American Mineralogist, Volume 62, pages 205-216, l9Z7

The identificationof Fe2+in the M(4) siteof calcicamphibores

DoN S. GoloueN e.NoGroncn R. RosslleN Diuision of Geological and planetary Sciences Califurnia Institute of Technology Pasadena,Califurnia 9 I I 25

Abstract

Evidenceis presentedfor the occurrenceof Fe2+in the predominantlyCa-filled M(4) siteof calcicamphiboles. Absorption bands in theelectronic spectra at 1030nm in p andat2470 in a providea sensitivemeans for identifyingFe2+ in thissite. The large e valuefor the 1030band, the largeenergy separation between both bands,and the similarityto the spectraof Fe2+in large,highly distortedcoordination sites in other mineralsprovide the basisfor theM(4) site assignment.The energysplittings and polarizationanisotropy of thesebands are analyzed, using a point-chargemodel basedon Cru symmetry.Spectral data indicatethat the Fe,+ contentin the M(4) site variessignificantly among the calcicamphiboles.

Introduction Fe2+in tetrahedralcoordination, also in a spectrum .Little is known about the Fe2+content of the M(4) of actinolite.Both ofthese bands occur in the spectra si{eof calcicamphiboles. M(4) Fe2+has beendifficult of the orthorhombic of the anthophyl; to identify becausecalcium generallyoccupies 85-95 lite-gedriteseries, (M g,Fe,Al),(Si,Al).O,,(OH)z; how- (1974) percent of this site (Leake, 1968);therefore, only a ever, Mao and Seifert assignedboth bands small percentageof the total in most samplescan to Fe2+in the M(4) site. occupy the M(4) site. Hence, it is difficult to analyze The high degreeof distortion of the M(4) site is for M(4) Fe2+in the complex Mdssbauerspectra, and expectedto producelarge -field splittings anal- to detectsmall quantitiesof Fe2+in a predominantly ogousto the spectraof Fe2+in the distortedM(2) site Ca-filled M(4) site by crystallographicanalysis. The of orthopyroxene, (Mg,Fe)rSirO.. Burns (1970), purpose of this paper is to identify Fe2+in the M(4) Runcimanet al.,(1973),and Goldman and Rossman site with electronic absorption spectra and to show (1976, 1977) studied a variety of orthopyroxenesin that this techniqueis sensitiveto small concentrations which bands at about 930 nm and 1850nm were of Fe2+in this site. assignedto transitionsto excitedstates derived from uE, The electronicabsorption spectra of the calcicam- the splitting of the octahedral state.These inter- phiboles consist of a superposition of absorption pretations provide a basis for the analysis of the bands that arise from ferrous iron in several sites of calcic spectra.A third absorption band at different coordination geometries.Amphiboles con- 2350cm-' was identifiedin orthopyroxene(Goldman tain three sites M(l), M(2), and M(3) that more and Rossman, 1976) and assignedto a transition uT"r(On) closely resembleregular octahedra than the fourth within the split stateof Fe2+in the M(2) site. site,M(4), which is highly distorted from octahedral We have examinedpolarized spectrato seeif a sim- geometry. Fe2+in the M(4) site of calcic amphiboles ilar band arising from Fe2+inthe M(4) site occursin has not been previously identified in absorption the 1200-4000cm-' region. spectra.Burns (1970)presented absorption spectra of two actinolites, Car(Mg,Fe)uSirOrr(OH)r, in the Sample description 350-1500nm region in which all Fer+ featureswere Tremolite: Mt. Bity, Malagasy Republic (Lacroix, attributed to Fe2+in the M(l), M(2), and M(3) sites. l9l0); CIT #8038;a2 cm prismatic,euhedral crystal It can be inferred from this discussion and from that is green,transparent, and free of inclusions.Mi- Burns (1965)that the main band at about 1035nm croprobe analysesof various chips from the crystal arisesfrom Fe2+in the M(2) site. White and Keester show a range of iron content from 1.6 to 2.8 weight (1966)assigned a band in the 2300-2500nm regionto percent FeO, but the microprobe analysis of the 205 GOLDMAN AND ROSSMAN CALCIC AMPHIBOLES sampleused for opticalspectra indicated 2.06 percent Test-e l. Microprobe analYses FeO (Table l) and the wet chemicalanalysis of the Mdssbauersample indicated 2.09 percent FeO. Weig percent of oxides Si02 57,65 57.01 42,9L Actinolite:Zillerthal, Tyrol, Austria; CIT .88 #4916; Ti0 2 .07 1.06 12.69 dark green,prismatic, euhedral laths up to l5 mm A120 3 1,31 Cr203 .24 long and up to 4 mm acrossthe basalsection in a Mco 23.62 2L.36 14.44 MnO ,22 .09 talc-mica schist.The absorptionspectra were taken I'eO 2.06 5.72 10.25 in areason the orientedslabs that were free of mi- Ca0 L3.42 12,30 13.00 Na20 .6r .4L caceousinclusions common to thesesamples. The Kzo .20 .05 r.57 F .2L .06 !.66 microprobeanalysis (Table l) is similarto thosere- c1 .04 in Leake 99.L9 98.43 99.94 ported for actinolitesfrom this locality ,09 .03 .70 (1e68). .01 99.O9 98.40 99.24 Pargasite:Pargas, Finland; CIT #2573;large crys- Formul a Proportionsx crystalsare black, Si 7.8r 7.87 6.28 tal clustersin whichthe individual A1(rV) to .13 t,72 along(010), and haveother well-developed (vr) .04'l .t+11 tabular A1 ,.021 -- | +. t | 4.40 | :.rs crystalfaces. These occur in marbleand com- | I Ti .01 | -l .10 5.16 | monly havecalcite inclusions. The spectrawere ob- Cr - | ''"' ne F -l I Mn .03 | .01| tained in inclusion-freeareas. The compositionof .23J .66J L.26J this samplediffers from thosereported for other par- L.95 1R? 2,03 Na .11 .69 gasitesfrom this localityin Leake(1968) in having .01 higherFe and lower F and Al. Theseminerals did not showchemical zonation in 3. Pargaslte, Pargas, Finland the areain which the opticalspectra were taken on *The formula proportlons are obtained by nomallztng the total- posltive io 46 assuming all Fe ls Fe2*' the (100)slabs. "t..i. Experimentalmethods All spectraldata presentedin this paperhave been the two components of each doublet were con- obtained at room temperatureon self-supportingstrained to be equal, although the half-widths of the (010)and (100) slabs. The method of preparationand different doublets were allowed to vary. Due to the data analysishave been described(Goldman and small percentageof Fe8+in this sample,the position Rossman.1977). of peak C was fixed, the areas of C and C' were The M

