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The Crystal Chemistry of the Tapiolite Series

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The Crystal Chemistry of the Tapiolite Series 631 The Canadian M ineralo g is t Vol. 34, pp.631,-647(1,996) THE CRYSTALCHEMISTRY OFTHE TAPIOLITE SERIES MICHAELA. WISE Departnunt of Mincral Sciences,Snr.ithsonian hstitution" Washington,D,C, 20560, U.SA. PETR dERNf Depamnentof GeologicalSciences, Unitersity of Maninba. Winnipeg,Manitoba R3T2N2 ABsrRAc"r Ferotapiolite-manganotapiolite-groupminslzls [@e,Mn)Ta2O6]axe formed as :rccessoryphases in rare-elementgranitic pegmatitesexhibiting moderateto high degreesof fractionation.Despite their relative paucity, 94 setsof unit-cell parameters and 194 chemical compositionswere compiled to characterizethe crystal chemistry of the tapiolite series.X-ray-diffraction studiesindicate that the degreeof orderand compositional variations stongly influenceunit-cell dimensionsof tapiolite.Natural tapiolite showswide rangesof structuralstate. Crystallization of disorderedphases at low lemperaturesis suggested;however, the available data are not unambiguous.Tapiolite chemistry is typically uniform (Fe > Mn, Ta > Nb), and compositional variations are the result of limited, but effective homovalentard heterovalentsubstitutions: (1) Mn2+Fe2+-r;(2) NbsiTas+-l; (3) Fek(ti,Sn)4rFe2+_r(IaM)5*-r and (4) @,sn)4,1Fe,Mn;{,6M,Ta)5*_2. ln association witl otler M,Ta,Sn-bearing minerals,tapiolite shows a distinct preferencefor Fd+ and Ta over Mn and Nb. Emicbment in Nb, Ti and Sn appearsto be commonin tapiolite from moderatelyfractionated pegmatites, whereas extreme Mn enrichmentis typical of highly fractionated pegmatitesof the petalite subtypeor metasomaticunits. Keywords:tapiolite series,crystal chemistry,order - disorder,tantalum" granitic pegmatite. SoM:uaRs Irs mindraux de la s6de ferrotapiolite - manganotapiolitetGe"IUn)TqO6l se pr6sententsous forme d'accessoiresdes pegmatitesgranitiques d 6l6mentsrares faisantpreuve d'un degr6de fractionnementintermddiaire tr 61ev6.Malgrd leur raret6, les r6sultatsde 94 affinementsde la maille 6l6mentaireet de 194 analyseschimiques sont disponibles,et serventi caract6riser la cristallochimie des min6raux de cette s6rie. Irs 6tudespar ditraction X permettent de voir que le degr6 d'ordre et les variations compositionnellesaffectent fortement les parambtresr6ticulaires. L'ensemble des 6chantillons t6moigne d'une grandevari6t6 du degrd d'ordre. Nous pr6conisonsune cristallisation des phasesddsordonn6es i faibles temp€ratures, quoique les donn6esdisponibles sont assezambigu6s. La compositiondes mindraux de ce groupe est typiquementuniforme (Fe > Mn, Ta > Nb); les variations en composition peuvent s'expliquer par le degt6 des substitutionshomovalentes et hdt6rovalenteszuivantes, somme tout assezrestreint (1) Mn2+Fl+-1, (2) M5+Ia5+-1, (3) Fe3r(ti,Sn)4fFe2+-r(TaM)5*-r, et (4) @,Sn)e.(Fe,NrIn)11(Nb,Ta)5+_r.Tout commeles autresmindraux porteurs de Nb, Ta et Sn qui lui sont associds,les min6raux de cette s6rie semblentprdfdrer le Fe2+et le Ta au Mn et au Nb. Un enrichissementen Nb, Ti et Sn sembleassez courantdans les mindrauxde cette s6rie dansles pegmatitesgranitiques i degr6de fractionnementintermddiaire, tandis qubn enrichissementextreme en Mn est tlpique de rochesgranitiques pegmatitiques fortement 6volu6es, du sous-t''pede la p'6talite, ou bien d'unit6sm6tasomatiques. (lraduit par la R6daction) Mots-cVs: min6rauxde la s6riede la tapiolite, chimie cristalline, ordre - ddsordre,tantale, pegmatite granitique. INTR.oDUcrroN tantalite,wodginile and cassiterite,tapiolite is diffrcult to identify in hand specimens. Consequently, its Minerals of the tapiolite series, (Fe,Mn)Ta2O6, presence is often overlooked. However, recent occur primarily as accessoryphases in rare-element systematicstudies of 6[s minera]ogy of Nb and Ta granitic pegmatitesthat displaymoderate to high levels suggest that tapiolite, although subordinate to of fractionation, and rarely in some Ta-mineralized columbite - tantalite in overall abundatrce,is more ganites. Comme6y associatedwith columbite - commonthan was previously believed (demf & Ercit 632 TIIE CANADIAN MINERALOGIST 1985,Wise 1987).As part of au ongoingsystematic foil and heated in a Fisher Isotemp muffle firnace study of Nb and Ta minerals,we aim here to provide (Model 186) in air at 1000"Cat I atm for 16 hours. a review of the mineralogy, crystal chemistry and After heating, sampleswere cooled to room tempera- geochemistryof the tapiolite series. ture, X-rayed, and their unit-cell parametersrefined. The chemical data for this study were obtainedby PREVrousWoRK electron-microprobeanalysis at our laboratories,and selected data were collected from the literafure. Until recently,most studieson the tapiolite minerals Electron-microprobeanalyses were obtained at the were merely descriptive reports on its occurrences, Univenity of Manitobausing a model MAC 5 elecfron with little or no X-ray-diffraction and chemical microprobein the energy-dispersionmode @D). The information given. The tapiolite structure was first principal data-setwas augmentedby data obtainedon solvedby Goldscbmidt(1926), but later refined by von an ARL-SEMQ electron microprobe in wavelength- Heidenstam(1968), Weitzel & Klein (19'74),and Wise dispersionmode (WD) at the SmithsonianInstitution. (1987).Hutchinson (1955) (1958) for (ED) microanalysis were: andHutton werethe Operating conditions '1.