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Coupled Substitution of H and Minor Elements in Rutile and The American Mineralogist, Volume 78, pages I 181-1I9I, 1993 Coupled substitution of H and minor elementsin rutile and the implications of high OH contentsin Nb- and Cr-rich rutile from the upper mantle DrvrrrnrosVr-lssopour,os S. S. Papadopulosand Associates,Inc.,7944 Wisconsin Avenue, Bethesda,Maryland 20814-3620,U.S.A. Gponcn R. Rossnn.q,N Division of Geological and Planetary Sciences,California Institute of Technology,Pasadena, California 91125, U.S.A. SrnpHrN E. Hlccrnry Department of Geology, University of Massachusetts,Amherst, Massachusetts01003, U.S.A. Ansrucr Infrared absorption spectraof rutile crystals from a variety of geologicalenvironments (carbonatite,hydrothermal vein, kyanite * rutile + lazulite association,xenoliths that are kimberlite hosted and dominated by Nb- and Cr-rich rutile) exhibit strong absorption in the 3300-cm-' regtrondue to interstitial protons bonded to structure O. In general the proton is located at sites slightly displaced from t/zYz0of the unit cell, although some samplesshow evidenceofadditional protons at tetrahedral interstitial sites.H contents of rutile range up to 0.8 rvto/oHrO, the highest concentrations occurring in mantle-derived Nb- and Cr-rich rutile of metasomatic origin. The role of H in rutile was examined, particularly with respect to its relations to other impurities. Protons are present in the rutile structure to compensatefor trivalent substitutional cations (Cr, Fe, V, Al), which are only partly compensatedby pentavalent ions (Nb, Ta). The possibility of using the H content of rutile as a geohygrometeris illustrated for the caseof coexisting hematite and rutile. INrnonucrroN crystalsto an OH stretch mode. The study by von Hippel (1962) pro- Rutile is known to have a greal affinity for H, and et al. suggestedthat there were two interstitial positions therefore presentsa unique opportunity for a casestudy ton between O" and Oo, each with a fourfold of H incorporation mechanismsin nominally anhydrous degeneracy(Fig. l). This was reinterpretedby Johnson et (1968), minerals. Becauseof its industrial importance, there has al. considering the dichroicity of the OH absorp- been much effort directed toward understanding the de- tion, which constrained the OH dipole axis to be in the fect chemistry of rutile, motivated by its specialdielectric basal (001) plane of the rutile structure. Johnson et al. and optical properties, which have found use in numer- (1968) consideredthe two possiblesites that satisfiedthis t/zt/20 ous applications.The existenceof structurally bound requirement, and 7200(Fig. l) and concluded that OH t/200 in rutile was first recognizedby Soffer (1961). Since then the site was energeticallymore favorable. The ob- peaks several studies bearing on H in rutile and its effect on served satellite were reinterpreted as impurity-as- (Al, groups, electrical and optical properties of synthetic as well as sociated Ga, Ni, Mg) OH suggestingthat H plays natural rutile have appeared (vide infra). Because the a role as a charge compensator in trivalent cation structural site and speciation of H in synthetic rutile is doped rutile, in support of the suggestionof Hill (1968). (1974) rather well constrained, a thermodynamic treatment of Furthermore, Ohlsen and Shen and Andersson et (1973,1974) H in rutile is feasible, provided realistic reactions for H al. found evidencefrom electronspin res- incorporation can be inferred from onance and infrared spectraofFe-doped rutile for Fe3*- compositional rela- t/zt/zYz. tions among the various minor components present. In OH defect association,consistent with Fe3+at the proton this study, we analyzedrutile in xenoliths from the upper Ti4+ site and the near the Yz00interstitial site. (1968) mantle, carbonatite, kyanite quartzite (kyanite * rutile Johnson et al. were not able to resolve impurity- peaks + lazulite association), and other geological environ- associatedOH in the infrared spectra of rutiles ments and infer from the compositional systematicsin doped with Fe, Cr, and V and concludedthat either there groups natural rutile, in light of previous studies of its defect was no association of OH with these cations or chemistry, the controls of H incorporation in rutile. the resultant shifts in the OH stretch band were too small to be detected. Pnrvrous woRK oN sTRUCTURALOH rN RUTTLE Beran and Znmann (1971) reported the occurrenceof Sotrer(1961) assigned a narrow doubleband near 3300 a sharp absorption band at 3270 crn-'due to OH groups cm I in the infrared spectrum of pure Verneuil-grown in two rutile crystals. In a recent study, Hammer and 0003-{04xl93/ | | t2-r 18 I S02.00 I 181 I 182 VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE c + I a Fig. 1. Structureof rutile. (a) Rutile unit cell