631

The Canadian M ineralo g is t Vol. 34, pp.631,-647(1,996)

THE CRYSTALCHEMISTRY OFTHE 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 ,tapiolite shows a distinct preferencefor Fd+ and Ta over Mn and Nb. Emicbment in Nb, Ti and Sn appearsto be commonin tapiolite from moderatelyfractionated , whereas extreme Mn enrichmentis typical of highly fractionated pegmatitesof the petalite subtypeor metasomaticunits.

Keywords:tapiolite series,crystal chemistry,order - disorder," granitic .

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 ,wodginile and ,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 - 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), and ilnenite CfiKa), and CaNb2O6and Turnock (1966) have attempted to experimentally (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 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 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, (Ercit 1986). Fifteen to twenty-five corrected and 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) FeO L6.m r alti elal (19&,) FgPj 5.S Libirh€nb0966) nq 1.fi ThisSdy Snq 3.n WiseO9{r'l) CaO 1.96 Sirysm0917) qq orE Librcfuke(1966) siq lJ Iavrinenloetat (fff) Al2q 029 Khv6to% & Add@gdslqja (19?l) wq 0i0 | -hri tt al (1983) 7& O2n ro Kming&FdipB (1981) Saor A2l v@KMing&Fadipe 09tl) $hg 1.40 Iehri €taL (l9E) Y.pt 0.01 mKMing&Fodtpe (r98r) Mp 0.6l Khv61qa & Ail:begeblsya 09?f) CO, O24 tubmlqtaO962) REq 0.16 Libvdtrb(1966) c@ 0.14 Ncdeorldold (1863) HrO- 0.90 tminenloAal. (191) HrCt 0.05 ChrL:hw&Borhedt-Kuplendqta (1967)

FeNbzOo MnNbeQ are of any quantitatiye importance (Table 1). Representative chemical compositions of selected Flc. 1. Composition of natural tapiolite minerals in the tapiolite mineralsare providedin Table 2. columbite quadrilateral. The two-phase field marks Tapiolite minerals are r:uely pure, and typically the approximate extent of single-phase, non-paired compositiorisof tapiotte and columbite - tantalite. The contain some Nb (Frg. 1). The content is new data reported here are complementedby those of generally less than 107oM2O5, although Permingeat Cernf & Harris(1973), Clark & Fejer(1978),I.aht et al. (1955) reported on ferrotapiolitn containtng 21.2Vo (1983),and Wise (1987). MzOs. However, the compositions of apparently coexisting tantalite and ferrotapiolite analyzedin his study are not in agreementwith recent data and

TABLE 2 RBFRESE\ITATIVBCIBMIO{L COMPSTNONSOF TAFIOIJIB

I\doo,wt% 0.38 0.70 0.E9 0.90 I22 1.60 2.28 4.n 6.V2 1020 F€o 11.90 u.93 13.20 t2.t3 11.81 11.49 il.29 9.45 ?.22 320 F€2q 2.97 094 0.00 0.52 t.52 l.6t 0.91 0.45 1.71 0.@ s{4q 0.00 0.04 0.06 0.@ 0.00 0.06 0.00 0.@ 0.@ 0.(m ItO, 4.87 7.17 0.49 0.00 0.12 0.93 0.10 0.00 0.57 0.00 Srq 0.00 l.16 0.07 3.y2 2.79 0.28 0.67 0.00 0.29 1.00 Nb:Or 6.9E 4.74 4.07 4.57 1.99 4.70 1.43 0.00 4.75 t.50 Ta2Or 72.17 72.60 E1.65 78.38 El.l4 n.41 t3.78 8r.97 7t.76 7630 Totd 99.27 99.28 100.43 l0l.l2 100.59 98.14 100.91 [email protected] 9D32 9920 Cdi@ baredd 12fiygEn 6ds lyh? 0.050 0.091 0.124 0.124 0.171 0.n4 0.323 0.616 0.837 1.4$ Fsa 1.533 1.533 1.E20 1.748 1.635 1.591 1.580 1.345 0.w2 0.436 Fe$ 0.3,14 0.109 0.000 0.063 0.190 0.201 0.115 0.058 o2tl 0.0m sce 0.000 0.005 0.009 0.000 0.@0 0.009 0.@0 0.000 0.000 0.000 Ti* 0.564 0.828 0.061 0.000 0.01t 0.il6 0.013 0.000 0.070 0.@0 Sr* 0.000 0.071 0.003 0.255 0.184 0.018 0.045 0.000 0.019 0.(b5 Nbs 0.486 0.329 0.303 0.337 0.149 0.352 0.108 0.000 0.353 0.627 Tas 3.023 3.033 3.661 3.427 3.654 3.488 3.814 3.981 3.5t? 3.3t4 Tel 5.99 5.999 5.982 5.W 5.W 5.W 5.999 6.001 s.w 5.921

