);F Fukao et o.l (1968) S Shankland (1969).

Three component bands, each with a different width at half height,may be resolved in each polarized spectrum. For a given olivine thepositions and widths of each band are almost identical in the threepolarized spectra, but the relative intensities differ from one spectrum toanother. Absorption maxima of each absorption band again decreaselinearly with increasing Fe2SiO+ content, but the half-widths of corre-sponding bands remain approximately constant over the compositionrange. The characteristic properties of the three component bands are:

band f : position 11800-11000 cm-l; half-width 2900+200 cm-rband II: position 9550-9250 cm-l; half-width 1500+ 100 cm-lband III: position 8700-8000 cm-l; half-width 1900+ 150 cm-1

AII three bands contribute appreciably to the 7 polarized spectra, withthe greatest contribution coming from band II. In fayalite, for example,the areas under the Gaussian curves are in the ratio band I:band II:band III:23.5:44.0;32.5 percent. Relatively small deviations from

.E

Lo

Efgo

o=

4 5 6 ' 7 A I 1 0 1 1

SPECTRA AND ORDERING IN OLIVINE

WAVENUMBER (CM-I)

l3ooo loooo ?ooo l3ooo loooo ?ooo l3ooo roooo ?000

t617

I

, t , , , | , , , , ' t ' | t t t ' ' ' t | ' t ' t

l3ooo loooo Tooo l3ooo loooo ?ooo 13000 10000 Tooo

u"ENuu8en (at-t )

Fro. 4. The polarized absorption spectra of olivines resolved into component Gaussian-

shaped bands. The figure shows each polarized spectrum of three olivines (specimens 11,

5 and 2) separated with a Dupont model 310 curve resolver into three overlapping absorp-

tion bands.

- l l I |

| I I

z

E

l r t t l r l l l

t t l l l l l l l r r r r l r t l r l r r l r l l

1618 ROGER G. BURNS

these values are found in the resolved 7 spectra of other Mgz+-psz+olivines (Table 4). In the a spectra bands I and III contribute moststrongly to the spectra, but a significant contribution comes from bandII. With decreasing FezSiOr concentration, the contribution from bandfI increases while the ratio of intensities of band I to band III remainsapproximately constant. The 0 spectrum of fayalite contains contribu-tions from bands I and III only. However, again there is an increasingcontribution from band II with decreasing FezSiOr concentration whilethe area ratio of band I to band III is approximately constant.

Interpretation of the Spectra. Magnetic susceptibility measurements(Nagata, Yukutake and Uyeda, 1957; Kondo and Miyahara, 1963;Santoro, Newnham and Nomura, 1966) indicate that Fe2+ ions in theolivine structure have the high-spin configuration with four unpairedelectrons. This would lead to the degeneratesT2o ground-state andsEoexcited state if Fe2+ ions were in regular octahedral coordination. How-ever, in the distorted six-coordinate sites of the olivine structure thedegeneracies of these crystal field states are resolved. In the M(1) site,which for illustrative purposes is assumed to have approximately Dusymmetry, the octahedralsT2o ground-state of Fe2+ is resolved into theIower 5.E, state and upper 5B2o state, while the octahedral |Eo excitedstate is resolved into the 6Ae and 5,B1, states. In the M(2) site, whichmay be conveniently approximated to Cr, symmetry, the ground-stateis 5,4 l and there are two excited states with 5E symmetry. The crystalfield states and electronic configurations of the ground states of theFe2+ ion in symmetry Dqn ar.d C3o crystal fields are shown in Figure 5.

The intense band at 10400-10800 A i.r the 7 spectra (band II) isindicative of Fe2+ ions in a noncentrosymmetric environment, andoriginates from a transition to the highest energy state of Fe2+ ions inthe M(2) positions. This transition is analogous to thebAr--+58 (upper)transition in a symmetry C3, site (Figure 5f). The bands at 8450-9150 A(band I) and 11000-12300 A (band III) represent transitions to the twohigh energy excited states of Fe2+ ions located in the M(1) positions.These transitions are analogous to the uEo-uBrn and 5En--->'Ars transi-tions in a symmetry D4r, environment (Figure 5e). The transitions 5,4r--+5E (lower) andsEn--->5Bzs for Fe2+ in symmetry G, and D+r, environ-ments, respectively, would be excepted to lie in the far infrared. Absorp-tion bands representing analogous transitions in Fe2+ in olivine were notobserved in the spectral range measured in this work, but one or othermay account for the absorption band at 1670 cm.-r reported by Whiteand Keester (1967).

