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The Site Distribution of Iron and Anomalous Biaxiality in Osumilitel

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The Site Distribution of Iron and Anomalous Biaxiality in Osumilitel American Mineralogist, Volume63, pages490498, 1978 Thesite distribution of iron andanomalous biaxiality in osumilitel DoN S. GoLDueN2^tNn Geoncn R. RossueN Diuision of Geological and Planetary Sciences Califurnia Institute of Technology Pasadena, California 91125 Abstract osumilitesfrom sakkabira,Japan, obsidian cliffs, oregon,and Nain, Labradorhave been studiedby optical,infrared, and Mdssbauerspectroscopy. Osumilite is an anhydrousmineral. A major proportion of the Fe2+occurs in the channel-likecavities. Biaxial Nain osumiliteis X-ray hexagonal,and doesnot showinfrared spectroscopic evidence for AllSi ordering.The biaxialityarises from the channelcontents. Introduction cluded that natural osumilitesare virtually an- hydrous.This conclusion has not beentested by inde- Osumilite,K(Mg, Fe,Mn )r(Al,Fe3+ )r(Si,Al )rrOro, is found in rhyoliteor rhyodacitevolcanic rocks and pendentmeans. high-grademetamorphic rocks in contactaureoles. The resultsof the spectroscopicstudy of cordierite Its crystalstructure (Brown and Gibbs, 1969) is simi- (Goldman et al., 1977)are applied to the study of lar to that of cordieriteand beryl,because it contains osumilite,because of the structuraland spectroscopic channel-likecavities that are formed by the tetrahe- similaritiesbetween both minerals.Goldman el a/. dral framework.In cordieriteand beryl the channels concludedthat (l) Fe2+occurs in boththe octahedral are formed by a superpositionof six-memberedtet- and channelsites, and (2) the bluecolor of cordierite rahedralrings, whereas the channelsin osumiliteare originatesat leastprimarily from intervalencecharge- formedby a superpositionof doublehexagonal rings transferbetween octahedral Fe2+ and channelFe3+. (Fig. l). The tetrahedralrings in theseminerals are They also establishedthe calibration for quan- linkedby octahedraand additionaltetrahedra. titativelydetermining the watercontent from infrared Osumiliteis generallydeep blue and displaysuni- absorptionspectra. These interpretations provide the axial optics.However, Berg and Wheeler(1976) re- basisfor the study of osumilite. porteda pink biaxialosumilite frorn Nain, Labrador. This paperreports the findingsof a combinedelec- The causeof the differencebetween uniaxial and tronic absorptionand Mdssbauerstudy of threeos- biaxialoptical behavior has not beenestablished. umilite samplesto examinethe site distribution of Water, as 1.73percent HrO*, was found in the iron and the origin of color. Changesin the electronic initial analysisof the Sakkabiraosumilite by Miya- absorptionspectra are compared to variationsin 2V shiro (1956).Brown and Gibbs found electronden- and inferencesare made regarding structural state. In sity in the centerof cavitiesbetween the doublehexa- addition,infrared spectra have been taken to examine gonal ring groups and attributedit to (Na,K,Ca) the water contentin osumilite,and to seeif spectral ions.They alsofound electrondensity near the center differencesexist betweenthe uniaxial and biaxial of the cavity within the double hexagonalrings and samples. attributedit to HrO hydrogen-bondedto the channel- Experimentaldetails wall oxygens.Olsen and Bunch (1970) found that the weightpercent of the oxidesin microprobeanalyses Samplesof osumilitefrom the type localityin Sak- of osumilitessummed near 100 percent,and con- kabira,Japan (Miyashiro, 1956) (Stanford University ff7753);Obsidian Cliffs, near McKenzie Pass, Oregon I ContributionNo. 2956 (Caltech #7396); and Nain, Labrador (Berg and ' Presentaddress: Technical Center, Owens/Corning Fiberglas Wheeler,1976) are used in this study.The formertwo Corporation,Granville, Ohio 43023. samplesoccur in silicicvolcanic rocks, are dark blue, 0003-w4x/78/ 0506-0490$02.00 490 GOLDMAN AND ROSSMAN: OSUMILITE 491 counts,/channel,respectively. All Mdssbauerspectra and parametersare reported relative to metalliciron. A Mdssbauerspectrum of the Nain osumilitewas alsotaken after heat treatment in air at 813"Cfor l3 hours, but significantFe8+ resonance was not ob- served. Site distributionof iron Electronic absorptionspectra The electronicabsorption spectra of the uniaxial CORDIERITE OSUMILITE osumilitesfrom Sakkabiraand ObsidianCliffs are Fig. l. The tetrahedralframework in cordieriteand osumilite presentedin Figures2 and3.They consist ofabsorp- proposedby Brown and Gibbs (1969). tion featuresat 10,280,15,480, 22,220, and 24,150 cm-r (973, 646, 450, and 414nm) in c.rpolarization I exhibit uniaxial positive optics,and are inferred to and4650, 7020, and 10,280 cm- (2150, 1425, and 973 contain Fe3+in tetrahedralcoordination basedon nm) in ot polarization. Faye (1972) attributed the stoichiometry(Olsen and Bunch) and X-ray (Brown asymmetryand