(r)PEC Oe,N6enestTqltods (Wis 19t7), (2)NMNE014439 KEafvetnine, Sv€dla,(3)NMNn#16t96: Alb & Giz dm, B!dl, (4) BtNESrnltlle, N6ftms T€rdbi6 (ms 1987),(t BICSII.L Degmdls,Ndttmst Tffis t$s 19e4, (O NDINEf15439: Rto Gmd6 do Nsb, Ml, CI)Tm, MdroDa (Br{to19E6), (E) Allo do Gz Ehe, Ml (Edt 1986),(9) NMNS fl4ellt TaboaTat$!, Arealta, (10 rfdM pe8Edib, Er4iNi, Edad (&M d al. L98). 634 TIIE CANADIAN MINERALOGIST likely cannotbe taken seriously.Although the Ta./Nb greater tlan the ideal value of 2.00 are generally ratio variesgreatly in naturalspecimenso it is neverless consideredto contain ferric iron. In our data base, than 1.0. whereFe2Or',rr'as not determinedand the cation sum in An alleged M-dominant member of the tapiolite the A site was found to exceed2.00, the excessFe was series was describedas 'omossite",from Berg, near recalculatedto Fe3+"and the Fez+/Ile3+ratio determined Moss, Norway (Brtigger 1897).The crystalsexamined by stoichiometry. This procedure was proYen by Briigger were describedas being tetragonal,but legitimale during the study of , conducted they were not analyzed.Holotype material chosenby under similar analytical conditions and checked by Brdgger (1906) for chemical analysis was found to Miissbauerspectroscopy (Ercit 1986,Ercit et al. 1992). contain5l.92%oTa2O5 and 317o M2O5 [Mn/(Mn + Fe) On this basis,we find that most samplescontain minor = 0 and Tal(Ta + Nb) = 0.501.'Mossite" also was amountsofFe3+, and that the averagecontent is about reported by Pryce & Chester (1978) from the 2.5VoFqO3 (0.3 atomsper formula unit Fe3+). Greenbushesdeposit in Australia.Their X-ray powder- Most samples of ferrotapiolite-manganotapiolite diffraction data confirmedthe tapiolite structure,and a containminorto fraceamounts of titanium, the average measureddensity of 5.9 led them to believe that the of all compositionsbeing l.U%oTiO2 (0.12 atomsper materialis M-rich. No chemicaldata for this material formula unit Ti4+). Electron-microprobeanalysis of were reporied.Bowley (1923) reportedthe occurrence ferrotapiolite from Kararfuet, Sweden showed Ti of "manganomossite" from the Jinniefans nfns, contentsof 7.l7vo TiO2 (0.83 atomsper formula unit Western Australia. Analysis of this material gave Ti4). The close similarity of the ionic radius of Ti#, Mn/(Mn + Fe) = 0.72 andTalCIa + Nb) = 0.44. Fe3+and Tas+enables titanium to enter the tapiolite Re-examination of "mossite" and "mangano- structure via the complementary substitution mossite,,has led to the discreditationof theseminerals Ti4\(A2+Bs+)-r. Tapiolite minerals enrichedin Ti are (Hutton 1959,Ewing 1976,Dunn et aI. 1979).T\e generallypoor in Mn. material from Norway was shown by X-ray powder- Tapiolite mineralscommonly contain minor to trace diffraction studies to be a ferrotapiolite - ferro- amountsof Sn. Although3.927oSnO2 (0.26 atomsper columbite mixture @unn e/ al. 1979). Chemical formula unit Sn4) was found in fenotapiolite from the analysis of "mossite" from the Greenbushesdeposit Yellowknife pegmatite field (Wise 1987), the SnO2 showed it to have Fe > Mn and, more importantly, contentrarely exceeds2,0Vo. T\e existenceof a ferro- Ta > Nb. Hutton's(1959) and Ewing's(1976) studies tapiolite - cassiterite isomorphous series has been of "manganomossite" showed this material to be suspectedfrom numerous finds of FeTa2O6-rich metamict manganocolumbite.Thus it appears that .uriit"rit" and "staringite" (eernf & Ercit 198-5,broat the formation of tetragonal FeM2O6 is unlikely in et al. 1992);however, casesof Sn-rich tapiolite have narure. yet to be found. Divalent manganesecommonly replaces ferrous Calcium has been detected in a few cases and iron. but the extent of substitution is limited. probably substitutesfor divalent Fe in the A site. Compositions close to the manganotapiolite end- Calcium contents are generally minor, and samples memberare not known. The amountof Mn occurring with elevated Ca contents are also associatedwith in natural tapiolite minerals is typically less than microlite. It is quite possible that the Ca found in 2.0VoMnO, but may reach up to 5.96Vo.Lahti et aI. many analysesis due to submicroscopicinclusions of (1983) found tapiolite minslals with Mn > Fe (up to microlite. L0.2VoMnO) and proposed the name mangano- The Si, Al, U, W, Zro Sc, Sb, Mg, the rare-earth tapiolite. However, if plotted on the columbite elements(REE) and H2O contentsare negligible, and quadrilateral,most of their dataplot within the tapiolite tleir presence in the tapiolite structure is not + tantaliteregion definedby natural samplesand some consideredestablished. Silicon and aluminumcontents experimental data. Textural evidence suggeststhat are easily attributedto contamination,whereas H2O is manganotapiolitemay be a product of metastable likely due to the presence of iron or crystallization, as it is only found in microscopically hydroxides. Spectoscopic analyses of fenotapiolite fine-grained mixtures with wodginite (Cernf & Ercit also showedtraces of Be, Bi, Cu, Mo, Na, Ni, Pb, Sr, 1985). Te, V, Y, Yb and Zr (Rudovskaya 1962, Chen & Data on the levels of ferric iron in tapiolite minerals Sidorenko 1962" Barsanov et al. 1964, Litovchenko are scarce.Although Fe2+is predominant,Litovchenko 1966,Cech1973). (1966)found up to 5.8070FqO3 in a samplefrom the As a generalrule, the bulk compositionof tapiolite Ukraine.Litovchenko attributed the presenceof Fe3+to minerals is faidy restricted. Although both Mn and the oddation of Fe2+during albitization. Khvostova& Fe3+are known to substitutefor Fe2+,the extent of Arkhangelskaya(1971) and Wise (1987) also have substitution is limited (Fig. 2a). Compositions lie shown Fe3+to be an important componentin some predominantlyalong the p"z+-3e3+join and, with few cases. exceptions,rarely show extensiveMn enrichment.The Tapiolite compositionswith A-site populations uniform chemistv of tapiolite minerals is further TIIE CRYSTAL CHEIVtrSTRYOF TIIE TAPIOLITE SERIES 635

Fes (Nb + Ta)

(Sn + Tl) (Mn+ Fef

FIc. 2. Bulk composition(atomic) of tapiolite in the Fe2+- Mn - Fe& triangle (A) and the (Nb,Ta) - (Sn,Ti) - Ge2*-Mn) triangle @).

emphasizedby Ta-rich compositions showing only octahedral arrangement. The oxygen atoms form minor enrichmentin Ti and Sn (Fig. 2b). hexagonal-closest-packedlayers that lie parallel to (001). The oxygenatoms occupy the samepositions as Cnvsrnuocnepny in the rutile sffucture,and eachis surroundedby three cations. The prominent feature of the structure is the presence of distorted, edge-sharing octahedra that Minerals of the tapiolite seriesare tetragonal,space form straighr chains parallel to [001] (Fig. 3). These groupP42lmnm, Z = 2. Goldschmidt(1926) solvedthe chains have an intrachain stacking sequence, sffuctureof ferrotapioliteand showedit to be relatedto ..A-B-B-A-B-8..., where A representsFe2* and B the rutile stucture (Frg. 3). Both the Fe2+ and Ta representsTa in the ideal sructural formula. Adjacent cations occupy unique positions in the ordered chains are linked by sharedcorners of the octahedra. sffucture (equivalentto the Ti sites in rutile) and are This structurehas been termed the trirutile sftucturebv coordinatedby six atoms of oxygen in a distorted Goldschmidt(1926).