Confirmation of the assignment of absorption bands in the olivine

SPECTRA AND ORDERING IN

-5eg_5A19

t6t9

o'n_cs - t t

5E

5E

SBtg

SB29

5p-g

(e)

06 Dgn

Irrc. 5. Energy level diagrams for electronic states and 3d orbitals of Fe2+ in octahedral,

tetragonal and trigonal coordination sites. (a), (b), (c) electronic configurations of the

ground-states in sites with symmetry Or', Drr (elongated along the tetrad axis) and Cr'

(compressed along the triad axis). (d), (e), (f) crystal field states corresponding to the

ground-states shown in (u), (b), and (c), respectively.

spectra stems from spectral measurements of the iron-rich monticellite

kirschsteinite, Ca(Mg, Fe)SiO+, containing 69.4 mole percent CaFeSiO.l(Sahama and Hytdnen, 1957). A low intensity, asymmetric absorption

band is observed in all three polarized spectra and there is a conspicuous

absence of an in tense band at about 10800 A in the 7 spectrum. (Burns,

1965). Since Ca2+ ions completely fi l l the M(2) positions in the monticel-

l i te structure (Brown and West, 1927;Hanke, 1965; Onken, 1965), the

spectra can only arise from absorption b-v Fe2+ ions in M(1) positions.

Therefore, the absence of Fe2+ ions in M(2) positions can be correlated

with the nonappearance of an intense absorption band around 10300 A

in the 7 spectrum of kirschsteinite.

ROGER G. BARNS

Teer,r 3. PosrtroNs eNo Assrcmmxrs or. SprN-FoRBTDDEN pnars (rn A)

Positions of Peaks (A)SpecimenNumber

%MnzSiOr

2101 2I J

74l . )

t6ref. (a)

ref. (b)

Assignment:

Fe2+Mn2+

6300 (s200)6100 (s2s0)63s0 (s300)

(s2s0)

(4700) 4500(4730) 4s9o

4550

4750 4550

(4060)(6000) (4ss0) 40es6000 M75 41105900 44Jlr0 41005820 44J]0 41255860 4410 4130

5755 41.10 4060

0 . 23 . 0

2 6 . 24 t . o66 .874 .59s.7

90

48504970500049505000

3 D3',I--zlh tTb

a A b ,

4 F

r l o

Ref. (a) Farrell and Newnham (1965)Ref. (b) Keester and White (1968)

Unresolved shoulders are bracketed.

The weak peaks located in the visible region of each polarized spec-trum of Mgz+-psz+ olivines represent spin-forbidden transitions withinFe2+ ions. An assignment of these peaks, based on that given by Furlani(1957) for similar peaks in the spectra of hydrated Fe2+ compounds, isincluded in Table 3.

The displacement of absorption maxima of bands I, II and III tolonger wavelengths with increasing iron content of the olivine may beattributed to the increased unit cell dimensions resulting from the re-placement of smaller Mgr+ ions by larger Fe2+ ions in the olivine struc-ture.

Color and Pleochroism of Oliuines. Forsteritic olivines are generallycolorless or pale green in thin section. Fayalites display faint opticalpleochroism, with a:7: pale yellow and B: orange yellow. The spectraillustrated in Figures 2 and 4, however, show that all olivines of theforsterite-fayalite series are distinctly pleochroic beyond the visibleregion.

The pale green color of olivines arises from absorption of the redportion of the visible region by shoulders of the bands with maxima inthe near infrared. The cause of the orange-yellow color of fayalite in B

SP]JCTRA AND ORDD,RING IN OLIVINE 162I

polarized light is the appearance into the visible region of a charge trans-fer band located in the ultraviolet. The edge of this band covers moreof the blue region in B polarized light than similar bands in a and 7polarized light. As a result, fayalite a.ppears ,,redder,' in B polarized light.

