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 R-4 3290 10.6 1841.0 0j2 0.011 R-8 Pomfret, VT hydrothermalvein F{-5 3290 I 5 1476.0 0 10 0.0085 JAG83-30 Jagersfontein,South kimberlite-hostedrutile- n-o 3290 82.8 4260.0 0.28 0.025 Africa ilmenitexenolith R-7 3290 33.4 1775.0 0j2 0.010 JAG85-2 Jagerstontein,South kimberlite-hostedrutile- R-8 3290 112.2 7381.0 0.48 0.043 Africa ilmenitexenolith JAG83-30-1 3290 133.3 8360.0 0.54 0.048 JAG83-30-2 3290 156.3 9233.0 0 60 0.053 Synthetic JAG83-30-3 3290 1853 12260.0 0.80 0.071 "Pure" CommercialCrystal Laboratories,Naples, FL JAG85-2 3290 61.3 3844.0 0.25 0.022 Al-doped CommercialCrystal Laboratories,Naples, FL Synthetic Fe-doped CommercialCrystal Laboratories,Naples, FL Pure 3278 3.24 88.7 0 0057 0.0005 3324 047 Al-doped 3278 J./J 200.2 0.013 0.0012 3324 2.20 sions, and other visible defectsthrough pinhole apertures Fe-doped 3378 2.00 59.9 0.0039 0.0003 between 100 pm and 1000 in diameter. For mounted - Absorption intensity per centimeterfor indicatedpolarization samples, the spectra were corrected for the effects of .'The (001)section. the glass slide and mounting medium by subtraction. Samplethicknesses in the region where the infrared spec- tra were taken were measuredwith a digital micrometer bandsin the regionof4000-2000 cm 'are given in Ta- + to I rrm. ble 2, and representativespectra are shown in Figure 2. Selectedsamples were reacted with DrO in a Teflon- The pure and Al-doped synthetic samples show a main lined pressurevessel (190'C and saturatedvapor pres- peak at 3278 cm-t and a satellite peak at 3324 cm-t. sure) or sealed in a Au capsule and placed in a rapid- oC in 'C After annealing at 1450 air for 60 h, the Al-doped quenchcold-seal vessel (500 and 100 MPa) for 24 h. crystal showedan overall increasein absorbance,and the IR spectrawere measuredbefore and after deuteration to relative intensity of the satellite peak doubled. The Fe- confirm that the peaks observed in unreacted samples doped crystal shows only the main band at 3278 cm I were all due to OH vibrations. (Fig. 2a). Quantitative chemical analyses of the rutile samples The IR absorption spectraof natural rutile crystalsare were caffied out with a Jeol7 33 electron microprobe ( I 5- broadly similar to those of the synthetic crystals,although kV acceleratingvoltage, 15-nA Faraday cup current) and satellite peaks are generally not resolved in spectra of on-line automated correction procedures.Prior to micro- natural rutile. The spectratypically show a single intense probe analysis the samples were screenedon an X-ray but broader band, with the peak at 3290 cm '. In two fluorescencespectrometer in order to determine the mi- samplesfrom Graves Mountain (R-l and R-2), in addi- present nor elements at detectable concentrations, as a tion to the peak at 3290 cm-', another broad peak at guide in the selection of elements to be quantitatively 3365 cm-l, of subordinateintensity, was observed.Po- analyzed. laized spectrareveal that, in contrast to the dipole giving I Rnsur,rs rise to the 3290-cm peak, the orientation ofthe dipole causingthis absorption is not confined to the basal plane IR spectroscopy (Fig. 2b). The polarized spectraconfirm that the absorption fea- The sample with the most intense absorption in the tures in the infrared region are intense and strongly ple- 3300-cm ' region is JAG83-30, Nb- and Cr-rich rutile ochroic, the greatestabsorption occurring when the elec- from a rutile-dominated xenolith from the Jagersfontein tric vector (E) of the source radiation is polarized kimberlite, South Africa. perpendicularto the crystallographicc axis of the sample. The substitution of H by D in an OH harmonic oscil- Calculation of H concentrationin rutile lator results in the appearanceof new absorption bands Calibrations of the infrared absorption intensity of the il v6o : vo./1.374. As expected,new peaks in the IR OH groups for estimating the H content in rutile have spectraofpartially deuteratedsamples appeared atvoD x beenpresented by Johnson etal. (1973) and Hammer and vo./1.35, confirming that all the features in the 3300- Beran (1991).Johnson et al. (1973) determineda value cm-' region are due to OH. of eo, : 15100(+ 500)L/(mol'cm'?) from D-H exchange The positions and relative intensities ofthe absorption experiments on synthetic rutile. Hammer and Beran I 184 VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE

