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The Infrared Spectroscopy of Vesuvianite in the Oh

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The Infrared Spectroscopy of Vesuvianite in the Oh 609 Thz CanadianMineralo gist Vol. 33, pp.609-626(1995) THEINFRARED SPECTROSCOPY OF VESUVIANITE IN THEOH REGION LEEA. GROAT Departrnentof GeolngicalSciences, University of BritishColumbie, Vancotner, British Columbia V6T 124 FRANK C. HAWTHORNE Departtnentof GeologicalSciences, University of ManitobaWinnipeg, Manitoba R3T 2N2 GEORGER.ROSSMAN Divisionof Geologicaland Planetary Sciences, Califomia Instinte of Teclnnl,ogy,Pasadena" Califurnia 91125, U.SA. T. SCOTI ERCIT Cannlian Museum of Nature, Mineral Sciences Section, Oxawa. Ontario KIP 6P4 ABsrRAsr Many imFortantsubstitutions in vesuvianiteinvolve variableH, and thosethat do not still perturbthe local environmentof the OH- anions.Consequently, infrared spectroscopyof the OH fundamentaland overtoneregions is an importantprobe of local order.We haveexamined a seriesof vesuvianitecrystals carefully characterized(by electron-microprobeanalysis, wet-chemical analysis and crystal-structurerefinement) by polarized single-crystalinfrared specuoscopy.The crystals span the complete range of chemical variation reportedin vesuvianite,and the spectrashow tremendousvariability. There are 13 recognizable bands(A-M) that can be divided into three types: (1) eight bandsdue to absorptionsat the O/1 site; theseresult from different local cation and anion configurationsat nearest-neighborand next-nearest-neighborsites; (2) four bandsdue to absorptionat O(10); theseresult ftom different local cation and anion configurationsat nearest-neighborand next-nearest-neighborsites, and (3) a low-energyelectronic absorption band. Boron is incorporatedinto the vesuvianitestructure primarily via the substitution B + Mg = 2H + A1. In boron-rich vesuvianite,the four bandsJ-M are not present,indicating that H has been completely replacedby B in the vicinity of the O(10) site. Althougb lacking the fine detail of the principal-stretchingregion, the overtone spectraare equallycharacteristic of this B<+H substitution.The spectrain the principal OH-stretchingregion extendover a very wide spectralrange (3700-3000cm*r;, and show two featurestlat arc of.gencraL lmportance in the qualtitative interpretation of such specffa:(1) the band width increasessignificantly with decreasingband-frequency (from -20 cm-1 at 3670 cm-l to -120 cm-l at 3060 cm{), and (2) the bandintensity is (nonlinearly)correlated with bandfrequency (in addition to H content). Thesetwo featuresare of significancein quantitativelyfitting spectraby numericaltechniques, and in relating bard intensities to compositionalfeatures. Keywords:vesuvianite, infrared spectr4 electron-microprobeanalysis, crystal structure,hydroxyl. Somntrnr Plusieurssch6mas de substitutionimFliquent ltydrogbne dansla vdsuvianite,et ceux qui ne I'impliquent pas directement afFectentquand m0me le voisinage des anions OH. Par cons6quent,la spectroscopiedes vibrations fondamentaleset des harmoniquesdans l'infrarouge peut fournir une information pr6cieused proposdu degr6d'ordre local. Nous avonse;aminf, ps1 spectroscopiefufrarouge polaris6e sur cristal unique une s6rie d'&hantillons bien caract6ris6schimiquement par analysea h microsonde6lectronique, par voie humide, et par affinement de la structurecristalline. l,es cristaux repr6sententla gamme complbtede variationschimiques d6crites dans la vdsuvianite,et les spectresfont preuved'une variabilit6 imposante.Le spectre comprendtreize bandesdistinctes (A-M) qtrr sont divisibles en trois categories:(1) huit bandesattribu6es aux absorptionsau site OH, qui d6coulentdes diff6rents agencementslocaux des cations et des anions impliquant les prochesvoisins et les deuxibmesplus prochesvoisins; (2) quatrebandes dues i l'absorptiondans le site O(10), qui d&oulent des diff6rentsagence- ments locaux des cationset des anionsimpliquant les prochesvoisins et les deuxibmesplus prochesvoisins, et (3) une bande atfibuable ! une absorptiondlectronique de faible 6nergie.lr bore est incorpor6dans la structuresurtout selonla substitution B + Mg ...- 2H + A1.Dans la vdsuvisniteborifAre, les quatrebandes ,l-M ne sontpas pr6sentes, ce qui indique que le B renplace complbtementle H dansle voisinagedu site O(10). Malgr6 l'absencede rdsolutionfine dansla r6gion de l'6tirementprincipal des ba:rdesOH, les spectresdes harrnoniquessont 6galementcaract6ristiques de cetle substitutionB<+H. Ceue r6gion des spectress'6tend sur un grand intervalle (3700-3000cm-r), et montre deux aspec$drimportance g6n6rale dans I'interpr6tation quantitativede tels spectres:(1) la largeur desbandes augmente de fagon importanted mesureque diminue la fr6quencede la bande,d'environ 20 cfr. I d,3670cm-l jusqu'i environ 120 cm-l d 3060 cm-1; (2) f intensit6des bandes montre une corr6lation non lin6aire avecla fr{uence de la bande,et en plus avecla teneuren H. Cesdeux g6ndralisationsont une importaacecapitale dansI'interpr6tation quantitative des spectreset dansI'attribution desintensitds des bandes aux traits compositionnels. (Traduit par la R6daction) Mots-cl6s:v6suvianite, spectre infrarouge, analyse i la microsonde6lectronique, sffucture cristalline, hydroxyle. 