Pleochroism in fayalite results from charge transfer or electronictransitions between neighboring ions in the structure. The B indicatrixaxis coincides with the c axis along which bands of cations in M(1) andM(2) positions extend throughout the olivine structure (Figure 1).rnteratomic distances between ions in adjacent M(1) positions in fayaliteare 3.05 A and M(l)-M(z) disrances aie about 3.2b A. Although theM(l)-M(z) direction does not coincide rvith the B axis, it does l ie in

s000 l00oo l5o@wovelength (A)

Frc. 6. Polarized absorption spectra of a ferroan tephroite (specimen l4). . .. aspectrum; -- p spectruml - T spectrum ; (optic orientationl a: [. p : 6. 7 : 6).

the (100) plane containing the B axis and infinite bands of cations (Fig.1). Overlap of t2o-type orbitals of adjacent Fe2+ ions situated in M(l)positions along the B axis and between Fe2+ ions in adjacent M(l) andM(2) positions in the planes containing the B axis and the bands ofcations leads to facilitated charge transfer in B polarized light. In thisorientation the electric vector of polarized light is most favorablyaligned to induce electron transfer through overlapping t2o-type orbitalsof neighboring cations along the chains.

Rnsurrs: SpBcrne oF THE FayarrrB-TEprrRorrE SBnrBs

The three polaized spectra of the tephroite (Feo.ezzMnr.raoCao.orsMgo.oro)SiOa (specimen 14) are shown in Figure 6, and in Figure 7 the 7 spectra ofa suite of olivines of the fayalite-tephroite series are illustrated. fn con-trast to the forsterite-fayalite series, the polarized spectra of man-

t622 ROGER G. BARNS

wavelength (A)

10,m0 15,000

0.3

o.2

0 .1

0

0.3

. o-2a

0.1

0.1

00.3

0.2

0.t

00.3

o.2

5,m t0,0m 15,000

wavelen$h (A)

Frc. 7. Polarized absorption spectra of Fe2+-Mn2+ olivines. The figure illustrates vari-

ations in the 7 spectra of specimens 70, l2,l4,and,16 with decreasing FerSiOrand increasing

MnzSiOl components.

ganiferous olivines show appreciable compositional variation. In the aand p spectrb bands I and III are located at shorter wavelengths inknebelite (specimen 12) than in fayalite (Table 2). In general, the

ts

SPECTRA AND ORDERING IN OLIVINE 1623

maxima of bands I and III move to longer wavelengths with increasing

MnzSiOa content. There is also a smaller contribution from band II to the

spectra than in Mgz+-p"z+ olivines of similar FezSiOs content'

The 7 spectra illustrated in Figure 7 display the most pronounced

compositional variations :

1. The intense band Iocated at 10780 A in fayalite migrates to Ionger

wavelengths with deceasing FezSiOr component in the manganiferous

olivine.

2. With decreasing FezSiOa content, the intensity of band II at 10780 A

decreases relative to bands I and III.

wavelengths with increasing Mn2Sio+ component (Table 3 and Figure 7)

4. Each of the three polarized spectra of picrotephroite (specimen 15)

and pure tephroite (specimen 16) is almost identical: the peaks occur at

the same positions in each spectrum, and their relative intensities are

approximately the same. The spectra, therefore, conform with the ob-

servation that tephroites are nonpleochroic. There is close agreement

between the polarized spectrum illustrated in Figure 7d and the diffuse

reflectance spectrum of tephroite described by Keester and White (1968).

Interpretation of the spectra. By comparison with the spectra of fayalite

(Figure 2), the peak. ut +Szo-iSsO A, 4950-5000 A and 6300-6350 A, in

addition to the broad bands in the infrared (8000-13500 A), arise from

transitions within Fe2+ ions. The peaks at 4095-4130 L, ++75-++00 A and

5320-6000 A are due to transitions within Mn2+ ions.

