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 (Faye, 1972).lt should therefore vary as Q/a)u, whereZ is the averagemetal-oxygen LtJ2.0 (J bond distanceof the coordinationsite. Using the Z- cordieritedata, a barycenterenergy of 8575cm-1 is C expectedfor octahedralFe2+ in osumilitefrom the || | l f, cf ideal (l/a)" dependence.These resultssupport the C] assignmentof the octahedralFe2+ bands in osumilite a aY1 't n spectra.In both mineralsthe two octahedralFd+ l! l.u CT bands are polarizedparallel to the c axis and have similarintensities. of Fe2+in osumilite 0.5 The barycenterenergy channel of about 7600cm-1 is slightlylarger than observedin cordierite (- 7200 cm-l). In both mineralsthe 0'0 higher-energyband is polarizedin the planenormal to the c axisand is muchmore intense than the lower- 2500020000 15000 10000 5000 energyband, which is polarizedparallel to the c axis. hJRVENUMBEB(cM-l ) As in cordierite,heating experiments support the as- Fig. 3. Room-temperatureelectronic absorption spectra of an signmentto channelFe2+ in osumilite.After an a7 osumilitefrom ObsidianCliffs. Crvstal thickness : 0.10mm. slabwas heated in air at 700"C for l3Vzhours. it was GOLDMAN AND ROSSMAN: OSUMILITE 493 cooled to room temperatureand spectroscopically I,.IFVELENGTH(NM ) examined.The bands assignedto octahedralFe2+ r-t00 500 700 1000 2000 retained the same intensity after heat-treatment, whereasboth bands assignedto channelFe2+ were reducedto about 85percent oftheir intensityprior to OSUMILITE heating.Berg and Wheelersuggested, from the pres Lobrodor ^B enceof graphiteand pyrrhotitein the rock, that Fe3+ lr' ll is "virtuallyabsent" in theNain osumilite.Using this ;l assumption LrJt n i,ri and the molar absorptivitydetermined ^'- ti it c_J t! il for octahedralFe2+ in cordieriteof 3.5,the intensity z. of the octahedralFe2+ band at 10,280cm-1 indicates cf that only about 60 percent of the total iron in the co Nain osumiliteis in the E. octahedralsite and 40 percent O is in the channels.Spectroscopic evidence for tetrahe- a fl dral Fe2+is not observed.The site distribution of iron co 0.5 fl ct tl in the Nain and ObsidianCliffs osumilites will now !i ;l be analyzedusing M6ssbauerspectroscopy. o f! ...j':':-':l'r. ll ':.. -.:-... I 'i'ia...,., ,./7 - t M'6ssbauer spect ros copy ./" -'i.-:. t Hzo The Mdssbauerspectrum of the Nain osumilite / (Fig.5) supportsthe contention of Bergand Wheeler 0.0 that Fe3+is negligiblein this osumilite.The Fe2+ 2500020000 15000 10000 s000 resonantabsorption is more intensein the low-veloc- hFVENUMBEB(cM-1 ) ity region,although both regionshave asymmetry in Fig. 4. Room-temperatureelectronic absorption spectra oi an their absorption profileswith the high-velocityab- osumilitefrom Nain. Optic orientation'."1 : c; a andp occurin the sorption being broader.It is unlikely that the in- planenormal to the c axis.Crystal thickness = 0.10mm. tensityasymmetry is producedby preferredorienta tion in the absorber,because these crystals do not mostly from Fe2+ in the octahedral site, based on the possessgood cleavage.These observationssuggest similarquadrupole splitting and isomershift found that Fe2+occurs in multiple environments. for octahedralFe2+ in cordierite(Goldman et al.), A two-doubletfit to the spectrumis presentedin and the innertwo doubletsarc mostly due to Fe2+in Figure 5a and the resultingspectral parameters are the channels.The smallerquadrupole splitting and listed in Table 2. The areasof the two lines of each isomershift values of the innerdoublet(s) are consis- doubletand the halfwidthsof all peakswere con- tent for the more distortedchannel environment. The strained to be equal. The large halfwidths of the resultsfrom the two- and three-doubletfits indicate peaks(0.42 mm/sec) and the inabilityto fit the main that