oxygensand that all Fe is Fe2+.Mdssbauer data (Papikeet al., 1969)with the principaldistortions presentedin this paperfor the Mt. Bity sampleand being due to (l) an elongationof the Fer+-O(6) the analysesgiven in Leake(1968) for actinolites bondswith a reductionin the O(6)-Fc,+-O(6)bond from the Zillerthal area indicate that less than l0 angle;(2) a twistingof the 0(6)-0(6) bond aboutb percentof the total Fe is Fe3+.Using an Fes+/Fe relativeto the O(2)-O(2)bond; (3) a relativecom- ratio of 0.l0 doesnot changethese formula propor- pressionof the Fe'+-O(4)bonds; and (4) a removal tions.The formulaproportions for thePargas sample of the centorof symmetry.The grune'ite M(4) site havebeen obtained similarly. Changing the Fes+/Fe will be used as the structuralmodel to analyzethe ratio up to 0.30 does not appreciablychange the bandsdue to Fe2+in theM(4) siteof actinolite.This resultingformula proportions. The sum of AlvI, Fe. assumesthat Fe2+occupies a "gruneriteposition" in Mg, Ti, and Mn is closeto 5.0,and the sumof Ca,K, the actinoliteM(4) site. and Na is approximately3.0 with Fe8+/Feratios of lessthan 0.30. Descriptionof spectra The spectraof the Mt. Bity tremoliteare presented Crystalstructure in Figure3 to illustratethe featuresthat characterize Calcic amphibolesare monoclinic with space most calcic amphibolespectra, These spectra will group Cr,*. The M(l), M(2), and M(4) siteshave serveas a basisfor theidentification, assignment, and point-group symmetry Cr, and the M(3) site has crystal-fieldanalysis of the absorptionbands due to point-groupsymmetry Crx. Fe2+in theM(4) site.Many of the featuresshown in The structuraldifferences between the M(l), M(2), and M(3) sites and the M(4) site suggestthat the spectroscopicfeatures of ferrous iron in thesesites csinP c will markedlydiffer. The (c*-6) and (a-c) Onrrp az"i (Johnson,1965) projections of the M(l), M(2), and M(3) sitesof actinoliteare presentedin Figure l. 2 '3a/ Thesesites approach octahedral geometry, with sim- / ilar averagemetal-oxygen (M-O) bond distancesof 2.105,2.098, and 2.098A, respectively(Mitchell el M(t) al., l97l). In comparison, X/ 3(, the M(4) siteof actinolite 2 shownin Figure2 is highly distortedand is eight- coordinatedwhen occupied by . I Ideally,absorption spectra would be analyzedus- ing the specificbond anglesand bond lengthsof a particularmetal in a coordinationpolyhedron. For a metal ion in a site that is occupiedpre- dominantlyby a differentmetal ion, thesestructural M(2) detailsare not known,as is the casefor Fe2+in the M(4) site of actinolite.The projectionsof the acti- nolite M(4) site relateprimarily the position of cal- cium.The C, point-groupdoes not constrainFe2+ to occupythe calciumposition; the only constraintis 3 that it must,on the average,occupy a positionon the two-fold rotationaxis parallel to D.The positionof Fe2+in the M(4) siteof monoclinicamphiboles has M(3) beendetermined in grunerite,FerSi'Orr(OH), (Fin- ger, 1969).A comparisonof the two sites(Fig. 2) indicatesthat Fe2+is displacedon the two-foldaxis 1{ towardthe O(2) oxygensrelative to thecalcium posi- ,tA tion. An importantconsequence of this displacement is the increaseof the Fe,+-O(5)bond distancesto FIc. l. (c*-b) and (a-c) Onrrr projectionsof the M(l), M(2), and M(3\ nearly 3.3 A. For this reason, coordination sites in actinolite based on the atomic the gruneritesite is coordinates given in Mitchell et al. (1971). All figures have been consideredas a highly distorted,six-coordinate site drawn to the same scale. 208 GOLDMAN AND ROSSMAN: CALCIC AMPHIBOLES

crucial for the site assignmentto be discussedin the nextsection. In addition to the featuresdiscussed in previous investigations,the sharppeaks of low intensityin the 400-550nm regionin Figure3 are identifiedas spin- forbidden,electronic transitions of Fe2+and Fe8+. Sharp bands at 2297nm and 2384nm in B and at 2315nm and 2387nm in 7 correspondto the loca- tions of the combinationbands described by White and Keester.