breadth of the bandat 15,480cm-1in andGibbs) considerations. Their polarized electronic the co spectrumof an Obsidian Cliffs sampleto a absorptionspectra were taken on polishedslabs cut superpositionof a dominant lower-energyband as- to containc. The Nain osumiliteoccurs in a gran- signedto intervalencecharge transfer betweentet- ulite,is pink,and exhibitsbiaxial positive optics with rahedralFe3+ and octahedralFe2+, and a subordinate 2V rangingfrom l3 to 40oas measured with a univer- higher-energyband assilned to tetrahedral Fe3+. sal stagefor differentcrystals in one hand specimen. Faye assignedthe weak band near 22,220cm-1 to 7 is parallelto thec axis;the orientationofthe a and tetrahedralFee+ and the band near 10,280cm-r to B indicatrix directionsin the planenormal to c is not octahedralFe2+. known. The 7 direction for spectroscopicmeasure- The electronicabsorption spectra of an osumilite ment was obtainedby first orientinga slabto center from Nain havinga 2V of 40" (Fig. 4) clearlyillus- the Bxa figureand thenrotating this slab90o abouta trate the threedistinct optical directions. The 7 spec- to obtain a centeredflash figure. After obtainingthe trum (which has the samecrystallographic orienta- 7 spectrum,the samplewas rotated 90o to reproduce tion as e in Figs. 2 and 3) consistsof bandsat 4686, the centeredBxa figure, and thinned for measure- 7020,and 10,280cm-r (2135,1425, and 973 nm). ment of the a and B spectra.Electron microprobe Peak maxima differ slightly for the main band in c data, obtainedat Caltechwith a Mec-V automated electronmicroprobe, were taken on slabsused for the Table l. Electronmicroprobe analyses of osumilite opticalspectra (Table l). Microprobedata for Nain osumiliteshaving 2Vs of 24, 30,and 36owere taken oxide weight percent to seeif variationsin 2V canbe relatedto composi- tional differences.but no suchcorrelation is evident. Labrador Labrador Labrador Japm oregon 2v=36" 2v=30' 2v=24" Microprobe analysesof a Nain osumiliteheated at 700'C in air for 13 hours did not differ from the analysesof the unheatedsamples. 494, 561and 520 St02 60.3 oL. z 60.8 60.6 60.7 Tro2 0.2 0.2 0.2 0.0 0.1 pg of the Sakkabira,Obsidian Cliffs, and Nain sam- Alz0e 22.0 22.2 22.4 22.0 2L.8 ples,respectively, were each ground with 200 mg of MsO 5.7 ).o 3.0 3.0 FeO* 6.3 6.4 6.6 9.4 9.7 KBr and pressedinto a pellet for infrared spectra. MnO 0.1 0.1 0,1 1.0 L.2 The pelletswere heatedin uacuoat 80oC overnight, Na20 O.2 0.4 0.3 0.7 0.2 Kzo 4.0 4.3 4.2 3.2 3.2 andrepressed to minimizeadsorbed HrO on the KBr. 99.0 100.5 100.1 99.9 99.9 All infrared spectrawere taken againsta KBr refer- ence pellet. Concentrationsfor the Obsidian Cliffs andNain samplesused in theM6ssbauer experiments 'ttotal Lron reported as Feo are about 2.3 and 1.3 mgFe/cm' with the off-reso- A. A. Chodos and D. S. Goldman, Analysts nanceregions having about2.9 X lff and l.l X 106 492 GOLDMAN AND ROSSMAN: OSUMILITE hFVELENGIH( NM ) and B polarizationsand occurat 10,360and 10,400 rl00 500 600 1000 2000 cm-'(965 and962nm), respectively.The bandin the 1.5 visible region in a and B has a broad maximum between18.000 and 21,000cm-l, and has a weak OSUMILITE shoulderat 22,220cm-' (450nm), correspondingto Jopon the position of the band observedin <,rin the other samples.Some of theintensity of the4686 cm-1 band occursin a, but it is only observedin mm-thick L! 1.0 sections. Z- Absorptionsdue to Fd+ canbe assignedusing the -jl Ct (r) .: i criteria establishedfor cordieriteby Goldman et al. m Thesecriteria include absorption band positions, in- Cf i: O '.: tensities,polarizations, and the energydifference be- U) ,' i..j tweenthe componentsof split bands.Therefore, the m nq i.' bandsat 10,280and 7020cm-' in 7 (or e) are as- Cf i,. i j.'' signedto Fe2+in the octahedralsite. Bands in the 10,200-10,400cm-r regionin a andp (or co)and the 4650-4700cm-' region in 7 (or e) are assignedto Fe2+in the channels.The two absorptionbands aris- 0.0 ing from eachtype of Fe2+are consideredto repre- sent electronictransitions to the componentsof the 2500020000 15000 10000 5000 split 58, state.The barycenter(median) energies of hRVENUMBEB(cll-1 ) the two transitionsfor octahedraland channelFe2+ Fig. 2. Room-temperature electronic absorption spectra of an are 8650 and 7590 cm-', respectively.For com- osumilite from Sakkabira. Crystal thickness : 0.10 mm. parison,the barycenterenergy for octahedralFe2+ in cordieriteis about 9200cm-1 (Goldman et al.).The largerbaryce4ter energy in cordieritereflects smaller hFVELENGTH(NM ) averageM-O distanceof the octahedralsite (2.12A; r.r00 500 600 1000 2000 Gibbs, 1966)than in osumilite(2.15A; Brown and J -U Gibbs, 1969).The barycenterenergy can be usedas OSUMILITE an estimatorof l0 Dq, the theoreticalenergy differ- 5Tr, uE, Oregon encebetween the and electronicstates, if the 2.5 splittingof thesT* stateis smallor similaramong the samplescompared
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