1.5 (a) o)

o) 8 r.o o Fe{oped

E a 9, I o.s Al{oped € - 1 jl,,1 i \'Pure" 0.0 0r_ 3000 3200 3400 3600 3800 4000 3000 32ffi 3400 3600 3800 4000 Wavenumbers(cm') Wavenwnbers(cm")

30 30 (c) (d) () R-6 d 820 F20 7 k 6 R-? o JAG83-3G.3 *10

R-E 0r_ 3200 3400 3600 3800 4000 3000 3200 3400 3600 3800 4000 Wavenumbers(cm') Wavenumbers(cm')

Fig.2. Polarized IR spectraofrutile, plotted as 1000-pm samplethickness (EIc in all spectra,except in b). (a) Synthetic rutile. Note absenceof satellitepeak at 3320 cm-r in Fe-dopedsample. (b) Rutile from kyanite + quartzite association(Graves Mountain), R-1. Solid line is for polarization El-c, dashedis for Ellc. Note the broad peak at 3365 cm-' that has a component outside of the (001) plane. (c) Crustal rutile from other hydrothermal vein and low-grade metamorphic associations:R-6, R-7, R-8. (d) Mantle- derived Nb- and Cr-rich rutile from South African kimberlites (Jagersfontein),JAG83-30-3 and JAG85-2.

(1991)determined eo" : 3270 L/(mol.cmr) for a natural tains minor components.Minor substitutionsin rutile are rutile sample with 0.09 wto/oHrO as measured by cou- usually dominated by Fe and V, although Nb, Ta, and lommetry. OH concentrationscalculated with this value Cr may be found in high concentrations @eer et al., 1962; of eo" have an uncertainty of x.20o/o. Putnis and Wilson, 1978; Rumble, 1976; Waychunas, The large disparity in the two values of eo" is unex- l99l). Rutile samplesfrom xenoliths whose assemblages plained. Although both experimentsappear to have been are believed to record metasomaticprocesses in the man- carried out with care, we choseto use the molar absorp- tle are characteized by higher total impurity concentra- tion coefficient of Hammer and Beran (1991) becauseit tions, dominated by Cr and Nb, with distinctly higher was based on a natural sample with an OH absorbance concentrationsof Zr (Joneset al., 1982; Haggerty, 1983, similar to thosereported here, and furthermore was based 1987, l99l; Tollo and Haggerty,1987). on the actual amount of HrO releasedfrom the rutile on thermal decomposition, and is more in keeping with eo, DrscussroN oF RESULTS values recently determined for other minerals (Rossman, 1987). The calculated OH contents of the rutile samples H speciationand location in rutile are given in Table 2. The polarization of the OH vibration precludesthe ex- The OH contents of the Nb- and Cr-rich mantle rutile istenceof fluid inclusion or interstitial HrO in any of our samplesare the highest and confirm the finding of simi- samples.H in rutile therefore is presenteither as protons larly high OH content in rutile from a mantle eclogite bound to structure O or interstitial OH-. Becausethe O xenolith by Rossmanand Smyth (1990). atoms of rutile are hexagonalclosest-packed, the incor- poration of interstitial anions is not considered likely; Minor componentsin rutile thus the dominant speciesmust be protons bonded to The compositions of the rutile samples studied are structure oxide ions. The fundamental OH vibrational summarized in Table 3. Natural rutile invariablv con- band in rutile is shifted to lower wavenumbers relative VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE I 185