The Mnz+ ion, which is paramagnetic in olivine (Santoro, Newham and

Nomura, 1966) with five unpaired electrons at room temperature, gives

rise to the non-degenerate 6-41s ground-state in an octahedral crystal

field. This state is not resolved into further states in fields of lower

symmetry, such as those in the M(l) and, M(2) coordination sites of the

olivine structure. Excited states of the x{n2+ ions include the aTb, aT2s,

aEn, and 4,41, states, which have lower spin multiplicities than the 6,41n

ground-state. Thus, electronic transitions in Mn2+ ions are spin-forbidden

irnd lead to weak absorption bands. The peaks in the spectra of man-

g:rnif erous olivines may be assigned as follows (Table 3) :

t624 ROGER G. BURNS

5820-6000 4., to uAro--.nTro

4475-4400 A, ro 6ALs-+472s

4095-4130 A, to uAro=-uEn, oAro

There is no resolution of the transition doublet 6Au--+aEo, a.41, in thespectra of manganiferous olivines such as that observed in the polarizedspectra of Fe3+ ions in epidote (Burns and Strens, 1967).

The 7 spectra show that there is a pronounced decrease in relativeintensity of band II centered atlOTSO_1t200 A with increasing MnzSiOrcomponent in the olivine. Since this band originates from absorption byFd+ ions in the noncentrosymmetric M(2) site, the spectra of man-ganiferous olivines show that Mn2+ ions selectively replace Fez+ ions inM(2) positions. As a result, Fd+ ions are enriched in the M(1) positions.

On the assumption that the configuration of the M(1) site has ap-proximately Da6 slmmetry, the displacement of bands I and III to lowerwavelengths (higher energies) relative to fayalite might suggest that themean metal-oxygen distance in the M(1) site is smaller in manganiferousolivines than in fayalite. Furthermore, the increased separation betweenbands I and III in knebelite (specimen 12) and roepperite (specimen 13)compared to fayalite might suggest that there is increased (tetragonal)distortion of the oxygen coordination polyhedron about the M(1) posi-tion in manganiferous and zincian olivines. rrowever, these deductionsdo not conform with recent structural refinements of manganiferousolivines. Gibbs and Brown (personal communication) find that the meanM (l)-O distance in the knebelite (Feo.514Mns.a6aMgo.ozz)zSiOn, containingboth Fe2+ and Mn2+ ions in M(1) positions, is slightly larger (2.168 A)than that in fayalite (2.158 A). Furthermore, in the roepperite(Mns.656Mgo .v2Zns rlaF es.o*)2SiOr, the .&1(1)-0(3) distances are smaller(2.190 A) than those in fayalite (2.230 A); and rhe M(1)-O(1) and, M(l)-O(2) distances, which are comparable (2.I22 A) in fayalite, are dissimilarQ.t48 A and 2.106 A; i" th. roepperite structure. These results showthat the true symmetry of the M(1) site is lower than the assumed Drrsymmetry, which accounts for the discrepency between predicted andmeasured configurations of the M(1) sites of manganiferous olivines.

Srrn PopurarroNs rN rnn Or,rvrNB Srnucrune

The polarized spectra of olivines show that it is possible to distinguishbetween Fe2+ ions in the M(1) and M(2) positions of the olivine struc-ture. Furthermore, the reduced intensity of band rr relative to bands rand III in the ? spectra of manganiferous olivines shows qualitativelythat cation ordering occurs in the fayalite-tephroite series. since band rris attributed to Fez+ ions in M(1) positions, the spectra indicate that

SPDCTRA AND ORDERING IN OLIVINE 1625

Mn2+ ions {avor the M(2) positions and that Fez+ ions are enriched in the

M(1) positions.The proportions of Fe2+ ions in the M(1) and M(2) positions, n1f tt'2,

may be estimated quantitatively from the area ratios of componentGaussian curves in the resolved crystal field spectra. Thus,