ACTINOLITEM(4) SITE Fe2+in M(4) The prominentabsorption bands at 1030nm in B and at 2470ind areassigned to transitionsof Fe2+in c sinB the M(4) site. The evidencesupporting this assign- l_o mentis derivedfrom (l) the similarityto the spectra of Fe2+in the M(4) sitesof the Mg-Fe amphiboles andin theM(2) sitesof ortho-and clinopyroxene, (2) the intensitycorrelation between these bands, (3) the barycenterenergy of the two bands,and (4) the in- tensityof the 1030nm band.

Similarity of specfta the 1030nm and 2470nm bands . tA The locationsof aresimilar to thosereported for otherlarge, distorted GRUNERITEM(4) SITE sites.The spectraof Fe'+ in the M(4) sitesof cum- (Burns, 1970)show a domi- Frc.2. (c*-b) and (a-c) ORrEP projections of the M(4) coordina- mingtonite-grunerites tion sites in actinolite and grunerite based on the atomic coordi- nant absorptionband in the 1000nm region.In the natesgiven in Mitchell et al. (1971) and Finger (1969), respectively. spectraof anthophylliteand gedrite, Mao andSeifert These projections illustrate the different positioning of iron in the (1973)assigned bands near 1000nm and 2500nm to All figures are grunerite site from calcium in the actinolite site. Fe2+in theM(4) site.Fe2+ in the distortedM(2) sites drawn to the same scale. of orthopyroxene(Burns, 1970;Runciman et al., 1973;Goldman and Rossman,1976, 1977) and Figure 3 have been described and assigned in pre- clinopyroxene(Bell and Mao, 1972;Burns et al., vious studies. 1972)also have bands in the 1000nm and 1800-2500 White and Keester (1966) presentedan unpola- nm regions.Therefore, it is apparentthat theselarge, rized spectrum of an actinolite and assignedan ab- distorted sites produce a crystal-fieldsplitting of sorption band at 1020nm to Fe2+in six-fold coordi- about 5500cm-' which resultsin one band in the nation, a sharp band at 1399 nm to the first 1000nm regionand the otherin the 2000nm region. vibrational overtone of the OH- stretching mode, Absorptionbands due to Fe2+in the smallerpyrox- and a set of sharp bands at 2320nm and 2392 nm to eneM(l) sitesdo not occurabove 1200 nm. If it can infrared combination modes. Burns (1970)presented be shownthat thesetwo bandsarise from the same the spectraof two actinolitesand assignedabsorption Fe2+ion, then theseobservations support an M(4) p"z+/pes+ bandsand bands at 727 nm in 7 and at 661 nm in B to site origin for the two calcicamphibole intervalence charge-transfer(IVCT) and described excludetheir origin from the M(l), M(2), andM(3) four sharp absorption bands in d at about 1400nm sites. from OH- vibrational overtones. Burns arising Band correlations attributed the absorptionsin the 800-1300nm region in to Fez+in the M(l), M(2), and M(3) sites.The band Sectionsof the Mt. Bity tremolitewere heated assignedby White and Keesterto tetrahedralFe2+ is air at 535'Cfor 8 hours.After thesample was heated, p its locatedin a at approximately2470 nm (4050cm-') in the 1030nm bandin retainedonly 50 percentof is ob- Figure 3. The correct interpretation of this band is unheatedintensity (Figure 4). A similarvalue GOLDMAN AND ROSSMAN: CALCIC AMPHIB}LES 209 hRVENUMBEB(cM-l ) 20000 10000 8000 s000 u000 t.u

tv) TREMOLITE UJ 1.5 LJ z. .G co ln I CC !l O $ ii , N v) t m 0.5 ii Cf il \

nn

500 1000 1s00 2000 2500 hFVELENGTH(NM)

FIc' 3' Room temperature spectra of tremolite from Mt. Bity, Malagasy Republic. a spectrum (. . . ), B spectrum G---), and r spectrum(-). aAc : 17". Crystal thickness: 1.0 mm.