TABue3. Chemicalcompositions of rutilesamples on the basisof two O atoms

Sample Mg Natural R-1 3 0.9873 0.0049 0 0110 0.0100 0.0004 1.0136 0.0012 0.0002 0.0003 0.0016 0.0001 0.0031 R-2 3 0.9893 0.0040 0.0004 0.0088 0.0005 1.0030 0.0012 0.0013 0.0001 0.0017 0.0001 0.0043 R-4 0.0002 0.9773 0.0017 0.0087 0.0110 0.9991 0.0001 0.0073 0.0002 0.0018 0.0041 0.013s h-c 3 0.0003 0.9853 0.0036 0.0004 0.0107 0.0025 1.0028 0.0001 0.0005 0.0001 0.0001 0.0001 0.0002 0.0010 R-6 2 0.9908 0.0008 0.0095 0.0008 1.0019 0.0010 0.0001 0.0004 0.0002 0.0020 R-7 3 0.0002 0.9813 0.0023 0.0030 0.0157 0.oo22 1.0046 0.0001 0.0005 0.0001 0.0009 0.0003 0.0004 0.0021 R-8 3 0.9930 0.0015 0.0005 0.0068 0.0003 1.0020 0.0001 0.0001 0.0001 0.0005 0.0001 0.0009 JAG83-30-1 1 0.0024 0.0008 0.9110 0 0020 0.0669 0.0046 0.0043 0.0200 0.0021 1.0140 JAG83-30-2 ,|I 0 0012 0.9170 0.0022 0.0623 0.0018 0.0042 0.0202 0.0019 1.0109 JAG83-30-3 0.0022 0.0036 0.8830 0.0021 0.0924 0.0148 0.0044 0.0194 0.0021 1.0240 JAG85-2 4 0.0012 0.0009 0.9155 0.0022 0.0572 0.0115 0.0040 0.0195 0.0010 1.0130 0.0008 0 0002 0.0008 0.0002 0.0054 0.0048 0.0001 0.0001 0.0002 0.0123

Pure 3 - 1.0000 -synthetic 1.0000 0.0001 0.0001 Al-doped3-0.00050.9993 0.9998 0.0001 0.0005 0.0005 /Vote:compositions calculated from electron microprobe analyses; - : below detection limit; Na, Ca, Mn, and Si were also analyzed for but not detected. second rows indicate standard deviation for replicateanalyses of the same crystat.

to the isolated OH- stretch (-3735 cm-,), due to H in rutile from mafic pegmatitesis also likely to be due to bonding, and the shift correspondsto an OH...O dis- minor element-associatedOH, possiblywith Mg for which tance of -0.28 nm (Nakamoto et al., 1955).The con- 26, : 3350cm ' in syntheticrutile (Johnsonet a1.,1968). straint placed on the OH dipole orientation by the polar- Unfortunately this cannot be confirmed becauseMg con- ized spectralimits the possiblelocation of H to only two centrations were not reported in Hammer and Beran's nonequivalent sites, t/zt/20and, Yu00. The O-O distances (1991)study. acrossthese sites are 0.2533and 0.3328nm, respectively The satellitepeak at 3360 cm-' in the spectraof R-l (Baur, 1956; Meagher and Lager, 1979; Sugiyamaand and R-2 (Fig. 2b) is not likely to be due to Mg-associated Tak6uchi, l99l). This suggeststhat the t/zr/zOsite (Fig. OH, as the Mg content of thesesamples is too low (Table lb) is the more likely location for H, and this was con- 3). The polarization dependenceofthe absorption bands, firmed by the study of Swopeet al. (1992). however, suggestthat part of the H in this sample may The Al-associatedOH peak is presentin both the pure be located aI the YzOt/csite, which is a distorted tetrahe- and Al-doped slnthetic crystals,but there is no impurity- dral site in the channelsof the rutile structure (Johnson associatedpeak in Fe-doped rutile (Fig. 2a). As Johnson et al., 1968). This is to our knowledge the first report of et al. (1968) pointed out, either there is no association OH vibrations not in the (001) plane of rutile and may betweenFe and H or the resulting shift is too small to be reflect specificphysiochemical conditions encounteredin resolved. the formation of the aluminum silicate + rutile + lazulite Hammer and Beran (1991) regardedthe presenceof association(Geijer, I 963). the satellite peak as an indication of formation at high In attempting to understandthe crystal chemistry of H temperature, since it was observed in rutile from mafic in rutile, important constraints are placed on possible H pegmatites (3360 cm '), as well as from eclogite and incorporation reactionsby the requirement of electroneu- blueschist(3320 cm-r), and was also observedby Ross- trality. Point defectsthat may be compensatedby inter- man and Smyth (1990) in mantle eclogites.In light of the stitial protons include Ti vacanciesand trivalent or di- foregoingdiscussion, this interpretation is not necessarily valent cation substitution. On the other hand, there may correct. The peak at 3320 cm-l is ascribed to Al-associ- also be defectsthat compete with interstitial H in charge ated OH groups, and this conclusion is supported by the compensation. These include pentavalent cation substi- relatively high concentration of AlrO. in the Dora Maira tutions, interstitial cations, including Ti4+, and anion va- eclogiticrutile (0.21 wto/o)of Hammer and Beran (1991) cancies. For the purposes of the present discussion, we and the eclogite xenolith rutile (0.68 wto/o)of Rossman will not considerintrinsic defectsin assessingcharge-cou- and Smyth (1990).The satellitepeak at 3360cm ' found pled compensation mechanisms of H incorporation be- 1186 VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE

causethe minor component concentrationsin natural ru- evidence of Fe2*. Preliminary X-ray absorption studies tile are much higher than intrinsic point defect (i.e., Ti of samplesR-l and R-7 indicate that Fe in thesesamples and/or O vacancy, Ti interstitial) concentrations. is predominantly trivalent, but in JAG85-2, near-edge featuresindicate both trivalent and divalent Fe (Waychu- Oxidation statesand sites occupiedby minor nas, personal communication). cornponentsin rutile Nb and Ta. Khodos et al. (1988) synthesizedrutile sol- In deducing plausible reactions for H incorporation id solutions at 1400'C with up to 25 molo/oNbO- but based on the electroneutrality condition, constraints re- concludedon the basis of inversion voltammetric studies garding the valence states and sites occupied by minor that above -3 molo/oNbOr5 Nbs+ is partly reduced to componentsin rutile are required. Nba+ and the more Nb-rich compositions should be re- Al. Stebbinset al. (1989),in an 27AlNMR studyof Al- garded as NbOr-TiO, solutions with the rutile structure. doped rutile, concludedthat only substitutional Al3+ was These results are in agreement with results from ESR present. Infrared spectra and electron microprobe anal- studies(Chester, I 96 l) and recentX-ray absorption spec- ysis of a slice from the same boule of Al-doped rutile troscopic investigations of TiOr-NbO, solid solutions, usedby Stebbinset al. (Al-doped, their Table 5) indicates which indicate that with decreasingmole fraction of NbOr, that OH groups compensatethe substitutional Al ions. the Nb5+/Nba+ratio increases(Antonio et al., l99l). An Y. ESR signals of synthetic V-doped rutile have been EXAFS analysiscarried out by the latter further indicates interpreted as being due to substitutional Vo* (Gerritsen that the averageNb-O distance decreasesapproximately and kwis, 1960) as well as interstitial Va* (Kubec and linearly with decreasingNb concentration and suggests Sroubek, 1972), dependingon the quenchingconditions. that an increasingfraction ofNb ions occupy tetrahedral Although V3+ possessestwo unpaired electrons,it is usu- interstitial sites with decreasingNb/(Nb + Ti). The con- ally difficult to detect by ESR. Therefore, the absenceof clusion that may be drawn from studies concerning Nb a V3+ ESR signal does not necessarilyimply the absence coordination and oxidation state in rutile is that Nb pre- of this cation. H insertion compounds of VO, (rutile dominantly occupiesnormal Ti sites, although it can be structure), with the general stoichiometry H,VOr, have tetravalent or pentavalent. Becauseofa paucity ofinfor- recently been synthesizedby Chippindale et al. (1991). mation about Ta in rutile, however, it is assumedthat, For compounds with x < 0.08, no changein XRD-based like Nb, Tas+ occupiessubstitutional sites. structure is observed, implying partial reduction of Va+ to V3+, concomitant with H insertion. Furthermore, Extent of substitution by minor componentsin Chavenaset al. (1973) have synthesizedVOOH in an rutile