Ftc. 3. Comparison of the rutile, disorderedtapiolite and ordered tapiotte structures. Shaded octa- hedra rqnesents Ti-bear- ing polyhedrain the rutile sffucture. Fe- and Ta- bearing polyhedra in the disordered structure. and Fe-bearing polyhedra in W the ordered structue. Rutile Disordered Ordered Unshadedoctahedra rep- resents Ta-bearing poly- Tapiolite Tapiolite hedra. 636 TIIE CANADIAN MINERALOGIST

Structural order - disorder reLationship pegmatite, Finland. IIis detailed study of several fragmentsextracted from a singlecrystal showedeither The trirutile structure described by Goldschmidt a trirutile-fype patternwith weak subcellreflections or representsthe ordered tapiolite structure, in which a monorutile-type pattem. Hutton's findings indicate ordering of the Fe2+and Ta cations in two unique that domains of varying degreesof order may exist sites (A and B, respectivd) resultsin a tripling of the within a single crystal. The data of Clark & Fejer rutile cell along the c axis.The positionsof the oxygen (1978) confirm the large variation in structural state atoms remain essentially unchanged, whereas the presentin the tapiolite series.In addition, Wise (1987) cationsorder to form the previously describedstacking noted varying degrees of order among different sequence.If tle Fe2+and Ta cationsbecome randomly specimensfrom a single pegmatitebody. distributed(disordered) over both the A andB sites,the Hutton (1958) found that order may be restoredby two sites become equivalent in a larger unit-cell heatingdisordered ferrotapiolite in air at 500-1200'C. equivalentto that of rutile (monorutilestucture). However,we havefound that heatingabove 1000'C in The effectsof order - disorderon natural samplesis air causes oxidation of Fe2+ and produces minor reflected in their X-ray powder-diffraction records FeTaOaalong with orderedtapiolite. Heat treafitrentof Grg. a, Table 3). Disorderedtapiolite has a diffraction natural samplesin air for 16 hours at 1000'C induces pattemwith supercellreflections weakened or missing, order with very limited oxidation of Fe2+. When i.e., hkl reflections where / * 3n. With increasing tapiolite minerals are heated (ordered), significant disorder, both the a and c cell dimensionsincrease, shifts in the relative positions of the diffraction c increasingat a much greaterrate than a. Disordered peaks occur. In addition, weak or diffirse supercell tapiolite typically showsunit-cell dimensionsa = 4.75 reflections become more intense and sharper (this, and c = (3 x 3.08) 9.24 A, comparedto a = 4.74 and Hutton felt, was clearly indicative of an order - c = 9.L9 A for orderedtapiolite. disorderrelationship). The departurefrom completeorder was first noted The causeof structuraldisorder in tapiotte minerals by Hutchinson (1955) in specimensfrom the Ross is not well understood.Disorder due to compositional Lake area,N.W.T. Hutton (1958) also noted similar variations or temperature changes have been relationships in ferrotapiolite from the Skogbiile suggested,but thus far these hypotheses have

11211 | 211

TWO THETA (CuKa)

Ftc. 4. Comparisonof X-ray powder-diffractionpatbms of natural (disordered)and heated(ordered) ferrotapiolite from the Yellowknife pegmatitefield [samplePEG-319-1 of Wise (1987)]. TI{E CRYSTAL CHEMISTRY OF TIIE TAPIOLTIE SERIES 637

TABLE 3, X-RAY POWDR-DIFHRACUONDATA FoR SY\ITIIE'NCAT{D NATURAL TAPIOLIIB

Syr&etic Synchstic Disordered Ord€rd FeTaOe (Mqde.o)TaaOo Ferrotapiolite Ferrotapiolite bHId(oba)Id(obs)Id(obs)Id(obs) @2 15 4.@3 14 4.654 2 4.621 9 4.610 10t 27 4.n0 31 424a 2 4.231 t6 4.n9 110 98 3.36 99 3.374 1@ 3.36 100 3.36s t72 17 2.71.5 t4 2.727 2 2.719 9 2.711 103 l@ 2.s77 100 2.594 44 2.5y2 57 2.575 2100 2E 2.378 25 2.383 16 2.377 16 2.376 004 4 2.3M 3 2.3t8 2 2.29 113 6 2.265 7 2.n9 5 2.275 6 2.263 2t0 4 2.tn 3 2.126 3 2.125 2V2 7 2.120 4 2.t08 2tL t0 2.072 15 2.077 7 2.W0 11.4 6 1.898 6 1.910 3 1.895 213 83 1..747 80 t.754 40 t.75r 83 1.745 105 , l. /lo 8 1.7?3 4 t.7t5 220 19 1.682 35 1.684 n 1.680 32 1.678 2U 5 1.62 ,)4 7 1.579 7 1.5U 4 t.577 301 : s |%2 4 1..565 ; 3 1.559 006 16 1.533 r0 1s6 J t.546 7 1.532 310 2t 1.504 27'1 1.507 10 1.503 15 1.5V2 ttz 7 t.430 1.433 4 1.427 303 24 1.408 20 1.413 t3 1.410 l0 l.z106 116 19 1.395 ,2 1.406 l0 1.406 t2 13E3 224 4 1.358 3 1.355 JIJ 3 1.350 1..352 321 s 1.306 6 .3@ 3 1.303 26 14 t.289 l5 .297 ,7 5 1.287 lW 3 7.267 4 .277 2 2 L2n5 314 5 t.259 't.212 .2@ 'r23 1,.257 323 17 23 .216 > t-LtJ 1.210 305 s 1.201 4 .2ffi 3 l.l9 400 7 1.189 E .1v2 3 1.187 g2 4 1.151 4 .154 i"Y 2 1.149 4t7 5 t.144 ) .147 226 14 1.133 11 .r39 3 1.138 15 l_131 330 8 1.121 8 .tn 5 1.208 8 l.n9 118 4 1.089 5 .w7 3 1.087 413 t7 1.080 t7 .083 6 1.080 6 1.078 316 18 l.M4 19 .080 4 t.o7a I t.972 420 8 1.064 9 .066 7 l.M3 4 1.62