about 68 and 53 percentof the total iron is peak intensitiesand the regionnear f0.6 mm/sec octahedralFe2+, respectively. Considering that the suggestthat this fit is inadequatelymodeling the data. three-doubletfit probablyunderestimates octahedral This posesa problem,because the electronicabsorp- Fe2+content because of increasedpeak overlap,and tion spectraof osumilitesuggest that Fe2+resides in uice uersafor the two-doubletfit, an approximate only two typesof sites,the octahedraland channel valueof 60 percentof the iron in the Nain osumilite sites.Although a three-doubletfit to the spectrum beingin the octahedralsite is reasonable,and agrees (Fig. 5b) providesa betteroverall spectral fit, a phys- with the site distribution predictedfrom the elec- ically meaningfulinterpretation for eachof the three tronic absorptionspectra. doubletsis unclear.In addition,the peakhalf-width The Mdssbauerspectrum of the ObsidianCliffs in the three-doubletfit (0.38mmlsec) is still broad osumilite(Fig. 5d) alsoshows asymmetry in the pro- for Fe2+in a singlesite. Spectralbroadening can file and intensity of the Fe2+ resonance,which is resultfrom variationsin next-nearest-neighborchem- again fitted with three doublets.The three doublets istry, as hasbeen proposed in clinopyroxenes(Dowty are only moderatelysimilar to the quadrupolesplit- and Lindsley,1973), orthorhombic Mg-Fe amphi- ting and isomershift values(Table 2) found in the boles (Seifert, 19'77),and calcic amphiboles(Gold- Nain sample,and the arearatio of the outerdoublet man, 1978).We suggestthat the outerdoublet arises to the total Fe2+resonance is 56percent, which is also 494 GOLDMAN AND ROSSMAN: OSUMILITE
VELOCI TY ( MM,/SEC) similar to that found in the Nain osumilite.In addi- tion, octahedralFe2+ (the outer doublet)accounts for about 45 percentof the total iron, in agreementwith value predicted from the electronic absorption spectra(41 percent).The ObsidianCliffs Mdssbauer spectrumcontains two additionalpeaks not found in the Nain spectrum,that occur near *0.9 and -0.8 mm,/secand haveequal intensities. Fitting a doublet to thesepeaks results in isomershift and quadrupole splittingvalues of 0.25and l.7l mm/sec,respectively. The isomer shift is indicativeof high-spinFes+, but this valueis much smallerthan expectedfor Fe8+in octahedralcoordination (- 0.4-0.5mm/sec). This doublet is assignedto Fes+in tetrahedralcoordina- tion, basedon the similar parametersreported for tetrahedralFee+ in ferri-diopside(Hafner and Huck- enholz,l97l) and sapphirine(Bancroft et al.,1968), although this assignmentis not consistentwith the data reportedfor iron orthoclase(Brown and Prit- chard,1969). Peaks placed at *0.5 and -0.1 mm/sec for "octahedral"Fes+ improve the fit, but their phys- z ical significanceis not certain. o The e values(molar absorptivity) from the elec- U1 a tronic absorptionspectra for octahedraland channel = a Fe2+in osumilite from the site distribution deter- z. CE (t mined from the Nain sampleare 3.5 and 130respec- F tively. Water Infrared absorption spectraof single crystalsof osumiliteindicate that it is virtuallyanhydrous. The fundamentalstretching modes of water occur in the 3300-3700cm-' region, but this region is free of absorptionfeatures in the Sakkabiraand Obsidian Cliffs osumilites.Although the initial analysisre- ported by Miyashiro found HrO*, all subsequent electron microprobe analysesof the Sakkabiraos- umilite(Olsen and Bunch,1970; G; Brown,personal comrnunication,1977; this work) havefailed to find evidencefor water. Recognizingthe possibilitythat the batchof osumiliteused by Miyashirocould differ in water content from that usedby other investiga- tors, we neverthelessconclude that osumiliteis cor- rectlyformulated as an anhydrousmineral. The Nain osumilite does have weak water