orthorhombic modification of the rutile structure, with The applicability of a charge compensation model in H ions at displaced Yz00sites ofthe unit cell. V in rutile- linking the H content of rutile to the thermodynamic con- type structures may therefore occur in the trivalent or ditions of H trapping requiresthat the minor components tetravalent state. be in homogeneoussolid solution in rutile and not in Cr. Gerritsen et al. (1959, 1960) attributed the ESR extended defects such as crystallographic shear (CS) spectrum of Cr-doped rutile to substitutional Cr3+ ions. structuresor exsolved phases. Ishida et al. (1990) combined ESCA, ESR, and diffuse CS structures occur in synthetic reduced rutile TiO, , reflectancespectroscopies to study the oxidation statesof (Hyde, 1976, and, referencestherein), rutile with high Cr in rutile and found predominantly Cr3*, as well as concentrationsof trivalent impurities such as Fe, Cr, V, minor Cra+, incorporated in the structure, the latter only Ga, and in natural rutile (Banfield and Veblen, 1991). at higher Cr concentrations. Chavenas et al. (1973), in The normal rutile structure consists of strings of edge- addition to VOOH, synthesizedCrOOH with the modi- shared [TiOu] octahedra (parallel to c) in two mutually fied rutile structure. In light of thesestudies, the trivalent perpendicular orientations and joined by corner sharing. substituting cation is expectedto be the dominant form Progressivereduction of Tio* or the introduction of tri- of Cr in natural rutile. valent impurities induces the formation of oriented pla- Fe. There are numerous studies dealing with the oxi- nar defects with the hematite structure and results in a dation state of Fe in synthetic rutile. As with the VOOH homologous seriesof related structures,termed Magn6li and CTOOH, Chavenaset al. (1973) synthesizedFeOOH phases,(Ti,M),Or,-1, with /?depending on the CS plane in the modified rutile structure. ESR (Carter and Okaya, density. 1960; Anderssonet al., 1973; Ohlsen and Shen, 1974) Experimental phase equilibria at 0.1 MPa in the sys- and Mdssbauer(Nicholson and Burns, 1963; Alam et al., tems AlO,5-TiO2,CrO,r-TiOr, VOr j-TiO2, NbO25-TiO2, 1966; Stampfl eI al., 1973 ; Sandin et al., lg7 6: yamarkin TaOr r-TiOr, and FeO-FeO, 5-TiO, indicate that the com- et al., 1978) spectra of synthetic Fe-doped rutile consis- positions of our rutile samples are generally within the tently indicate that the dopant is predominantly Fe3* and rutile single-phasefield (Brach et al., 1977; Bursill and occupiesnormal Ti sites. In contrast, wet chemical anal- Jun, 1984;Khodos et al., 1988;MacChesney and Muan, yses of Fe-bearing rutile reported by Henriques (1963) 1959;Mertin etal.,1970',O'Keefe and Ribble, 1972;Roth indicate that Fe2+predominates in some natural samples. and Coughanour, 1955; Slepetys and Vaughan, 1969; UV-visible spectraof the samplesin Table I show no Taylor, 1964; Websterand Bright, 1961; Wittke, 1967; VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE I 187