Datafs syddic ftr!@iohe ad E/dstictro&c6of Tbmoce(1966').oisqdered (unboed)d cdered(lEateCl tco@one fr@ fre pEc Ctrim (Wisepe7).

been inadequately investigated. Compositionally pegmatite(i.e., strongly reducing conditions and high dependentdegree of order or disorder was studied on temperaturesin a dry system). the related systems CoM2O6-CoTqO6 and The influence of minor amountsof Nb, Mn, Ti and NiMrOu-NiTqOu by Felten (1967). At temperatures Sn on the order- disordertransformation has yet to be above1400"C, M-dominant speciescrystallized with determined.Although Clark & Fejer (1978)found that the rutile structureowhereas the Ta-dominantphases the a dimensiondecreases with increasingTi and Nb, havethe firutile sfructure.Felten found that as the Nb they found no obvious correlationbefween degree of content increases,the peak height of the supercell order and composition. reflections decreases,and the peak broadens. He Temperature-related studies are only in their attributedthe broadeningof the supercellreflections to infancy. A single study by Komkov & Dubik (I974b) the crystallizationof trirutile domainswithin the rutile suggeststhat the degreeof order may be temperature- sffucture, the domains leseming larger as the Ta dependent. Under hydrothermal conditions at content increases.Whether or not this holds true for 200-600'C, they found that the degree of order of naturally occurringtapiolite requiresfurther study. FeTa2O6,based on the relative intensitiesof the (101) Crystals of syntheticFeM2O6 grown befween850 and (200) reflections, decreasedwith decreasing and 1400'C were shownto have a disorderedtapiolite temperatue. However, because of the strong sfrucfure (Aruga et al. 1985, Tokizaki et al. L986). compositional influence on the intensities of these However, the crystals were formed under conditions reflections,we questionthe validity of this methodand that are unlikely to occur during formation of granitic its conclusions. 638 TTIE CANADIAN MINERALOGIST

A fundamentalproblem in adequatelydescribing the Manganese tapiolite structureis the determinationof the degreeof order in natural specimens. The first aftempt to Manganotapioliteand Mn-rich ferrotapioliteare rare quantify degreeof order in the tapiolite serieswas that in nature; as a resulq we have had to rely upon of Komkov (1974), who notedthat the intensitiesof the synthetic phases to characterizethe effect of Mn (200)and (101) reflections diminished with increasing on the cell dimensions.Turnock (1966) synthesized disorder.In general, all hkl re.flecttonsin which / * 3 tapiolite between the compositions FeTa2O6and show diminished intensities with increasingdisorder. (Fe6.aMns.JTa2O6.New, imFroved unit-cell Becauseof its great sensitivity to structural sfuanges, dimensionswere determinedfor Tumock's synthetic fte (101)/(200) ratio of ferrotapiolite was usedas an indicator of order. However" we have found this method to be unsatisfactory, owing to the strong influence of Mn and Ti on the intensity of the (200) reflection. Clark & Fejer (1978) noted that the c dimension of disordered tapiolite minerals is siguificantly larger than that of ordered tapiolite. Ferrotapiohtewas considereddisordered if c is greater than9.24 A. However,examination of numeroussets of unit-cell dimensionsof chemicallyanalyzed samples showedthis conclusionto be partly erroneous.High Mn contentsalso canproduce high c valuesin tapiolite minerals that have a fullv ordered structure.Thus a quick and simple meansof Oeterminlngthe degreeof order is not available at present,and we must rely on detailed refinement of the structure to obtain this information.

Unit-cell dimensions

The unit-cell dimensions of ferrotapiolite - manganotapiolite vary over a wide range; their variation is dueto variabledegree of order andvariable chemical composition.Disordered tapiolite has larger a ard c dimensions(and subsequentlylarger unit-cell volume) than orderedtapiolite (Hutton 1958).This is further shown by heating the disordered tapiolite, which causes the structure to order, resulting in reductionsrn a. c andV. Chemicalsubstitution also accountsfor variationsin the unit cell, althoughits role is not well understood. Clark & Fejer (1978) investigatedthe variations in cell dimensions with composition and found trends with extent of Nb, Ti and Mn incorporation. The effects of Mn and Fe3+ in synthetic tapiolite were examined by Turnock (1966) and Wise (L987). They showed that Mn-for-Fe substitution strongly affects a, c and V, whereasFe3+ primarily affects the a dimension.Unfortunately, compositional effects are generally masked by variable degree of order. Our understanding of the overall variation in cell parameters remains somewhat uncle:ir. In order to clarify the above relationships, data FeTap6 lrol pr c€il l,hTa2O6 on synthetic and natural phases were used to characterize the compositional dependenceof the FIc. 5. Variation in unit-cell dimensionsand cell volume of unit-cell dimensions of tapiolite. The primary synthetic tapiolite with proportion of Mn. Unit-cell data substitutingconstituents are Mtr2+oFe3+, Ti4, Sne and refined from the materialssynthesized by Turnock (1966). Nb5+. Errors on our data are aboutthe size of the svmbols. TIIE CRYSTAL CHEI\ItrSTRYOF T}IE TAPIOLITE SERIES 639