absorptionsin the infraredregion (Fig. 6); however,the watercontent is calculatedto be lessthan 0.01 percent(by weight), -2-l 0t23rl usingthe molar absorptivityfor water determinedin cordierite by Goldman et al. The presenceof the Fig. 5. Room-temperatureMilssbauer spectrum of osumilite (1905 from Nain: (a) fitted with two Fe'+ quadrupoledoublets, (b) fitted combination modes near 5250 cm-r nm), with threeFe'z+ quadrupole doublets, and (c) heatedto 813. for l5 barely visiblein Figure4 in 1, indicatethat the mo- hours;(d) osumilitefrom ObsidianCliffs. lecularspecies HrO is present,although these data do GOLDMAN AND ROSSMAN: OSUMILITE 495
Table2. MiissbauerDarameters ring groups.The origin of the pink color in the Nain osumilite is unclear.The possibility that an inter- SaEple Peaks Isoner Quadrupole Half- Z Ara *' Fd+ and shlft splltring wadtn valencecharge transfer between octahedral Fe8+in the adjacenttetrahedral site is responsiblefor Nall AAr L.20 2.35 0.42 68 392* Laoraoor 5b- I.14 1.86 0.42 someof the bluecolor in the Sakkabiraand Obsidian It may be a NaLn L.2L 2,4L 0.38 53 313* Cliffs osumilitescannot be excluded. Labrador BB' 1.16 2 ,O3 0. 38 35 possiblecause for the asymmetryof the main charge- 1.17 1.48 0.38 L2 transferband. Naln AAl r.22 2.34 0.34 51 255* The of waterin the channelsappears Labrador BB' 1.19 I .88 0.34 30 concentration HeatedT cc' L.26 1.41 0,34 19 to correlatewith the easeat which channelFe'+ will
0bsidla AAt 1.19 2.55 U.JJ 45 539** oxidizeupon heating.Channel Fe2+ was not oxidized Cliffg BB' L.20 1.86 0.33 20 ObsidianCliffs osumilite. Oregon L.2L 1.40 0.33 15 at 869oCin the anhydrous DD' 1 .71 0.33 16 About l5 percentof the channelFe2+ was oxidized at EE' 0.40 0.60 0.33 4 700'C in the Nain osumilite,which containsminor amountsof water. Nearly 45 percentof the channel *200 channels fltted **330 channels fitted *813o 15 horr" Fe2+is oxidizedat thesetemperatures in water-bear- ing cordierites.Ninety percentof the water in the not excludethe simultaneouspresence of the OH- Nain osumilitewas lost from the sampleheated to ion. The complexityof the spectrain the fundamental 869oC,as determinedby the infraredspectra, yet the regionindicates that water(or OH-) occursin mul- value of 2V dld not change,further indicatingthat tiple environments.The spectradisplay anisotropic water doesnot causethe biaxial optics. differencesbetween EIc and E I lc, but in the plane The M6ssbauerspectrum of the heatedNain os- perpendicularto c the water bands fail to display umilite showsno changein the intensityasymmetry, anisotropy(( 4 percent).This resultwas verified for but doesshow broadening ofboth componentsofthe 8 sampleswith2V rangingfrom l9 to 36o.This result Fe2+doublet in the inner region (Fig. 5). The spec- contrastsstrongly with cordierite,for whichthe water trum changesto resemblethat of the ObsidianCliffs featuresare highly anisotropic,and indicatesthat the osumilite but without the Fe8+doublet. The half- water in the Nain osumilite is structurallyordered widths of the three-doubletsfit (Table 2) are still accordingto the "pseudo" hexagonalsymmetry of broad for Fe2+in a singlesite. the crystal. It seemsunlikely that the water would causethe biaxial optics if it is hexagonallyordered. This conclusionis further substantiatedby the poor correlationbetween water concentration and 2V (Fig. NRVELENGTHcxr> 7). 