Woerman et al., 1969). In addition, HRTEM images of 015 (a) samples R-6 and JAG85-2 (J. Banfield, personal com- .s munication) no initially, show evidenceof CS structures \ + although annealing under the electron beam resulted in 0.t l+ J+ nm) comparedwith the valuefor pure TiO, (a:0.45935 trr + l9), as would be expected for a substitutional solid 1 solution. O

oa2 0 03 H and its relation to minor componentsin rutile Nb+Ta pfu Following the argumentsand evidencepresented in the precedingsections, it would seemreasonable to assignthe minor componentsto substitutional sites,with oxidation statesof 3+ for Cr, Fe, V, and Al,2+ for Mg, and 5+ I OO rr for Nb and Ta. Coupled substitutions of the type M3+ + 0.r5 T M5+ - 2Tia+ and M2+ + 2Ms* * 3Tio+are requiredto * maintain electroneutrality. There is evidence for such o I charge-coupled in I substitution rutile, and there are sev- oo +r eral Ti-free oxide end-members that have been synthe- , 01 T-l- T IJ- sized or found in nature. In the absenceofH interstitials. o) the chargebalance in rutile is fa + + * + + 2[Mgr+]., l-. [cr.*]r, [Fe3+]r, [Vr*]r, [Al3+]rt o 005 : [Nb5+]1; * [Ta5+]r, (1) where representsthe mole fraction of M at Ti sites. [M]r, (b) Figure 3 illustrates the covariation of minor element concentrations in natural rutile from a wide variety of 0 005 0.1 015 0.2 environments, including samplesanalyzed in the present work and analysesfound in the literature. There is gen- Nb + Ta pfu impurities pentavalent erally an excessof trivalent over Fig. 3. (a) Correlation between Cr * Fe + V + Al and Nb pdr formula unit, which is presumably compensatedby + Ta contents in rutile. The line representsthe trend expected H+ interstitials (Fig. 3), If this is the dominant incorpo- for the charge-coupledsubstitution M3+ + M5+ : 2Ti4+. Solid ration mechanism for H in rutile, a one-to-one coffela- circles are from the present study, open circles are data from tion between the calculated charge deficit per formula unit Hammer and Beran (1991), and the square is from Rossman and the concentration of H per formula unit is expected: and Smyth (1990). In general,natural rutile exhibits an excess of trivalent over pentavalent substitutional cations. (b) Similar :2[Mr*],, - [H*] + 2>[Mr+]ri 2[M'*],,. (2) plot for Nb- and Cr-rich rutile from upper-mantle xenolith suites Figure 4 supports this hypothesis,although there is con- from South African kimberlites. The solid line defines the ex- pected trend for charge-coupledsubstitution as in a. Solid squares siderablescatter in the correlation. The scattermay result are for rutile associatedwith ilmenite from Orapa. Open sym- partly from substitutional cations in multiple oxidation bols are for samplesfrom Jagersfontein:squares : rutile asso- states(Fe, and possibly V, Nb). Calculation of a unique ciated with ilmenite; circles : rutile associatedwith lindsleyite distribution of cation oxidation states is not possible, mathiasite; diamonds : rutile associated with armalcolite [data however, from the microprobe data alone. The effect of from Tollo and Haggerty (1987) and Haggerty (unpublished the assumedoxidation state of an impurity on the cal- analyses)1. culated chargedeficit, however, is obvious. For example, if any Fe is presentas Fe2+or any Nb is tetravalent, then the chargedeficit per formula unit will be underestimated with wet chemical analyses of Magnet Cove rutile re- by Equation 2. On the other hand, if any V is tetravalent, ported by Henriques (1963). the chargedeficit per formula unit will be overestimated. Despite the scatter in Figure 4, an assessment of the The result of assumingall Fe to be Fe3* vs. Fe2+is shown validity of our choice of eo" in rutile can be made. The in Figure 4. It is interesting to note that for sample R-4 H contents calculated using eo, ofJohnson et al. (1973) (Magnet Cove), the chargebalance is satisfied only if all would be approximately one-fifth of the values reported Fe is taken as divalent (Table 3). This is in agreement in Table 2. The H contents calculated with Hammer and I 188 VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE

drous vapor phase. At 0. I MPa the two phasescoexist stablybelow -585 "C and fo,at or abovethat definedby hematite-magnetitecoexistence (Anovitz et al., 1985).At a given pressure, temperature, and fo, where the two 008 phasesare in equilibrium, a nonstoichiometric solid so- R lution of the hematite component in rutile (Fer" 006 Ti,_r,)O, , will exist. The substitution of Tio* by Fe3* requires compensation,in this caseby an intrinsic point defect. Although the intrinsic point defect structure of rutile is by no means well understood, O vacanciesare believed to predominate under relatively oxidizing con- ditions (Millot et al., 1987, and referencestherein), al- though local reconstructionofthe structurein the vicinity ,002 004 006 00r 0l 0r2 of the anion vacancy may occur (Bursill and Blanchin, Cr+Fe+V+Al-(Nb+Ta) pfu 1984).The solid solution of Fe,O. in rutile in the absence of H,O may thus be describedbv o12 FerOr,n"-,-2Fe+l + 3Oa + Vo (3a) (b) all Fe2+ 0t FerO.,n.-)- 2FeO, rr,uo* Vor,uo (3b) 008 R where Vo,^,, is a doubly ionized anion vacancy and the 006 subscriptson the right side in Equation 3a refer to rutile structure sites.With the assumption of ideal behavior for low defect concentrations,the equilibrium for this reac- tion is