TABLB 4. UNTI.{5LL DIME{SICilIS OF SYI$SSIIC TAPIOUIts The a--cplot

. (A) c(A) vG3) A plot of the a versus c cell dimensionsmay be Fcoaqe|lle considereda convenientmethod of representingthe FETa"O6 4XW4) 9.19p2/9\ 208.1?Cl) degreeof orderfor naturalsamples (Fig. 8a).However, of cell dimensions of Ittotr'@ @ee@d qigni(lrp|l$s the complex interdependence (eo.rl/t!o.rTa2o6 4.'t587(3) 92tn{\ 2m61Q) tapiolite minerals on compositionand degreeof order (e0.il[no.rTa?O6 4X6llP) 943(q M.44{2) makes such a simple graphical representation of (Rq.5ltlq.u)Tqoo 4X6ff78) 91560(8) 2rO.W) (F%.aI\4&.dTa2O6 4]@ 92m{A AOJW (Foo.jJU&.s)Ta?O6 4X6W, 9,811C/) 2IAATQ)

Ftf&ry{l|o (tt?'o.eFs3'0.@)Tqo6 43ffi{4) 9$9qr0 a19$) (Fe&o.sFe*0.2)Ta2O6 4Xqq3') 9.1943t1\ m7%{2, fittut4ffi (Felelto.1xfa3.6Tjo,ro b 4JlW) 9.r42lr) 2f6.Jqq)

products Gig. 5, Table 4). The results show a steep increasein the c dimensionand a moderateincrease in a with increasingMn content.Tapiolite describedby eech (1973) and Lahti et al. (1983) show cell parameterstypical of manganotapiolite.However, it should be stressedthat large values of c in natural tapiolite are more likely the result of structural disorder.Caution thus is advisedwhen estimatingthe Mn contentof tapiolite using cell dimensions.

Fenic iron

Variations in the unit cell due to incorporation of ferric iron are not well characterizedowing to the lack of Fe3+determinations in chemicalanalvses. The ionic radius of F-e3*(0.66 A; is much smaller than that of Fe2*(0.78 A;, andthus its effectwould probablycause a decreasein the cell sLe. This assumption was verified by Tumock (1965), who synthesizedFe3+- bearing fenotapiolite. New unit-cell dimensions are shown in Figure 6. The experimental data show a gradualdecrease in the a dimensionand a more limit- ed decreasein the c and V parameterswith increasing proportion of ferric iron.

Titanium and tin

The effect of titanium incorporationon the unit cell of ferrotapioliteis similar to that of ferric iron Gig. 7), n €x) &50 i.e., a, c and V all decreasewith increasingTi content FeTa2O6 ft ol p€r carl FeTaO4 (Wise er al. 1988).In contrast,preliminary investiga- - tions of the ferotapiolite cassiteritesystem shows Ftc. 6. Variation in unit-cell dimensionsand cell volume of that the incorporationof Sn into the tapiolite structure synthetictapiolite with proportion of Fe3+.Unit-cell data causesa decreasein a and an increasein both c andV refined from the materialssynthesized by Tumock (1965). (unFubl.data of M. Wise). Errors on our data are aboutthe size of the symbols. &0 TIIE CANADIAN MINERALOGIST

referencepoint for the extapolation of Mn- and Fe3+- rich disorderedphases. Alnost all of the availabledata for natural samplesfall well within these bounding lines. The relative degreeof order in natural specimens can be seen by t}te comparison of the unit-cell dimensionsfor natural and heatedsamples. Tie-lines joining thesepoints representa heating vector or path of ordering (Frg. 8b). However, the generaldirection of these tie-lines is significantly different from that generatedby the syntheticdisordered phaseso and it is difficult to determinewhich ordering trend is correct. Consequently,the determinationofthe degreeoforder in the tapiolite series remains unresolved until synthetic disordered phases are produced and an accurate method of quantifying degree of order in natural samplescan be derived.

PHASEREr-AnoNs

Our present understandingof phase stabilities amongferrotapiolite-manganotapiolite and coeristing M,Ta-bearing oxide minerals is poor. A number of authors have attempted to characterizethe stability range of the tapiolite series through experimental studies(Moreau & Tramasure1965, Tumock 1966, Schriicke 1966, Komkov & Dubik 1974a).Despite their efforts, the picture remains hazy (aun! et al. 1992). Although a few studieshave been conducted hydrothermally(Komkov & Dubik L974a),most of the experimentalwork has been conductedin dry systems and under geologically unrealistic conditions. Ferrotapiolitewas produced under controlled fugacities of oxygen by Turnock (1966) and Komkov & Dubik (1974a). Solid solution between FeTa2O6and FeM2O6 is limited. Beugnies& Mozafari (1968) found that the incorporation of FeM2O6 is limited to 36 mol.Von ferrotapiolite, although approximately 20 mol.Vo FeM2O6 is the maximum amount found so fm in naturaltapiolite. The solubility of FeM2O6in synthetic FeTqO6 is virtually nil at 885"C.Is limit has been T0.2 Md per FeTa2O5 cent determinedto be 20 mol.Voat 1050'C (Moreau & Tramasure 1965). At higher concentrationsof Nb, FIc. 7. Variation in unit-cell dimensionsand cell volume of intergrowths of ferrotapiolite synthetictantalian rutile and titanian tapiolite (Y,Iiseet aI. + ferrocolumbite were 1988).For tapiolire(95 and 100mo1.7a FeTqO), c andV observed. ln contrast Schr6cke (1966) found thal are divided by 3 for a direct comparisonto the rutile the FeM2O6 solubility increases to 56 mol.Vo at unit-cell. Materials synthesizedat 1100"C, P = 1 atu, 1100"c. CO/CO2= 3 gagflow. Errors on our dataare about the size Solid solution toward MnTarOuhas been shown to of the symbols. increase significantly with increasing temperature (fumock 1966). His data show that the marimum extent of solid solution varies from near 58Vo at 1000'C to about 68Vo at 1500'C, at atuospheric structural state difncuft to interpret. Superimposed on pressure.A two-phaseregion of tapiolite + tantalite the a-c plot axe data for synthetic ordered phases exists at concentations greater than these limits in outlining the effect of Mn, Fe3+ and Ti on the a and natural tapiolite; Mn-for-Fe substitution extends to c dimensions. The data for disordered FeTqO6 taken about40 mol.Vooalthough most samplesshow less than from Komkov & Dubik (L974b) are used here as a 20 mol.Voof the Mn end-member. TIIE CRYSTAL CHEMISTRY OF T}IE TAPIOLITE SERIES &l

aa MnTarO, tt r a /.66% a t 60%

a It aa 9.25 t ol a aa ll aa' I c(fu aa tr' a aa t' a i. a- .t FeTaOo Synthetc Ordered FeTarO.