-2 first to identify an order - disorder relationship in acceleratingvoltage 15 kV, beam diameter mm, nafural tapiolite minerals. Clark & Fejer (1978) and andcollection time 200 secondsfor eachspectrum. The Wise (1987) provided a thorough study on the following standardswere usedfor the study: chromite composition and unit-cell dimensions of tapiolite and fayalite (FeKcr), CaNb2O6 and Ba2NaNb5O' mineralsfrom various localities. (Nblc[), manganotantalite(MnKcr, TaMa), cassiterite Moreau& Tramasure(1965), Scbriicke (1966) and (Snl,CI),rutile and ilnenite CfiKa), and CaNb2O6and Turnock (1966) have attempted to experimentally microlite (CaKcl). Energy-dispersion spectra were define compositionalfields for tapiolite. Furthennore, collected with a KEVEX Micro-X 7000 ED Turnock(1966), Komkov & Dubik (L974a),atdZ:mio spectrometerand reduced with KEVEX computer et al. (1977)nyestigated the conditionsfor the stability softwareutiliz.ing the MAGIC V compulerprogfam of of tapiolite. Howeverothe presentstate of experimental Colby (1980). WD analyseswere carried out at an evidenceis far from satisfactory(Cem! et al. 1992). operating voltage of 20 kV and a sample current of 0.015 nA, with 20-s counts. The following standaxds E)pm.n/mt.rrAr.Msurox were used: MnNb2O6 (MnKct, NbZa), CaTaaOrt (CaKa, TaMa), cassiterite (SnZ{r), ilmenite @eKcq We used X-ray powder-dffiaction methods to TiKcr), Sc;TiO5 (ScKc) and MnWOo (WMcr). Data characterizethe degreeof cation order in unheatedand reduction was done using the procedureof Bence & heated tapiolite minerals. Heating induces order in Albee (1968). Back-scatteredelectron imagery was disorderedand patially disorderedmembers of the frequentlyused for examiningtextural featuressuch as series. Samples were scanned on an automated exsolution lamellae and inclusions. as well as for Philips powder-ditractometer system @W 1710) at routine checks for compositionalinhomogeneity and 6o 20lminute for the purposeof mineral identification zoning. At least two data sets (rim and core) were and at 0.60o2Olminute for the ref,inementof unit-cell collected for each crystal. In most cases,however, dimensions, with graphite-monochromated CuKa tbree to six points were analyzedacross the mineral radiation, operating voltage of 40 kV and operatiqg grain. Compositionsof some grains occasionally current of 40 mA. AnnealedCaF2la = 5.46379(4)AJ showedcation sumsin excessof sfructurally available calibrated against a NBS silicon reference standard sitesif all the iron is calculatedas Fe2+.Recalculation (barch640a; a = 5.430825A,) was used as an intemal of the unit-cell contentswas adjustedby convertingas standard in all slow scans. Peak positions and much Fe2+to Fe$ as required to eliminate the cation intensities were determinedwith the automatedPW surplus,f.e., ferric iron for fenotapiolile was calculated 1710 systemssing a computer-controlledcontinuous by normalizingto 12 oxygen atomsand 6 cations. scanning program (CSP), which searchesfor the tops of diffraction peals. The gross positions of Cnvsrer Cnmnsrnv peaks147s1e calibrated to true CuKcl, positions (CuKcl, = 1.54O64A) and calibrated against the internal The general formula of tapiolite minerals can be standard using the computer program FD(, which expressedas ABrO6 where A representsFd+ andMn2+, reducessystematic error in positionaldata to 10.01' 20 and B, Tas+and Nb5+Ga > Nb). Ferric iron, titanium (Ercit 1986). Fifteen to twenty-five corrected and tin occur in minor quantities in both sites. The reflections were used for the indexing and unit-cell presentnomenclature ofthe tapiolite seriesis basedon refinement using o modified version of the CELREF a study by Lahti et al. (1983). Ferrotapioliterefers to least-squares refinement computer program of the Fe2+ end-memberFeTa2O6, whereas mangano- Appleman& Evans(1973). tapiolite refers to the Mn-rich member. Of the more Fragmentsof ferrotapiolite crystals 2 to 5 mm in than 20 elements reported to occur in tapiolite size were placed in silica "boats" lined with Ag-Pd mingrals,only six of theseo@e, Mn, Ta"Nb, Ti andSn) TIIE CRYSTAL CHEMISTRY OF TIIE TAPIOLITE SERIES 633 TABLEI. MA:OMITMOXIDE@IIENNREPOI{rDIN ?cniat$ NATURALTAPIOLTE Oxide T/t% R&tf{e TsPj 86.m ErcitO98O Nbp5 ll.tr WiseOgET) o 6 ooS oQb ft{nO 1020 I .hri €t al (1983)
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