tained for the 2470nm band, although the vibrational simultaneouslypresent or absent,and hence,they absorptions superimposedupon this band preclude mustarise from the sameFe2+ ion. an exactdetermination of the reduction factor. These data suggestthat both the 1030 nm and 2470 nm Barycenterenergy bands arise from the same Fe2+ion. The M(2) Fe2+absorption bands at about930 nm The spectra of two additional calcic amphiboles and 1850nm in orthopyroxeneare derived from the have been obtained to verify that both bands origi_ splittingof theuEE(On)state due to thelow symmetry nate from the sameFe2+ ion. Spectraof an actinolite of this site.Based upon the interpretationof Run- are presented in Figure 5 in which the 1030and2470 ciman et al., Mao and Seifert concludedthat the nm bands are approximately 3.1 times as intenseas similarbands in anthophylliteand gedritewere due the analogous bands in Figure 3. Stoichiometrycon_ to the splittingof this state.The resultsof a studyby siderations for this sample (Table 2) indicate that Faye(1972) support these interpretations, and hence, there is an excessof AlvI, Mg, Cr, Mn, and Fe that supportthe assignmentof the 1030nm and2470 nm cannot be accommodatedin the five M(l), M(2), and bands as electronictransitions to the separated uEr(On) M(3) sitesin the half-unit cell. Hence,the excessmusr componentsof Fe2+in the M(4) site.Faye be accommodated in M(4), which has only l.g2 correlatedthe barycenterenergy of the two com- of 2.0 possiblesites occupiedby Ca. In addition, ponentswith the averageM-O bond length of six- there is not enough Ca and Na to completely fill coordinatesites in a variety of minerals.While the M(4) site. Furthermore, the spectraof pargasite, slightlymodifying the average M-O bonddistance to NaCar(Mg,Fe'+ )n(Al,Fe3+)Si6AlrOrr(OH,F,Cl)r, (Fig. 6) accountfor the effectof the substitutionof other indicatethat the absenceof the 1030nm band is cor- metalions into eachsite, it wasfound that the bary- related with the absenceof the 24i0 nm band. Re- centerenergy decreased linearly with increasingav- gardless of the Fe3+/Fe2+ ratio assumed for this erageM-O bond length.Using the averagefor the six sample, the resulting formulas from the microprobe M-O bond lengthsin the gruneiteM(4) siteof 2.29 analysis indicate that the M(4) site is fully occupied A (Finger,1969), a barycenterenergy of 6880cm-r is by Ca and that the sum of the remaining octahedral predictedfrom Faye'scorrelation. This is the same cations is approximately 5.0. In summary, these valuethat is obtainedfrom the tremolitespectra in spectra indicatethat both absorptionbands are either Figure3. For comparison,the smallerM(2) sitein 2to GOLDMAN AND ROSSMAN: CALCIC AMPHIBOLES hRVENUMBER(cM-l ) 20000 10000 8000 5000 u000 2.5

2.O TREMOLITE lrl CJ z tq cf CN CC cl a 1.0 m cf 0.5

o 0.0 500 I 000 1500 2000 2500 NFVELENGTH(NM )

Frc.4. Spectratremolite from Mt Bity, Malagasy Republic, before heating (-) and after heating in air at 535oC for 8 hours G------).The bands at 1030nm in 0 and 24'70nm in a are reducedto nearly halfoftheir original intensityafter heatingwhich suggestsa common Fe'z+origin. Crystal thickness = 1.0 mm. The B-spectra have been displaced vertically for clarity.

the orthopyroxene bronzite, having an averageM-O I nte nsit y conside rations bond distanceof 2.22 A, has a barycenterenergy of about 8160 cm-1. In addition, Faye's correlation ar- The intensificationof absorptionfeatures in dis- gues against the possibility that the 2470 nm band, torted siteslacking centers of symmetryis a well- and hence,the 1030nm band, arisefrom Fe2+in the known phenomenon(Burns, 1970; White and Kees- M(l), M(2), or M(3) sites.The predictedbarycenter ter,1967).The greaterintensity ofthe 1030nm band energiesof absorption bandsarising from thesesites, in B, in comparisonwith other Fe2+features in the having averageM-O bond lengthsof about 2.1 A, ate 800-1300nm region,provides an indicationthat this about 10,000cm-t, which precludesthe occurrenceof band is due to Fez+in a non-centrosymmetricsite. one component at 2500nm (4000cm-'). The remain- Although the M(l) and M(2) siteslack centersof ing Fe2+bands in the near-infraredregion occur be- symmetry,they are not nearlyas distortedor non- tween 850nm and 1150nm. Taking theselimits asthe centrosymmetricas the M(4) site.Therefore Fe2+ in maximum-extent of the 6E, splitting for Fe2+in the theM(4) siteis the mostlikely candidate for the 1030 M(l), M(2), or M(3) sites, a barycenter energy of nm band. This assignment,indicated by the band 10,200cm-' is obtained.The agreementwith Faye's positions,can be evaluatedby comparingthe absolute correlation supports their assignmentto Fe2+in the intensityof the 1030nm band(expressed in termsof M(l), M(2), or M(3) sites.It is concludedthat both molar absorptivity,eroro) with e valuesfrom sitesof the 1030nm and the2470 nm absorptionbands in the variouscoordination geometries. Although an accu- spectra of calcic amphiboles arise from Fe2+in the rate valuefor e,oro cannot be determinedbecause the M(4) site and that these bands result from the split- Fe2+contents of varioussites are not known,electron ting of the uEr(O) state in the low-symmetry envi- microprobeand Mdssbaueranalyses of the Mt. Bity ronment. tremoliteprovide constraints to derivea lowerlimit. GOLDMAN AND ROSSMAN' CALCIC AMPHIBOLES 2ll TdFVENUMBER(cM-t ) 10000 8000 s000 q000

5.0 ACTINOLITE Ulrr=." n LJ z. E r.o t 3 r.o CD c t.o

0.0 500 1000 1s00 2000 2500 NFVELENGTH(NM ) FIc. 5.Room temperature spectra of actinolitefrom Zillerthal, Tyrol, Austria. a spectrum(. .),B spectrum(------), and 7 spectrum (-). 7r'c : 16o.Crystal thiekness : 1.0 mm.