utith t/2t/20afld, equivalentinterstitial sites labeled (open squares). (b) A 2 x 2 I unit-cellprojection onto (001)showing the locationofH at dis- i placedrht/20 site. Larger white and stippledcircles represent O, smallerblack and gray circles represent Ti, andsmall white circle : representsH. Black and white atoms are at z 0 and stippled b atomsare projected up from one-halfunit cellbelow. +_a_-+ Beran (1991) presented IR spectra of rutile from peg- element concentrations.The first possibility is likely, giv- matitic veins in mafic intrusives, fissuresin low- to me- en that the xenoliths in the Roberts Victor kimberlite dium-grade metamorphic terrains, as well as high-pres- were entrained from the upper mantle, whereasthe Dora sure environments (eclogite and blueschist). This study Maira eclogiteswere formed at shallowerdepths. The sec- establishedthe widespreadoccurrence of OH in rutile of ond possibility is not as likely, since H readily diffuses crustal origin. The rutile samplesall exhibit a sharp band through the rutile structure (Johnson et al., 1975). The at 3280 cm-r, and some show additional peaks at 3320 third possibility is given credenceby the fact that the less 1. or 3360 cm The OH contents of the samples,deter- impure crystal had the lower OH content. It thus appears mined from calibrated spectra,ranged from 0.04 to 0.2 that a combination of physicochemical environmental wt0/oHrO. The concentration of OH groups was found to conditions and the availability of substitutional cations be correlated with minor component concentration, and must be responsiblefor variations in the OH content of thdre was some indication that the environment of for- natural rutile. mation played a role in determining OH content. Rossmanand Smyth (1990) presentedinfrared spectra Ml.rnnrAl-s AND METHoDS of Nb- and Cr-rich rutile from mantle eclogite xenoliths Polarized infrared absorption spectra and chemical with surprisingly intense absorption peaks at 3300 and compositions of both natural and synthetic singlecrystals ', 3320 cm interpreted as being due to an elevated con- of rutile were measured.The natural samplesinclude ru- tent of structural OH. A neutron difraction study of a tile from a variety of geological settings:chlorite schist, rutile crystal from the same sample (Swopeet al., 1992) kyanite quartzite deposits (aluminum silicate * rutile * contributed the first direct evidencefor the location ofH lazulite association),hydrothermal vein, carbonatite,and in rutile. In contrast to the earlier studies of synthetic kimberlite-hosted rutile-silicate and rutile-ilmenite nod- rutile, it was found that the H position was much closer ules of upper mantle origin (Table l). The synthetic sam- to Vzt/20than to Y200.Because of its implications, the dis- ples consist of nominally pure and impurity-doped (Fe covery of elevatedOH content in mantle rutile prompted or Al) crystals. the present study to examine additional mantle rutile from The larger crystals were cut and polished into plates other associationsin order to determine whether mantle 0.03-l mm thick oriented such that they contained the rutile crystals are typically hydrous. c axis parallel to the polished surfaces.The finer grained The eclogitic rutile studied by Rossman and Smyth samples(aggregates of grains < I mm in greatestdimen- (1990) containsan order of magnitudemore OH (-0.7 sion) were mounted on glass slides with thermoplastic wto/oHrO) than the eclogitic rutile of Hammer and Beran cement prior to polishing the secondside, and crystalsof (1991) (0.08 wto/oHrO), suggestingthat rutile from the suitable orientation were subsequentlylocated with a po- two eclogite samples(l) equilibrated in significantly dif- laizing microscope. ferent physiochemical environments (e.g., HrO activity, Polarized infrared absorption spectra were obtained pressure,or temperature),(2) did not record equilibrium, with a Nicolet 60SX Fourier-transform spectrophotom- or (3) equilibrated under similar conditions, but different eter using a LiIO. polarizer. Spectra were measured on amounts of OH were incorporated due to different minor regions of the samplesthat were free of fractures, inclu- VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE 1183 TlaLe 1. Rutilesamples studied TABLE2. lR absorptionin rutilein the 3300-cm-' region Sample Locality Association Linear absorbance Integrated Natural absor- lon Ic llc bance H.O H GravesMountain, GA kyanite + rutile + ') '). lazulite Sample (cm (cm (cm'). (cm') (wto/") (pfu) R-2 Graves Mountain, GA kyanite + rutile + Natural lazulite R-4 Magnet Cove, AR carbonatite R-1 3290 32.9 4435.0 0 29 0.026 R-5 Polk County, Tt\ isolated crystal 336s 22.9 3.3 R-6 PA-MD border magnetite + apatite + R-2 3290 2V.6 4756.0 0.31 0.027 chlorite schist 3365 24.0 R-7 Namibia hydrothermalvein
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