A t / A , : Cn t /nz

where .4r and Az are the areas under absorption bands originating fromFe2+ ions in the M(1) and M(2) positions, respectively, and C is a con-stant. The constant C rr,ay be evaluated from the area data. of one stan-dard olivine for which the ratio hf nzis known. On the assumption thatCis constant over the entire Mgz+-Fe2+ and Fe2+-Mn2+ composition range,the site populations of all Fe2+ olivines may be determined from the areasof component Gaussian absorption bands in, for example, the 7 polarized

spectra.Since Fe2+ ions completely fill all available cation sites in FegSiO+,

the crystal field spectra of pure fayalite must contain contributions fromequivalent amounts of Fe2+ ions in the M(l) and M(2) positions. Nopure fayalite was available in the present study. However, curve resolu-tion of several'y spectra of specimen 10, (Fer.gzMno.ooCao.orMgo.or)SiOn,and specimen 11, (Fe1.e6Mne 6Cae.61)SiOa, gave identical and repro-ducible area data. Thus, the average areas obtained from eleven spectraof specimen 10 and six spectra of specimen 11 measured on Cary spectro-photometers at Berkeley, Cambridge and Ottawa are

band I : mean 23.5/e; range 22.5-25.07oband II : mean a+.O/6; range 42.5-45.5/sband III: mean 32.5/e; range 31.0-33.57o

Specimens 10 and 11 contain signifi.cant amounts of Ca2+ and Mn2+ions, and available evidence suggests that these ions occupy preferentiallythe M(2) positions of the olivine structure. In order to evaluate C it wasassumed that all the Ca2+ and Mn2+ ions are located in the M (2) positions

of these fayalites, leading to reduced amounts of Fe2+ ions in these posi-tions relative to the M(1) positions. Thus, the ratios ntf nzfor specimens10 and 11 are assumed to be 0.99/0.93 and 1.00/0.96, respectively.

By taking the average area of bands I and III to be proportional to theamount of Fe2+ ions in the M(1) positions, constant C was evaluated from

Ar/ A, : t@, * An)/ Ar, : Cnt/nz

This led to values of C of 0.60 and 0.61, respectively.Alternatively, on the assumption that Fe2+ ions are equally distributed

1626 ROGER G. BURNS

Teel.n 4 DrstnrsurroNs ol Fe2+ foms rw Or,rvrNns r.RoM THE Ctysrel Ftrto Slncrne

SpeciNumber

0 . 0 80 . 1 3o . 1 70.430 .540 . 7 t0 . 6 90 . 7 80 .860 .93*0 .96 .0 . 5 7o . 2 9o . 2 l

n Assumed distribution.b Includes 0.172 Zn2+.

over the M(l) and M(2) positions of each fayalite, the value of C is 0.64.On the other hand, calculations based on the widest ranges of area datafor bands I, II and III lead to values of C between 0.67 and 0.58. Takingthe relative merits of all these factors into account, the best value of C isconsidered to be 0.61.

Having obtained a value for C, the ratios qf n2 may be calculated forother Fe2+ olivines from the area data in the resolved 7 spectra. The re-sults of these calculations are summarized in Table 4. Columns 4, 5 and 6of this table list the average areas obtained from at least four resolved 7spectra of each olivine. These area data are reproducible to within fivepercent of the figures quoted. The amounts of Fe2+ ions in the M(1) andM(2) positions are given in the last two columns of Table 4.

Although the site populations in Table 4 have an accuracy of between 5and 10 percent, the data suggest that in olivines containing negligibleamounts of Mn2+ and Ca2+ ions, the Fe2+ ions are equally distributedover the M(l) and M(2) positions of the olivine structure. However, thereis a consistent trend towards slight relative Fe2+ enrichment in the M(2)positions, but in no specimen does the amount of iron in this positionexceed 51 percent of the total Fe2+ ion content. These results may becompared with recent refinements of the olivine structure which show noevidence of ordering of Mg2+ and Fe2+ ions (Birle et al., 1968; Hanke,1965). In those Mgz+-Fe2+ olivines containing appreciable amounts of

- | e " ' ln

M(2)

12

45o

789

101 11 2I J

t4

Band I Band II Band III

0 . 1 6 80.2460 . 3 2 20 .8581 . 1 5 01 . 4 0 61.4481 . 5 6 0| 7021 . 9 2 21 .9581 3 80 830o . 6 2 2

3 0 028.52 9 . 53 1 . 53 2 . 430. 53 2 . 03 2 . 030 .03 2 . 5, t z . t

3 2 . 537 .O2 t . 0

I e " ' r n

M(r)