An absolute lower limit for e,oroof l6 is obtained bauer spectrum (below) indicate that some iron is from the formula, assumingall iron is Fe2* and that Fe3+.If the 1030nm band is dueto Fe2+in M(4),the all of it contributesto the band. However, the pres- presenceof Fe3+in this sampleis suggestedin the t0.5 ence of ps2+/Fs3+IVCT bands in 7 and B in the mm,/secregion, although a distinct peak is not ob- 600-750 nm region, as well as aspectsof the M6ss- served.

hFVENUMBER(cM-l ) 20000 10000 8000 5000 q000 2.O

I I \l PARGASITE uJ 1.5 (-) Z. cr co 1.0 E. O U) o o.s cf

0.0 500 1000 1s00 2000 hFVELENGTH(NM )

FIc. 6 Room temperaturespectra of pargasitefrom Pargas,Finland. a spectrum ( .), B spectrum (------), and 7 spectrum (-) 1Ac - 24". Crystal thickness : 010 mm 2t2 GOLDMAN AND ROSSMAN CALCIC AMPHIBOLES

The M6ssbauer data for the Mt. Bity tremolite et al.,1967b; Hafner and Ghose, 1971).These spectra provide further constraintson the lower limit of e1s36. are characterizedby two distinct quadrupole dou- The room temperature Mdssbauer spectrum of this blets:the outer doublet has a quadrupolesplitting of sample(Fig. 7) showstwo distinct peaksin the high- about 2.7-2.8 mm,/secand is assignedto Fe2+in the velocity region(2.0-2.5 mm/sec) that are attributable M(l), M(2), and M(3) sites;the inner doublet has a to Fe2+ in at least two distinct environments.The quadrupolesplitting of 1.5-1.8 mmlsec and is as- presenceof Fe3+in this sampleis suggestedin the * signedto Fe2+in the M(4) site. 0.5 mm,/secregion, although a distinct peak is not Making the reasonableassumption that the band observed. at 1030nm is due to Fe2+in one site only, the exis- The Mdssbauerstudies of the calcic amphibolesby tence of two doublets in the spectrum of Mt. Bity Bancroft et al. (1967c), Hiiggstrdm et al. (1969), tremolite establishesthat e,o3o must be greater than Burns and Greaves(197 1), and Bancroftand Brown 16,i.e. at least27 if the band is due to the site giving (1975)have assumed that Ca, Na, and K completely riseto peaksAA' , and at least40 if it is due to the site filf the M(4) site, and hence, peak assignmentshave giving riseto BB' .lf the Mdssbauerpeaks in question beenmade only for Fe'+ in the M(l), M(2), andM(3) representmore than one site, the value of e,oro could sites,with the peakshaving the smallestquadrupole only be higher. spfitting assignedto Fe2+inlhe M(2) site. Fe'+ in the Thus, if we assignAA' to M(l) andM(3), andBB' M(4) site has been identified in the Mdssbauer to M(2) in accordancewith previous work on calcic spectra of cummingtonitesand grunerites(Bancroft amphiboles,we obtain minimum valuesof e,o3o equal

VELOCITY (MM/sEC)

3.tl

3.to a F Z. 3.O9 ol C) 3.OB I o a 3.O7 z a 3.06 J J 3.05

3.04 TREMOLITE

Frc. 7 Mdssbauerspectrum of tremolite,Mt. Bity, MalagasyRepublic, taken at room temperature The spectrumis presentedrelative to Feo. GOLDMAN AND ROSSMAN CALCIC AMPHIBOLES 2t3 to ( 1)40 if the bandis dueto M(2); (2) 27 if it is due havea vibrationalorigin. Another band due to M(4) to M(l) or M(3), and(3) verylarge (well over 40) if it Fe2+does not occurin thisregion above 2000 cm-'. is due to M(4).If we assignpeaks BB' to M(4) and Both the 1030nm bandin 0 andthe 2470bandinq. AA' to M(l), M(2), and M(3), we obtain minimum in the spectraof pargasiteare absent. The absorption valuesfor e,oroof (l) 40 if the band is due to M(4), band in a at 1030nm is also absentin Figure 6, and(b) 27 if it is dueto M(l), M(2), or M(3). althoughit is presentin both the tremoliteand acti- The e valuesfor Fe2+absorption bands in other nolitespectra. Furthermore, this band was reduced in sites(Table 3) canbe comparedwith the lowerlimit intensityin the heat-treatedtremolite spectra. To ex- determinedfor ero3o.The smallersites such as the aminethe possibilitythat this band may be due to M( I ) sitesin olivineand orthopyroxene have e values Fe2+in the M(4) site,the intensitiesfor the a and0 of lessthan 10,even though these sites have different bandsat 1030nm werecompared for fourteencalcic distortionsand departuresfrom centrosymmetry. amphibolesamples (Fig. 9). The trendof thesedata The larger sitesin the pyroxenesand amphiboles suggeststhat the a andp bandsare correlated.Con- havelarger e values.Based on this comparison,an e sequently,the absorptionband at 1030nm in a is value for the 1030nm band of greaterthan 27 is assignedto M(4) Fe2+.The possibilitythat the a consistentwith an assignmentto Fe2+in Ihe M(4) band resultsfrom the mixingof intensitiesfrom the site, but'probably incon$istentwith assignmentto intense0 peakdue to experimentalproblems does not any othersite. appearlikely, because there is littleevidence for mix- ing of polarizationintensities in the 2000-2500nm Otherconsideralions region. Due to similar typesof distortionsof the calcic amphiboleM@) siti andthe orthopyroxene M(2) site crystal-fieldanalysis involvingelongation of two adjacentM-O bonds, The crystallographicpoint-group symmetry of the and due to the similarcrystal-field splittings of the calcicamphibole M(4) site is C2 with the two-fold uEr(On)state for theseminerals, the spectraof the rotation axis parallel to b which bisects the Mt. Bity tremolitewere obtainedin the 1200-4200 O(6)-M(4)-O(6)and O(2)-M(4)-O(2)bond angles. cm-' region (Fig. 8) to searchfor a low-energyab- For spectroscopicanalysis, the Z crystal-fieldaxis is sorptionband analogousto the 2350cm-r band in taken along D to coincidewith the axis of highest orthopyroxene.The absorption bands in the symmetryof the site. The relativeordering of the 3600-3750cm-' region arisefrom OH- stretching crystal-fieldstates and the polarizationproperties of motions,and the absorptionsbelow 2000 cm-1 also the allowedtransitions for Cz,in whichZ is a dihe-