K*(P, 7:,f,) : X[FeOr5(rur)]2.x (4) where X[FeO, is the mole fraction of FeO, in rutile 0 r1-,)] , and x is the departure from stoichiometry due to anion vacanciesin rutile (Ti,_r*Fer,)Or_,.At constant pressure, Cr + 2xFe+ V + Al - (Nb + Ta) pfu temperature, and fo, in the dry system, the solubility of Fig.4. H contentvs. calculatedcharge deficiency. Symbols FeO, , in rutile is fixed. In the presenceof a hydrous va- as in Fig. 3a. Solidline representsexpected trend for l:l com- por, however, the incorporation of HrO and formation pensationofcalculated charge deficiency by interstitialprotons. of interstitial H+ in rutile may be written (a)Charge deficiency calculated assuming all Feis Fe3+.(b) Charge - deficiencycalculated assuming all Fe is Fer+.Within the range HrO,, 2H,r + Oa-. (5) of uncertaintyin oxidationstate of Fe,most of the samplesfall The interstitial protons can compensate substitutional arounda l:l relationbetween H contentand calculatedcharge Fe3+and the oxide ion can fill the anion vacanciescreated deficiency. by the nonstoichiometric solid solution to maintain neu- trality in the crystal. By combining Equations 3 and 5, the solubility of hematite in rutile in the presenceof HrO Beran's (1991) €oHare clearly in better accord with the can then be describedas expectedcorrelation with minor elements. FerO36"-y+ H2O(,)* 2HFeOr,.,,r (6) Application of the H content in rutile as a geohygrometer for which the equilibrium constant may be expressed Becauseof its affinity for H incorporation, rutile may be of use as an indicator of the fugacity or activity of HrO X[HFeOr,,,,,]' accompanying geologic processes.In principle, this re- &(P, T,f",): (7) J H)O quires the formulation of a thermodynamic model based on the relations found between H content and minor Keeping in mind that X[HFeO,,,",,]is equal to the H con- component concentrations,taking into account the vari- tent per formula unit as determined from IR measure- ous reactions and electroneutrality constraints. To make ments, Equation 7 can be rearrangedto give such a geohygrometeruseful, experimental calibration of H (pfu) : T, (8) the effectsof pressure,temperature, O-H speciesfugacity, tI&(P, fo,).fn.of"'. and composition would then be necessary. Therefore, the H content of rutile is proportional to the Consider a simple hypothetical situation, the coexis- square root of fn,o in the vapor phase with which it is tence of rutile with hematite, at equilibrium with a hy- equilibrated. At high /",o, rutile may eventually reach VLASSOPOULOS ET AL.: H AND MINOR ELEMENTS IN RUTILE I t89 saturation with respect to the FeOOH component, thus The storageof small amounts of H in the mantle in reachingan upper limit to both the H and Fe concentra- nominally anhydrous minerals was proposed by Martin tions in rutile. and Donnay (1972), and our present state of knowledge Applicability of such a geohygrometerto real systems, is summarized by Bell and Rossman (1992). The relative however, is complicated, requiring a more elaborate importance of a given mineral as regardsthe H budget of treatment taking into account the various minor com- the mantle will obviously depend on the amount of H ponents.For a given rutile-bearing assemblage,/",o could that can be accommodated in the phase and its distri- in principle be estimated if independent constraints on bution and concentration in the mantle. T, P, and fo,are avaTlableand the T, P, and composition The H contents in mantle-derived rutile (Table 2) are dependenceofKu is known. among the highest reported for nominally anhydrous Potential problems in applying this geohygrometerto minerals(Rossman, 1987; Bell and Rossman,1992). Ru- natural systemsmay arise in establishing whether the P tile, although a common phase,is believed to be present and Z determined by thermobarometry are the same as in the mantle only at accessorylevels (Haggerty, 1983, that under which rutile last equilibrated with a hydrous 1987, l99l; Tollo and Haggerty, 1987; Rossman and phase.In light of the fact that protons diffuse much faster Smyth, 1990). Therefore, despite the fact that rutile has through rutile than any