0 . 0 80 . 1 20 . 1 50.430 6 10 . 6 80 . 7 60 . 7 80 . 8 40.99 '1 .00n0 . 8 10 . 5 2o . 4 l

SPECTRA AND ORDI'RING IN OLIYIND 1627

Mn2+ ions (specimens 6 and 7) there is a consistent trend towards slight

Fe2+ ion enrichment in M(l) positions.In the Fe2+-Mn2+ olivines there is significant enrichment of Fe2+ ions

in M(l) positions, indicating that Mn2+ ions occupy preferentially the

M(2) positions. However, the data for specimen 12, Ior example, show

that all the manganese does not enter the M(2) positions and that at

least 20 percent of the Mn2+ ions must occupy M(1) positions. These

results are in good agreement with those obtained by Gibbs and Brown

(personal communication) for the knebelite (Fer.ozsMno szaMgo.o++)SiOr,in which site populations determined from counter data using Finger's

RFINE program were estimated to be M(2):Mno.oaaFeo...l;z; M(l): Feo.oorMno.2eaMgo.o+r. No decisive conclusions can be drawn about the

site occupancy of Znz+ ions in the roepperite (specimen 13). Iron con-

tinues to show relative enrichmentrn M(l) positions as a result of Mn2+

(and Zn'+?) ion preferencefor M(2) positions, and all the zinc could be

accommodated in either or both the M(l) and M(2) positions. These re-

sults for the relative enrichment of iron in roepperite differ from those

obtained by Gibbs and Brown (personal communication) for the speci-

men (Mnr.gssMg6 3aaZno.zraFeo.rza)SiO+, in which site populations were

estimated as M(2): Mno.soaFeo .oesZno o+z and M(l): Mno.+azMgo.stsZno soFeo.o*s.

Interpretation oJ the Site Poputation Data. The average metal-oxygen dis-

tances in Mgz+-p.2+ olivines show that the M(2) site is slightly larger

than the M(1) site. Thus, size criteria would lead to the predicton that

larger cations will favor the M (2) positions relative to the M(1) positions(Ghose, 1962; Burns,l97Aa). As a result, 1u1nz+ (ionic radius 0.82 A,

Shannon and Prewi t t , 1969) and 6 'ut+ (1.00 A) ions are expected to enter

M(2) sites pref erentially and lead to Mgz+ (0.72 A) and Fe2+ (0.77 A) ion

enrichment in M(1) positions. The data in Table 4 for specimens 5 and 7

and the manganilerous olivines (specimens 12, 13 and 14) are in accord

with this prediction. Analogous cation distributions based on size criteria

have been found in other olivine-type structures (Newnham, Santoro,

Pearson and Jansen, 1964; Santoro and Newnham, 1967)'

On this basis Fe2+ ions might be expected to show a pronounced en-

richment over M92+ ions in M(2) positions of the f orsterite-fayalite series'

I{owever, the recent refinements to the crystal structure (Birle et al.,

1968; Hanke, 1965) and the data in table 4 indicate that the occupancy of

Fe2+ ions in the M(l) and M(2) positions approaches a random distri-

bution, with only a very small tendency towards reiative enrichment in

M(2) postions. The absence of a distinct site preference for Fe2+ ions in

the olivine structure compared to other silicates may be explained by

crystal f ield theory. (Burns, 1970b).

1628 ROGER G. BURNS

The ground-state electronic configuration of the Fe2+ ion in octahedralcoordination corresponds to five electrons occupying singly each of thefive 3d orbitals and the sixth 3d electron f.lling one of the low energyorbitals of the l2s group. This sixth electron induces a crystal fieldstabilization energy of fA" on the Fe2+ ion in octahedral coordination.The crystal field splitting parameter A is inversely proportional to thefifth power of the metal-oxygen distance, suggesting that

Au <z; I Atr o':

and that the stabilization energy of Fe2+ in the larger M(2) site is smallerthan that in the smaller M(l) site. Therefore, a simple crystal field cri-terion would lead to the prediction that Fe2+ ions (and other transitionmetal ions acquiring crystal field stabilization energy) would be enrichedin the smaller M(l) site (Burns, 1970a).