Tenlr 3. r valuesfor common minerals

BAND (cm c([/nole cm) REFEREITCE* oLrvrNE l{(1) 10,930 2.4 (r),{1},[2],t31 (FAYALTTE) !4(2) 9,290 8.6 oRTHOPYROXENE M(2) 10,930 4I (4),{4},Is] (BRONZITE) oRTHOPYROXEIIE M(1) 8,560 tu10 (1),{6}, tsl (ORTHOFERROSILITE) Mg-Fe AMPHIBOLE M(4) a10,600 N20 (7), {8},181 (GEDR]TE) Mg-Fe AI,IPHIBOLE M(4) 9,980 ru80 (1),ie], tel (GRI]NERITE)

GARIIET 8-fo1d 7,830 (10),i10] (PYROPE-ALMANDINE) COR.DIERITE 6-fold 8,200 (11),{11},[11] 1. Burns (1970) 7. Mao and Seifert (1974) 2. Bush et al. (1970) 8. Papike and Ross (1970) 3. Birle et al. (1968) 9. Bancroft, Burns and Maddock (1967) 4. Goldmaa and Rossnan (1976b) 10. White and Moore (1972) 5. Bancroft, Burns and Howie (1967) 11. Goldman and Rossman (in preparation) 6. Kuno (1954) * ( ) = SDecrral DAta t I = Site poDillarion Data { } =Chemical Data 2t4 GOLDMAN AND ROSSMAN: CALCIC AMPHIBOLES hFVELENGTH(NM ) 3000 u000 s000 2.O

U-J 1.5 (-) z. ct m 1.0 E. O m 0.5 C

0.0 rJ000 3500 3000 2500 2000 1500 hFVENUMBEB(cM-l )

Frc. 8. Room temperature, mid-infrared spectra of tremolite from Mt. Bity, Malagasy Republic, showing that a second M(4) Fe'+ r. 'yAc = absorption band does not occur in this region above 2000 cm a spectrum (. . . ), B spectrum (------), and 7 spectrum 1-) l7o. Crystal thickness= 1.0 mm. dral axis, are presentedin Figure 10. The classifica- tions are obtained from the charactertable for C, in tion of statesis derived from the "descentin symme- Cotton (1963). Tetragonal compressionbetween O6 try" method from 01 to Da7 to Cr,(Cr") to C"(Cr") and Dnn and acute dihedral angles aboul Z are used using the correlation tables in Wilson et al. (1955), based on the structural data. The upper two states and the polarization propertiesof the allowed transi- and the lower three states are derived from the oc- tahedral uE, and 522, states of Fe2+, respectively. 120 Although C, explainsthe polarization of the 1030nm band mostly in Z(B), it cannot explain the complete polarization of the2470 nm band in a. This transition too is expectedto occur in both a(X) and 7(I/). There- fore, the polarization properties of thesebands are BO indicative of a higher, effective electrostatic symme- try. tB'lo3o An The effective electrostatic symmetry of the calcic a amphibole M(4) site is taken Io be Cou(Cr"),based aa upon the similar interpretation of the analogous 40 bands in the spectraof orthopyroxene by Runciman et al., and Goldman and Rossman (1977), and an- 20 thophyllite and gedriteby Mao and Seifert.From the gt ' g classificationof statesand the allowed transitionsfo? nL"o Cr,, Ihe 1030nm and2470 nm absorption bands are 4 to t2 assignedto the At - Ar and A' - Br transitions, -c respectively.The A, - B, transition within the split I roso uTrr(On)state, which was identified at 2350 cm-' in the spectra of orthopyroxene, does not occur at Frc 9. Intensity correlation of the calcic amphibole absorption energiesgreater than 2000 cm-'. For C, symmetry, bands in B and u at 1030 nm suggestingthat the a band is also due to Fe2+ in the M(4) site. All points represent room temperature this transition is expectedto be polarized in 7(I) as soectra normalized to 1.0 cm thickness. shown in Fisure 10. GOLDMAN AND ROSSMAN: CALCIC AMPHIBOLES 215