9.15 4.740 4,750 a(A)4.755

4,740 4.746 4.750 o 4.755 4.760 4.765 4.770 a(A)

Ftc. 8. The a-c plot for the apiolite series,illustrating the probabledegree of order. (A) Vectors indicate generalchange in a and c dimensionscaused by chemical substitution.The data on synthetic orderedFeTa2O6, FeTaOo and Mn-substituted tapiolite of Tumock (1966) were refined by Wise (1987). The syntheticdisordered FeTa2O6 is that of Komkov & Dubik (1'974b} Dashed line representsextrapolation for Mn- and Fek-rich disorderedphases. Plots of natural tapiolite from Anderson(1984), Barsanov a al. (1964),Burke et al. (1969),eech (1973),eemf & Harris (1973),Chen & Sidorenko (1962), Chul'brov & Bonshtedt-Kupletskaya(1967), Clark & Fejer (1978), Ercit (1986), Iftvostova & Arkhangelskaya (1971),von Iftorring et al. (1,969),1-ahtet al. (1983),Vorma (1965),Wise (1987),and unpublished data of the authors. @) Generaltrend of ordering of tapiolite from the Yellowknife pegmatitefield. Arowheads representordered (heated) tapiolite. Data of Wise (1987).

Under conditions of increasing temperature, of Fe3+present in tapiolite is dependenton the partial pressureand oxygenfugacity, tapiolite-groupminerals pressure of oxygen under which the tapiohte may becomeoxidized and incorporateFe3+. Synthetic crystallizedand the degreeof Mn incorporationin the tapiolite wasfound to takeup to 22mol.VoFeTaO, into strucflre. solid solution (Iumock 1966). A rutile-type phase Solid phasesin the binary system FeTqO6-TiO2 occursat higher concentrationsofFeTaOa. The rmount have been svnthesizedat 1000'1300oC.The system &2 TI{E CANADIAN MINERALOGIST shows completesolid-solution between the rutile and Fenotapiolite versw ta.ntalite ferrotapiolite end-members.In the Ti1-fer3Ta2,6O2 phases,the rutile structure converts to the trirutile Ferrotapiolite is invariably conspicuouslyenriched strucfureonly in the 0.95-1.00range of .r at 1100oC in Ta and Fe in comparisonto coexisting tantalite. (Wiseer c/ 1988). Compositionaldata for ferrotapiolite + tantalite pairs Experimentaldata for the systemFeTa2O6-SnO2 are show very restricted Mn-for-Fe2+substitution in nonexistenl.Solid solution betweenferrotapiolite and ferrotapiolite, and only slightly greatersubstitution of cassiteritehas been suggested (Amark 1941),but so far M-for-Ta (Fig. 9). In contrast, the coexisting has not been studied. At very high temperatures, orthorhombic phase shows a broad range of Mn/ ferrotapiolite- cassiteritesolid solution may exist, but (Mn + Fe) values(0.2G{.90), along with an extensive at lower temperatures,the tapiolite phase would be rangeof Ta(Ta + Nb) values(0.52-0 .98) (Cenf et al. expectedto exsolve cassiterite@dwards 1940). The 1.992). Tie-lines linking coexisting grains of extent of Sn in natural tapiolite is approximately ferrotapiolite and tantalite sweepacross the two-phase 4 mol.VoSnor. Amark (194t) found that up to region and vary greatly in their slope. Internal 25 mol.Voof ferrotapiolite might enter the cassiterite consistencyin slopesof tie-lines linking ferrotapiolite structure.According to Quensel (1957), elevatedTa - ferrotantalite pairs is best representedby material contents favor the solubility of Sn in M,Ta oxide from Spittal and Nyanga #2 (cf. Cem! et al. 1992), minerals. He further suggestedthat substantial M suggesting near-equilibrium conditions. In confrast, reduces the miscibility between SnO2 and the strongly divergent slopes and crossing tie-lines "columbite" component,and the result is a mixture of suggestiveof disequilibrium are typical of the Snnis separatephases. River, Scheibengrabenand Yellowknife data(cl, Cernf et al. 1992).Compositions of ferrotapiolite- mangano- Coexisrtngphases tantalite pairs from Tanco, Bikita and Alto do Giz are similar, suggesIng similar conditionsof crystallization Tapiolite minerals rarely occur as the only M,Ta- (Ercit 1986). bearingmineral in a given pegmatite.A mineral of this Weitzel (1976) suggestedthat the preferenceof Fe seriesis usually accompaniedby columbite- tantalite and Ta for the tetragonal phase relative to the or wodginite and, less commonly, cassiterite or orthorhombic phase is due to the difference in microlite. The influence of coexisting phases on polarizing effect of Ta5* and Nb5+.In addition, the tapiolite composition is at present unknown. difference in ionic radius between Mnh and Feh Insufficient chemical data on coexisting pairs has (0.83 and 0.78 A for Mn andFe, respectively;Shannon hindered the understandingof element partitioning. 1976) apparcntly confiols their preference for a The little that we do know is inferred from only a particular sffucture-type.The Mn cation is too large to handfrrl of analyses.Figures 9 and 10 summarizethe be easily accommodatedby the tapiolite structure,and present understanding of Fe, Mn, Nb and Ta therefore enters the orthorhombic structure of partitioning in pairs of coexistingminerals. columbite [the averageA-O distancesfor ferrotapiolite and manganotantaliteue 2.07 A lvon Heidenstam 1968)and 2.18 A (Griceet al. 1976),respectivelyl.