However, distortion of the M(1) and M(2) sites from octahedral sym-metry leads to the stabilization of one or two of the t2o orbitals (Figure5b&c). By filling one of these low energy orbitals the sixth 3d electron ofFez+ induces an additional crystal field stabilization energy on the cation.Although the nature of the distortions of the M(l) and M(2) coordina-tion sites difier in detail (Figure 1), the energy level diagrams shown inFigure 5 suggest and calculations show (Burns, lg70b, p.83) that theresolution of 3d orbitals of iron and the overall stabilization energy maybe comparable in the two coordination sites of forsteritic olivines.

rt is noteworthy that the olivine structure differs from those of otherferromagnesian silicates by having two moderately distorted six-coor-dinate sites. The ordering of Fe2+ ions found in pyroxene and amphibolestructures may be correlated with the preference of iron for the mostdistorted octahedral sites in these structures (Burns. 1968).

CoNcrusroNs

The present paper illustrates further applications of polarized absorp-tion spectral measurements to crystal chemical problems of silicate min-erals' First, the compositional variations of absorption bands form thebasis of another olivine composition determinative curve, from which theFezSiOr content of unanalysed olivines of the forsterite-fayalite seriesmay be estimated. The maxima of all absorption bands in the polarizedspectra move to longer wavelengths with increasing Fe2SiOa componentin the olivine. rrowever, since Mn2+ ions also produce shifts of absorptionmaxima to longer wavelengths, the determinative curves are best suitedto olivines containing negligible manganese.

second, contributionsfrom Fe2+ ions in each of the six-coordinate posi-

SPECTRA AND ORDERING IN OLIVINE 1629

tions may be distinguished in the crystal fi.eld spectra of olivines. There-fore, areas under absorption bands can be used to estimate Fe2+ sitepopulations in the olivine structure. In the forsterite-fayalite series, Fe2+ions are almost equally distributed over the M(I) and rl1(2) positions ofthe olivine structure, but there is a suggestion that Fe2+ ions show veryslight relative enrichment in M(2) positions. In manganiferous olivines,Fe2+ ions are concentrated in .41(1) positions through the preference ofthe larger Mn2+ ions for the larger M(2) coordination site. As a result,small amounts of Mn2+ (and Caz+) ions in Mgz+-psr+ olivines Iead toslight relative enrichment of Fe2+ ions in M(1) positions instead of M(2)positions.

Third, energy level diagrams for each Fe2+ ion in the olivine structuremay be calculated from the positions of absorption bands. The sitepopulations of the Fe2+ ion in the olivine structure may be interpreted bycrystal field theory.

Fourth, measurements of polarized absorption spectra enable the con-cepts of color and pleochroism to be defined more rigorously. The ap-pearance pf absorption bands outside or on the fringes of the visible re-gion produce pale green or yellow colors in Mgz+-p"z+ olivines. How-ever, the polarized spectra show that olivines are strongly pleochroic, butthe effect is not seen under the microscope because radiation in the nearinfrared is affected. In tephroites, even though absorption bands occur inthe visible region, the very low intensity of the bands and small differ-ences in the polarized spectra lead to the absence of color and pleo-chroism in these minerals.

Therefore, the FezSiOq content, Fe2+ site populations, electronic energylevels and pleochroic scheme may be determined from one set of polarizedabsorption spectra of an olivine mineral.

Norr Annro rm Pnool

After this manuscript was submitted, other papers have been published which describeoptical spectral measurements of olivines at elevated temperatures and pressures. Theseinclude:AnoNsor, J. R., L. H. Ber,r,orrr, S. W. Ecrnoan, R. K. McCoNNET,L) AND P. C. VoN

TniiNr' (1970) Infrared spectra and radiative thermal conductivity of minerals at hightemperatures. "I . G eo p hy s. Res., 7 5, 3 M3-3 456.

Prrr, G., enn D. C. Toznn (1970) Optical absorption measurements on natural andsyntlretic ferromagnesian minerals subjected to high pressures. Phys. Darth Pl,anet.Interiors,2, 179-188.