of the minor cations present a very great afrnity for H incorporation, it is not expected (Kingsburyet al., 1968; Hoshino et al., 1985; Sasakiet to be a significant mantle HrO repository, exceptpossibly al., 1985),the H content will essentiallybe fixed below on a very small scale,in regions of rutile-dominated as- temperatures at which minor component diffusion be- semblagessuch as the rutile-ilmenite and rutile-silicate comes negligible. The temperaturesat which H becomes nodules reported from Jagersfontein,Orapa, and other fixed dependon the cation in question, but for most tran- localities (Ottenburgs and Fieremans, 1979; Haggerty, sition metals diffirsion becomesinsignificant below - 500 1983,1987; Tollo and Haggerty,1987; Schulze, 1990). 'C. Complications may also arise from postentrapment The high H content of mantle-derived rutile may also modification of the H content caused by reduction or have implications for mantle silicate mineral relations, oxidation of Fe, V, or Nb, but this problem may be over- on the grounds that stishovite, the SiO, modification iso- come by examination of the compositional systematics morphous with rutile, may occur in the mantle transition in a suite of samplesfrom the same occurrence. zone as the result ofincongruent pressure-inducedphase Somequalitative conclusionscan neverthelessbe drawn transformations oforthopyroxene to B spinel or 7 spinel from a comparisonofH content and the calculatedcharge structuresthat require the stoichiometric releaseof SiO, deficiencyin specificsamples. At relatively low values of as stishovite(Fei et al., 1990;Gasparik, 1990). ,fr,o, intrinsic speciesare the dominant charge-compen- sating defects,and this should result in H contents that Acxi.,towI,nncMENTS are lessthan those predicted by Equation 2. At relatively We would like to extend our appreciationto Ulrich Klabunde (E.I. du high values of fr,o, on the other hand, H contents should Pont de Nemours and Company, Inc.) for providing the syntheticsamples be closely predicted by Equation 2. and stimulatinB discussionsand to J.L. Rosenfeld for a natural sample (R-8). We also thank Jill Banfield (University of Wisconsin,Madison) for H in Nb- and Cr-rich rutile from upper mantle her interestin this study and for accessto data prior to publication, Glenn xenoliths Waychunas(Stanford University) for contributions from XAS that sig- nificantly constrainedour interpretations,and David Bell for performing A rigorous thermodynamic treatment of H in rutile is the microprobe analysesand discussions.Helpful commentswere provid- beyond our present scope,particularly as regardsthe es- ed by Harry Green and Alison Pawley The researchwas funded in part sentially unknown effects of pressure on H and minor by NSF grantsEAR-91-04059 to G.R.R. and EAR-91-16551to S E.H. Division contribution no. 5284. element incorporation. The systematicsamong the upper mantle rutile values plotted in Figure 4 allow some in- ferencesto be made. The H contents of JAG83-30 and RrrnnnNcns crruo the Roberts Victor eclogiterutile of Rossmanand Smyth Alam, M., Chandra,S, and Hoy, G.R. (1966)Mdssbauer studies of spin (1990) closely match the calculatedcharge deficiency per relaxation of Fer* in TiOr. Physicsl*tterc, 22, 26-28. (1973) formula unit, whereasthe H content of is sig- Andersson,P-O , Kollberg, E L., and Jelenski,A. Extra EPR spec- JAG85-2 tra of iron-doped rutile. PhysicalReviews, 88, 4956-4965. nificantly lessthan required for chargebalance. From the - (1974) Chargecompensation in iron-doped rutile. Journal ofPhys- foregoing model of H incorporation in rutile, the limited ics C: Solid StatePhysics, 7, 1868-1880 data presented here strongly srggest the presenceof a Anovitz, L.M., Treiman, A.H., Essene,E.J., Hemingway, B.S, Westrum, hydrous fluid phaseand perhapslocal variability in E.F., Jr., Wall, V.J., Burriel, R., and Bohlen, S.R (1985) The heat fH,o phase in capacity ofilmenite and equilibria in the systemFe-Ti-O. Geo- the upper mantle during or after the formation of Nb- chimica et Cosmochimica Acta, 49, 2027-2040. and Cr-rich rutile. 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