Ferrotapiolite versuswodginite and

In pairs of coexisting ferrotapiolite + wodernite, FeTarO, MnTarO, fenotapiolite preferentially accepts Fe2+ and Ta, whereas wodginite prefers Sn, Mn and Nb (von Knorring et al. 1969, Vorma & Siivola 1967, Btcit 1986). Ixiolite, which seemsto be closely related to wodginite compositionally, also show similar partitioning tendencies.This mineral pair is scarce,and the available data indicate a sffong preferenceof Mn for ixiolite relative to ferrotapiolite (Fig. 10a). Pafiitioning of Ta betweenthe two phasesis essentially equal.

F enotapiolite versuscas siterite and microlite

FeNbrO, MnNbrO. Data for fenotapiolite + cassiteritepairs also are Ftc. 9. Compositionsof coexisting (or associated)natural scarce.In generaloFe2+ is concentratedin cassiterite ferrotapioliteand ta.ntalite pai$. After Cemf et aL (1992). relative to slightly manganiferous ferrotapiolite, TIIE CRYSTAL CHEMISTRY OF TIIE TAPIOLITE SERIES 643

FeTarO, MnTarO, A ts

A g F 60aE zll + z o + F ag

t! |! F F

FeNbrO. MnNb.O.

Ftc. 10. Compositionsof nairal ferotapiolite coexistingwith (A) cassiterite,ixiolite and wodginite, and (B) with microlite.

whereasthe behavior of Ta and Nb is quite erratic pegmatites, commonly intimately associated with (Fig. 10a).In pairs of this type, Sn is conspicuously albite, muscovite, , spodumene,tourmaline, absent or occun in minor quantities only in ferro- triphylite, and other M,Ta,Sn-bearing tapiolite. In coexistingferrotapiolite and microlite, the minerals (columbite - tantalite, wodginite, cassiterite, Ta content is slightly higher in ferrotapiolite than microlite). Rarely is ferrotapiolite the sole representa- microlite (Fig. 10b), although some ferrotapiolite + tive of Ta mineralizationin a pegmatite(von Knorring microlite pairs show impartial Ta distribution. & Fadipe1981). Tapiolite mineralsare usually found in muscovite+ PARAGBI,IESIS quartz + albite zones or in replacementbodies of lithian muscovite of Li-poor or Lienriched granitic Primary tapiolite- s e rie s nine rals pegmatites (von Knorring &' Fadipe 1981). Manganotapiolite- fenotapiolite occurs in the fine- Tapiolite minerals are restricted to rarc-element gmined albite + quartz+ muscoviteborder zoneof the granitic pegmatitesthat showmoderate to high degrees Tianinen pegmatite, Finland (Lahti et al. 1983). of fractionation, to placers derived from granitic Manganese-rich ferrotapiolite from the Mar5ikov pegmatites and, rately, to some Ta-mineralized pegmatite, Czech Republic, belongs to a late albite granitescharacterized by low levels of F and limited metasomatic complex (Cech 1973). Manganoan enrichmealin Fe and Mn. Granitic pegmatitesof the compositionsare typical of ferrotapiolitebelonging to beryl-columbite and complex_(Li, Be, Ta?M) or the simpsoniteparagenesis of low-pressure,petalite- (Li, Rb, Cs, T>Nb) subtypes(Cernf 1991)commonly subtypepegmatites, which may alsoinclude wodginite, carry tapiolite, although the less fractionated manganotantaliteand microlite. These samples of pegmatitesof the gadolinite subtype and the highly tapiqlite are chemically more evolved than those not fractionated spodumenesubtype may also carry a associatedwith the simpsoniteparagenesis, i.e., they tapiolite-series mineral as a minor constituent. are strongly depletedin Nb, Ti and Sn, but Mn- and Ferrotapioliteoccurs as a raJeaccessory mineral in the Ta-enriched (Ercit 1986). In REE-bearing miea- intermediate zone and core margin of zoned bearing granitic pegmatite,ferrotapiolite is reportedly 6M TIIE CANADIAN MINERALOGIST associated with REE phosphates (monazite and The alterationof tapiolite mineralsto secondaryNb, xenotime), epidote and allanite (Hutton 1958, Ta-bearing minenls also does not appear to be a Rudovskaya1962). common process. The replacement of tapiolite Ferrotapiolite may occur as inclusions within mineralsby microlite has beendocumented by Pough wodginite(Vorma & Siivola 196T,vonKnorringet al. (1945) and Eid & von Knoning (1976). Fibrous 1969), as subhedralgrains in stibiotantalite(eemf & microlite replacesferrotapiotte along its margins and Hanis 1973) or as grains within columbite - tantalite continuesalong fracturesuntil most of the mineral is or cassiterite.The associationferrotapiolite - stibio- replaced. tantalite - antimonian microlite of the Odd West pegmatite,Manitoba, occurs in a highly fractionated CoNcr,unmcRE\4ARKs Li-emiched pegmatitein a relatively early assemblage of albite +_ quartz + muscovite + tounnaline + X-ray-diffraction studies have provided valuable cassiterite(Cernf & Hanis 1973).Numerous homo- information regarding the snuctural variations of the geneoussamples of ferrotapiolite were shown to be tapiolite series. Althongh we .re able to recognize associated with contemporaneousor secondary order- disorderrelationships in natural specimens,the microlite (Adusumilli 1980, von Knoning & Fadipe mechanismsresponsible for suchstructural changes are 1981). Others are intimately associated with still undefined,primarily becauseof the pronounced 'staringite'o @urke ar al. 1969, Adusumilli 1980). effect of compositionon the unit cell. The latter is not Ta-rich cassiterite may also contain inclusions of well documented,and the presgntdata only underscore ferrotapiolile (Vorma & Siivola 1967, Cem! et al. the difficulty in separatingcompositional factors from 1986, Wise 1987). Fenotapiolite intergrown with those causedby the degreeof order. Comparisonof tantalianrutile was found associatedwith saccharoidal chemical and experimental data suggeststhat the albite, beryl and Ti-dch columbite - tantalite degreeof ordermay be controlledby the crystallization (Anderson1984). history and only slightly affected by chemical variations. An experimental approach emphasiz.ing Exsolution of fenotapiolite changesin lemperature,rate ofcooling, pressure,etc., complementedby structure refinement of disordered Exsolution of ferrotapiolitefrom an unstablehomo- phasesoobviously is needed. geneous precursor was described by (em! et al. Limited but effective homovalentand heterovalent (1989a) from Uganda. Other possible occurrencesof substitutions,acting in most case$concomitaatly, and similar exsolution were reported from Australia (Jutz competition of coexisting phasesplay dominantroles 1986) and Aushia (dern! et al. 1989b); however, in defining the compositionalrange of tapiolite. Major deformation of the ferrotapiolite - ferrotantalite deviations from the typical compositions are due aggregatesdisturbed the primary textural relationship not only to the bulk M/Ta value of the crystalllzing in the latter case.Another occurrenceof an exsolved melt or fluid, but also to the progress of pegmatite ferrotapiolite - ferrotantalite pair was recently fractionation and the presenceof associatedM,Ta examined from tle Chantrey Inlet, Northwest phases. Territoriespegmatite field (Tomascaket aI. 1994). Microscopic rounded grains of ferrotapiolite in AcKIIowI.EDGEIVIEI\rTS Fe,Ta-bearingcassiterite were interpretedas products of exsolution by Ramdohr (1955). More recent The authorsthank Drs. T.S. Ercit. A.C. Tumock and observationsof this exsolution include, for example, A.J. Andersonfor useof their data.We gxalefu[y thaxk thoseof demf et al. (1981,1985), Groat et al. (1992), the following colleaguesfor providing us with samples and unpublisheddata ofthe authors. forXRD and elecfton-microprobestudy: H. Annemten, W. Bid, F. dech, E.E. Foord, C.A. Francis, C. Late seconlary tapiolite Guillemin, the late O. von Knorring, B. Lindquist, the late H. Meixner, G. Niedernayr, M. Novak, the late The formation of ferrotapioliteby lale hydrothermal T.G. Sahama,R. Segnit,S.-A. Smeds,E.Ii Ucakuwun, or metasomatic alteration of pre-existing Nb-Ta and J.S. White. This researchwas startedwithin the minerals seems to be an uncommon process. At frameworkof the Ph.D.thesis research of M. Wise. and present, only two replacement-inducedsequences was supported by the NSERC Operating Grants have been documented. Ferrotapiolite, possibly and DEMR ResearchAgreements to P. Cernf, and by recrystallizedfrom remnantsof an earliergeneration of financial and logistical support from the Tantalum tapiolite, is found in cavitiesof massivemicrolite from Mining Corporation of Canada Limited and the the Bulema pegmatite,Uganda (von Knorring & Geology Division, Indian and Northern Affairs Fadipe1981). Ercit & Cernf (1982),Voloshin (1983) Canada, Yellowknife. We are grateful for tle and Voloshin & Pakhomovskyi(1983) found tapiolite constructivecomments by E.E. Foord J. Grahamand replacingsimpsonite. R.F. Maftin, which greatly improvedthis paper. TIIE CRYSTAL CHEMISTRY OF T}IE TAPIOUTE SERIES @5