AcKNomEDGT&]NTs

The spectral measurements were commenced at Berkeley, continued at Cambridge andcompleted at Ottawa when the author was a visiting research scientist at the Mines Branch,Department of Energy, Mines and Resources, during the summer of 1969. At Berkeley,

1630 ROGER G, BURNS

thanl<s are due to Professors W. S. Fyfe and A. Pabst for helpful advice and discussronI wish to thank Professor W. A. Deer and Professor Lord Todd for making facilities avail-able to me in the Deaprtment of Mineralogy and Petrology and University ChemicalLaboratories, respectively, at Cambridge. I appreciate the invitation, facilities and helpfulsuggestions ofiered to me by Dr. E. H. Nickel at Ottawa. Dr. D. C. Harris generously per-formed electron microprobe measurements on some of the specimens. Mr. G. H. Faye,Profesor G. V. Gibbs, and Dr. G. E. Brown kindly read the draft manuscript and ofieredmany valuable criticisms and unpublished data. I wish to thank several mineralogists forofiering me olivine specimens for this and other studies. Professor A. Pabst, Dr. S. O. Agrell,and Professor E. A. Vincent made available specimens and petrographic thin sections fromcollections and Berkeley, Cambridge and Oxford, while Dr. B. Mason, Professor C. S.Hurlbut, Jr., and Professor Th. G. Sahama donated various rare and unusual olivines.

Rrrrner.rcrs

Blr,cAN, A S , .Lxo H. G Dmcr.a.mn (1959) Efiect of pressure on the spectra of olivine andgarnet. "I. Appl. Phys. 30, tM6 lM7.

BeNcnonr, G. M., R. G. BunNs, aNo R. A. Howrr (1967) Determination of the cation dis-tribution in the orthopyroxene series b1' the Mdssbauer efiect. Natule,2l3,122l-1223.

lNo A. G. Meonocx (1967a) Determination of cation distribution in thecummingtonite-grunerite series by Mtjssbauer spectra. Amer. MinuoJ 52, 1009-1026.

(1967b) Applications of the Mijssbauer efiect to silicate min-eralogy. Part 1. Iron silicates of known crystal structures. Geochim. Cosmochim. Acta,3r,2219-2246.

Brlov, N. V., E. N. BEr.ovA, N. H. ArvnnreNovA, AND P. F. SurnNov.q (1951) Determina-

tion of the parameters in the olivine (forsterite) structure with the harmonic three-

dimensional synthesis. Dokl. Akad. Nauk SSSR, 81, 399-402.Brnln, J. D., G. V. Grres, P. B. Mooro, AND J. V. Sutrrr (1968) Crystai structures of natu-

ral olivines. Arner. Minual.53, 807-824.Bn,rcc, W. L., eNo G B. Bnowr.r (1926) Die Struktur des Olivins. Z. Kristal,l 'ogr.63,

538-556.BnowN, G. 8., awo J. Wrsr (1927) The structure of monticellite (MgCaSiOr). Z. Kri,stal-

I,ogr.66, 15+-161.Btrnus, R. G. (1965) Electronic spectra of silicate minerals. Applications of crystal field

theory to aspects of geochemistry. Ph.D. Dissertation Univ. Calif ., Berkeley, Calif.-- (1966) Apparatus for measuring polarized absorption spectra of small crystals. .I.

Sci. I nslrum .43, 58-60.-- (1968) Crystal field phenomena and iron enrichments in pyroxenes and amphiboles.

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Mineral Assoc., 6th Gen. Meeting, Prague. Chem. Geol.5r 275-283.- (1970b) Minnalogi'ed Appli'cat'ions oJ Crystal Fi.eld, Theory. Cambridge University

Press, England.--- AND R. G. J. SrnuNs (1967) Structural interpretation of polarized absorption

spectra of Al-Fe-Mn-Cr epidotes. M i.neraJ. M og. 36, 204-226.

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SPECTRA AND ORDERIN(; IN OLIVINE 1631

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Emscniirz, M , eNo G,tNmr., U. (1967) Mcissbauer studies of Fe2+ in paramagnetic fayalite.Soli.d. State Commln. 5, 267 .

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Manuscript receiaed., February 2, 1970; Mcepted, Jor publicati.on, April 2, 1970.