2800 3000 3200 16 Origin of color OSUMILITE Heating experimentson cordierite (Goldman el Lobrodor t2 a/.) suggestedthat its bluecolor originatesfrom inter- lrJ (J valencecharge transfer between octahedral Fe2+ and z. CE channelFe3+. A similarresult has been obtained with p8 osumilite.The Nain osumilitewas heatedfor one o (t) hour at 813' and thenone hour at 869'C.Intensities (D cEq \rrc of the octahedralfeatures remained the sameafter Ellc-j heattreatment, but significantchange occurred in the spectrumnormal to the c axis (Fig. 8). The channel Fd+ bandnear 10,400 cm-r reducedin intensity,and the sampleturned from pink to blue. Analogousto 3800 3600 3'100 3200 3000 the resultsfor cordierite,this suggeststhat a portion I.IRVENUMBERccn-t I of the channelFe2+ oxidizes upon heating,and that Fig. 6. Infraredabsorption spectra of a crystalofNain osumilite (2V : at 78 K. Thesespectra exhibit only moderately the blue color in osumiliteis at leastin part due 40") taken to betterspectral resolution than the room-temperaturespectra, and intervalencecharge transfer between octahedral Fe2+ areabout 40 percentmore intense. Data presented for 1.0cm-thick and Fe3+inthe cavitiesbetween the doublehexagonal sample. 496 GOLDMAN AND ROSSMAN: OSUMILITE
ro channelFd+ near 10,400cm-r would have equal OSUMIL ITE intensities,both being in the plane normal to the c Lobrodor axis. Ee o Becausethe Nain samplesexhibit a wide rangeof (D 2V,the relationshipbetween 2V andthe0/a intensity o- ratio ofthe channelFez+ bands was explored, and the 6 q) resultsare presentedin Figure9. The hexagonalSak- o o c are also in- o kabira and Obsidian Cliffs osumilites €+ a cluded in this correlation,which showsthat the in- o o tensityasymmetry in the planenormal to c increases -o as2V increases.The trend is not linearand appearsto 2 havean upwardcurvature. This trend intersectstypi- cal valuesfor cordieritewhich haveasymmetry values near2.0 with 2V valuesof 80-90". oro203040 The correlationin Figure 9 was not expectedini- intensitydifferences are due 2V (degrees) tially, becausethe band to the effect of the coordinationabout the channel Fig. 7. Correlation of the 2V anglewith water content as Fe2+ion on its electronicstructure, whereas 2Z is measuredby the intensityof the El_c bandat 3600cm-r. determinedby differencesin refractiveindex along the differentcrystal directionsin the visible region, which are controlledby the electrondistribution of Structural state the entire $tructure.Cordierite has a completelyor- Thereremains a discrepancybetween the electronic deredAl/Si tetrahedralframework, alarge 2V, anda absorptionand X-ray data concerningthe structural large band-intensitydifference, whereas the Sakka- symmetry of the Nain osumilite. Zero and upper- bira osumilite has a disorderedtetrahedral frame- level X-ray precessionphotographs of a crystal of work, 2V equal to zero, and no differencein band Nain osumilitewith 2V : 30" are consistentwith a intensity.This suggeststhat the Nain osumilitemay hexagonalP6/mcc spacegroup. This resulthas been havevariable degrees of Al and Si orderin the double independently confirmed by Professor Gordon hexagonalrings which control the overall optics of Brown (personalcommunication) at Stanford Uni- the crystal.A greaterdegree of order is expectedin versity.However, the biaxialoptics and threedistinct the slowly-cooledNain osumilite comparedto the absorptiondirections indicate that the overall sym- quickly-cooledvolcanic osumilitesfrom Sakkabira metry must be lower than hexagonal.In the Sakka- and ObsidianCliffs. bira samplethere are only two absorptiondirections The infrared spectraof the three osumiliteshave and the 2V is zero.Therefore, if the Nain osumilite beentaken to seeif differencesexist between the Nain were truly hexagonalthe bands ih a and B due to sampleand the hexagonalosumilites from Sakkabira and ObsidianCliffs (Fig. l0). Although differences l,',l8VENU|'1BEB Conclusions 0.r.1 This study has shown that osumilite should be o.2 formulatedas an anhydrousmineral. Evidencealso indicatesthat an appreciableproportion of the iron occursin the channels.Other ions for which 0.0 direct 1500 1000 500 spectroscopicprobes do not existmay alsobe located I.ISVENUMBEB