610 TIIE CANADL{N MINERALOGIST IurrotucnoN TAELE 1. VET'UVTANIIE SAMPLES IJSED FOR INFRARED SPECTROSCOPICWOFK n l@lity Olalitalivo ctmi@l d@dpdon Although the generalarrangement ofthe vesuvianite stucture has beenknown sincethe work of Warren & V4 Tmbac* Lake, N,W,T. F-rlch, low-Al, t gttsFEs+ Modell (1931), there are still details of its crystal V6 L@g trk6 M@, Olden Twp., Ofr. Hch, higFAl chemistry that remain obscure. Groat et aI. (I992a) V6 York Riw, Dungafiqn Twp., Ont. F lch, Iow-Fe showedthat the generalformula of vesuvianitecan be V1 l Jel{roy Mlna, Rlchmond Co., Ouo. Flr@. higFAl, lry-Fe. Tl-fr@ v13 Jsllby Mine, Fichmond Co., Ors. Fn@, hlgh-Al writtenX,nYBTsZnO6sWrc, whereX= Ca Na REE, V23 Laqsl,Ouo. Ffr@. trlgh-F€s,Gs$ Pb2+,sb3+, I= A1,Mg, Fe2*,Fe3*, Mn, Ti, Ct,Ct,Zn, V28 Tompleton Twp.. Ottsw Co., O@. lnt-F, Tidch T = n, B, Z = Si, and 17= OH, F, O2-.A particularly V30 Templeton Twp., Ottawa Co., Ouo. lnt-F. Brlch, low€H important aspectof this work was the recognition (or V31 Wakefidd twp., Ottawa Co., Ore. lrt-F, I-riah rather the re-emphasis)of the importanceof boron in V33 Mmo Clalm, Y.T. tsncn vesuvianitefrom somelocalities. Groat et al. (1992a) Vsg tuiccia, Roms, ltaly Int4, Int-B, hlgttsREE proposedthat B is incorporatedinto ttle vesuvianite v46 Lag@ d€l Ja@, Ctthu6h@, Medoo lm-F, irt-B v49 Bal@histan,Poklstan Fdch Ti-ir@, high-Al, low-Fo structure particularly Hlndlb8gh, via a substitutionthat is difficult \/75 Wlui Riwr, Yalqbkays, Rusda F{r66. Rrlch, lw-oH to recopize solely on the basisof electron-microprobe ALA Val d'Ala. Piomorne, ltely data: B + Mg * 2H + N. This situation is further cEM Teuqnia complicatedby the fact that B also canbe incorporated by a mechan:ismthat involves the substitutionof REE for Ca at the X positionsin the structure. Groat et al. (L994a) showedthat B occurs at two distinct sitesin the structure,sites that are unoccupied adhesive. in B-free vesuvianite.The local details around these sites are somewhatobscured by extensivepositional Chernicalanalysis disorder. Nevertheless,it is apparentthat B locally replacesH in the structure. Many important substi- Chemical analyseswere done with a JEOL 733 tutions in vesuvianite involve variable amounts of electronmicroprobe fitted with one energy-dispersion hydrogen incorporated via (OH)- = 02- exchange. spectrometer and four automated wavelength- Becauseof this, infrared spectoscopy in the (OID- dispersionspectrometers. A detaileddescription of the stretching region should be a usefirl probe of local analytical procedures was given by Groat et al. stereochemicalvariations in the vicinity of the O(10) (L992a).For some samples,H2O and Fe2+/Fe3+ratios and OH sites in the vesuvianite structure. were determinedas describedby Groat et al. (L992ay Consequently,this study was initiated in parallel with Becauseof the amount of samplerequired for these the concurrentwork on the chemistryand structureof measurements,bulk sampleshad to be used for all vesuvianite(Groat et al. I992a,b, 1993,l994ub). crystals, including those showing sector zoning; thus we haveno direct information on the variation of these E:rpsRNB{TAL componentsin single crystals. The samplesused in this work are listed in Table 1; In{rared spectroscopy the letter coding correspondsto that of Groat et al. (1992a).Oriented sectionsof single crystalswere cut Infrared spectrawere measuredon a Nicolet 60SX using, where possible, the external morphology as a FTIR spectrometerfitted with a rotatableglan-Foucault guide, and the orientations of the resulting sections LiIO3 crystal polarizer; aperhres (50-200 pm in were checked by optical interference figures. For diameter) were used to limit the area of the sample sampleswith no exlernal morphology, crystals were exposedto the beam.Crystal fragments were examined oriented by X-ray precessionphotography, sunk into with a polarizing-light microscope,and points were a block of 5-minute@epoxy and then sawninlo slices selectedfor measurementon the basisof crystalquality of appropriate orientation and thickness. Sections (absenceof alteration,cracks and inclusionswithin the 2.04.5 mm thick were groundand doubly polishedfor field of the aperture);the crystal was tlen orientedon optical and electron-microprobemeasurements. The a polarizing microscope prior to insertion into the sections were then removed from the glass slides, spectrometer. remountedwith Crystalbond@adhesive, handground to a thicknessin the range 1G-90pm, dependingon the Rssulrs H content,and
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