The operator-equivalent method described by v2v c2 Hutchings (1964) to derive energy expressionsfor the crystal-field states was A used by Goldman and Ross- zHl zA man (1977) to analyze the spectra of Fe2+ in the 97lO cna-l M(2) site of orthopyroxene. In orthopyroxene, all three allowedtransitions of Cr, wereobserved experi- mentally, and hence the three crystal-field fitting pa- rameters were calculated;A(: 10 Dq), M, and N. From these parameters, the energy of the forbidden At - Az transition was calculated.However, the en- ergy of the A, - 8, transition due to Fe2+in the M(4) 4O5Ocw-l site of calcic amphibolesis not known, although the mid-infrared spectra of the Mt. Bity tremolite in- B2 .2OOOcr,t-l dicate that it doesnot occur above2000 cm-'. Using A2 this value as an upper limit for theAr - B, transition, . lOOOou-l = A1 the following results are obtained: A 4629 cm-l O ctu-l and A, - Az : 976 cm-'.lf Ar - B, occursat 1400 FIc. 10. Energy level schemes for Cz and Cr, symmetries in cm-1, the crystal-fieldparameters change as follows: which the crystal-field Z axis is a dihedral axis. The Cz, energy level : - : A 4760cm-l, and Ar Az 524 cm-t. scheme explains the polarization properties of the M(4) Fez+ ab- The value determinedfor A, the energyseparation sorption bands. C,, the crystallographic point-group symmetry of between the octahedral 5T,, and 6Ee states in the site, cannot explain the polarization of the 247Onm band 4600-4800 cm-r range has been obtained from the :1":n point-charge model in which the crystal-field poten- tial was derived in terms of the average M-O bond distanceof the M(4) site.Although a A of this magni- of theA, - Bztransition. However, the two elongated tude for a six-coordinatesite at first seemslow, its M-O bonds in orthopyroxeneare about 2.5 A, value is acceptableupon consideringorthopyroxene whereasthey are nearly 2.8 A in grunerite.This spectraldata. The At - Ar At - Bt and Ar + Bz longer bond distanceexplains the lower energyof 1, transitions of bronzite occur at 10,930cm 5400 A, - B" transitionin the amphiboleM(4) site. cm-1, and 2350cm-1, respectively.Using the average Finally,it wassuggested that Fe2+in the M(4) site M-O bond distanceof theM(2) siteof 2.22 A, A was alsoproduces a bandin a at 1030nm. Since l, - l, calculated to be 6522 cm-l. In comparison, the is expectedto be polarizedentirely in B, the presence smaller A value for M(4) Fe'+ in calcic amphiboles of the a band at 1030nm is difficult to explain. reflects the larger size of the M(4) site as expected Similarly,in the spectraof M(2) Fe2+in orthopyrox- from the l/f dependenceof A. The energiesof the ene, the polarizationintensities of the subsidiary transitions to the A, and ^8, states are about 1000 componentsof the main bandswere not accounted cm-' and 1400cm-r smaller than the energiesof the for usingthe mostintuitively reasonable selection for analogoustransitions in orthopyroxene,respectively. the Cr, axes,although the polarizationintensities of This indicates that the energy of the Er(Ox) staIe, the main bandswere correctly explained. It must be from which A, and B, are derived,is also at least 1000 rememberedthat theseresults are based upon a theo- cm-r lower in the calcic amphibole M(4) site, and reticaltreatment that is highly idealized,both with hence A is expected to be reduced from the or- regardto the presumedeffective symmetry and the thopyroxene value by about this magnitude. There- assumptionof equivalentpoint-charge contributions fore, a A for M(4) Fe2+in the 4600-4900 cm-l range from eachof the ligands.Nevertheless, on a com- is reasonable. parativelevel, the crystal-fieldanalysis of the calcic Goldman and Rossman(1977) indicated that the amphiboleand orthopyroxenespectral data produce O-M-O bond angles about the crystal-field Z axis consistentresults. play a significantrole in determiningthe energyof the Ar - Bz transition within the split uTrr(On)state. The Conclusion O(6)-M(4)-O(6) angle of 62o is l0o smaller than The electronicabsorptions arising from ferrous the O(6)-M(2)-O(3) angle in orthopyroxene. The iron in the M(4) siteof calcicamphiboles have been smaller angle about Z is expectedto raise the energy identified.The absorptionbands due to M(4) Fe2+ 2t6 GOLDMAN AND ROSSMAN: CALCIC AMPHIBOLES occur at 1030nm in p, with some intensity in a, and Dollase,W. A (1973) Mdssbauerspectra and iron distribution in 4l-63. at 2470 nm in d. The spectra of a wide variety of the epidote-group minerals. Z. Kristallogr., 138, Faye, G H. (19'12) Relationship between crystal-field splitting Fe2+ in calcic amphiboles indicate that the content parameter, "Au1," and Mro.r-O bond distance as an aid in the the M(4) site is variable.In particular, M(4) Fe2+has interpretation of absorption spectra of Fe'+ materials Can. been found in absorption spectra of all actinolite Mineral . I l. 4'73-48'7. samples studied in this laboratory. The indication Finger, L. W. (1969) The and cation distribution from the absorption spectra that the M(4) Fe'+ con- of a grunerite. Mineral. Soc. Spec. Pap. No. 2,95-100. Goldman, D. S and C. R. Rossman (1976) Identification of a variation may be important tent exhibits significant mid-infrared electronic absorption band of Fe'z+in the distorted petrologically, due to the temperatureand pressure M(2) site of orthopyroxene, (Mg,Fe)SiO". J. Chem. Phys. Lett., dependenciesof the various elementalpartitions that 4t, 474-475. involve iron, either between coexisting minerals, or - and - (1977) The spectra of iron in orthopyroxene among the crystallographicallydistinct sites within revisited the splitting of the ground state.Am. Mineral ,62, l5l-t57. each mineral. Hafner, S. S and S. Ghose (1971) Iron and magnesiumdistribu- tion in (Fe,Mg)'SLOz(OH)". Z. Kristallogr., Acknowledgments 133,361-376. We wish to thank J D. Hare (Caltech)and R. G. Burns (MIT) Hlggstrdm, L., R. Wappling and H. Annersten(1969) Mtissbauer for discussingmany aspectsof the spectroscopicinterpretation. We study of oxidized iron silicate minerals, Phys. Stat Solidi, 33, also appreciate the use of the Mdssbauer spectrometer made avail- 74t-7 48. able to us by W. A. Dollase (UCLA) and the many discussions Hutchings,M. T. (1964)Point-charge calculations of energylevels with him concerning the analysis of the M6ssbauer spectrum. of magnetic in crystalline electric fields. Soird State Phys., 16,22'7-2'.13. References Johnson, C. K. (1965) Onrsp, a FonruN thermal ellipsoidplot program for crystal structure illustrations. U. S Natl. Tech. Inf. Bancroft, G M. and J. R. Brown (1975) A Mdssbauerstudy of Seru. ORNL-3794. coexisting hornblendes and : Quantitative Fes+/Fe2+ ra- Kuno, H (1954) Study of orthopyroxenesfrom volcanic rocks. tios. A m. M ineral., 60, 265-272. Am Mineral . 39. 30-46. -, R. G. Burns and R. A. Howie (1967a)Determination of Lacroix, A. (1910) Mineralogie de la France et de ses Colonies. the cation distribution in the orthopyroxene seriesby the Miiss- Tome lV, p 786. bauer effect.Nature, 2 I 3, 122l-1223. Leake, B. E. (1968) A catalog of analyzedcalciferous and sub- and A. G. Maddock (1967b) Determination of calciferous amphiboles together with their nomenclature and cation distribution in the -gruneriteseries by associated minerals. Geol. Soc Am Spec. Pap. No 98 Mcissbauer spectra.A m M ineral., 52, 1009-1026. Mao, H. K and F. Seifert (1974) A study of crystal-field effectsof -, A. G. Maddock and R. G. Burns (1967c)Applications of iron in the amphiboles and gedrite. CarnegieInst. the Mdssbauer effect to silicate mineralogy. Part I. Iron silicates Wash Year Book. 73,500-503. of known crystal structurc. Geochim. Cosmochim. Acta, 3i,, (1971) of 2219-2246 Mitchell, J T., F. D Blossand G. V. Gibbs Examination the actinolite structure and four other C2/^ amphiboles in terms Bell, P. M. and H. K Mao (1972) Crystal-field effectsof iron and of double bonding. Z. Kristallogr., i,33,273-300. in selectedgrains of Apollo 12, 14, and l5 rocks, Papike, J. J. and M. Ross (1970) Gedrite: crystal structuresand glasses,and fine fractions. Proc. Third Lunar Sci. Conf., Geo- intracrystalline cation distributions. Am Mineral., JJ, 304-305. chim Cosmochim.Acta, Suppl. 3, Vol 1,545-553. and J. R. Clark (1969)Crystal-chemical character- Birle, J. D., G. V. Gibbs, P. B. Moore and J. V Smith (1968) ization of clinoamphiboles based on five new structure refine- Crystal structuresof natural olivi nes.A m M ineral., 53, 807-824. ments.Min Soc Am. Spec.Pap No.2,117-136. Burns, R. G. (1965) Electonic spectra of silicate minerals: appli' Runciman, W. A , D. Sengupta and M. Marshall (1973) The cations of crystal feld theory to aspects of geochemistry. Ph D. polarized spectra of iron in silicates. L Enstatite. Am Mineral., thesis, Uniuersity of Caldornia, Berkeley, Caldornia. 58, 444-450. - (1970) Mineralogical Applications of Crystal Field Theory, White, W. B and K. L. Keester(1966) Optical absorptionspectra Cambridge University Press.Cambridge, England. of iron in the rock-forming silicates.Am. Mineral ,5i,,7'14-791. -, R. M. Abu-Eid and F. E. Huggins (1972) Crystal field - and - (196'7)Selection rules and assignments for the spectra of lunar pyroxenes. Proc. Third Lunar Sci. Conf., Geo spectra of ferrous iron in pyroxenes. Am. Mineral.,52, chim. Cosmochim.Acta, Suppl. 3, Vol 1, 533-543. t508-1514. - and C Greaves ( l97l ) Correlations of infrared and Miiss- - and R. K. Moore (1972)Interpretation of the spin-allowed bauer site population measurementsof actinolites.Am. Mineral 1692-1710' 56, 20tO-2033. bands of Fe" in silicate garnels Am. Mineral., 57, (1955) Molecular Bush,W. R.,S S Hafnerand D. Virgo(1970) Some ordering of Wilson, E. B., J. C. Decius and P. C. Cross Vibrarions. McGraw-Hill, New York. iron and at the octahedrally coordinated sites in a magnesium-rich olivine. N ature, 227, 1339-134l. Cotton, F. A (1963) Chemical Applications of Group Theory. W\' Manuscript receiued, April 28, 1976; accepted lev-Interscience.New York. for publication, September21, 1976.