Rm'aRB,tcFs & Encrr, T.S. (1985): Somerecent advancesin fls minsralogy and geochemistryofNb and Ta in rare- ADusurvffJ, M. (1980): Tapiolitesfrom northeastBraztl,. Int. elementgranitic pegmatites.Bull. Min6ral. l0E, 499-532, Geol.Congr. 26th,ll5 (abst.). & WISE,M.A. (1986): Mineralogy, AMARK,K. (1941): Minerals of the Varutrask pegmatite. paragenesisand geochemical evolution of Nb-, and )Om(. An X-ray study of stanniferouscolumbite from Ta-oxide minerals in granitic pegmarites.Int. Mineral. Varutraskaad of the relatedFinnish mineralsainatite and Assoc.,I4th Gen Meet.,Abstr, Programs,Tl. rxiohte. Geol,Fdren. StockltolmFiirh, 63,295-299. & - (1992): TlJle tantalite - ANDRSoN, A.J. (1984): The Cross l"al

BURKE,E.A.J., KEFI, C., Ftsrrus,R.O. & Aousul,m.r,r.M.S. DIINN,P.J., Genws, R.V. & Knrsnarsmr, R. (1979):Mossite (1969): Staringite, a new Sn-Ta mineral from discredited.Mizeral. Mag. 43, 553-554. north-ea$temBraz:l. Mineral Mag. t7,447452. EDWARDS,A.B. (1940): A note on sometantalum - niobium dncr, F. (1973):Manganoan tapiolite from northernMoravia, minszfg from Western Australia. Aust. Irxt. Minino Czechoslovakia"Acta Univ. Carolirne - Geol. Rostvol.. M etall, Proc. 120,7 3L:7 44. No.l-2.3745. ED, A.S. & voN KNoRRNc,O. (L976): Geochemicalaspects dBnrf, P. (1991): Rare-elementgranitic pegpatites. 1. of the tantalum mineral microlite from some African Anatomy and internal evolution of pegmatite deposits. pegmatites.In Ann. Rep.,Inst. Afr. Geology,Univ. Leeds, Geosci.Can. 18.49-67. Leeds,U.K. (56-58).

CHaptuaN,R., CHAcKowsKr,L.E. & ERcm,T.S. Encn, T.S. (1986): The SimpsoniteParagenesis; the Crystal (1989b):A fenotantalite- fenotapiolite intergrowthfrom Chemistry and Geochemistry of Extreme Tantalum Spittal a"d.Drau, Carinthia"Austria. Mineral. Petrol. 41, Fractiorwtion. Ph.D. thesis, Univ. Manitoba, Winnipeg, 53-63. Manitoba- 646 TIIE CANADIAN MINERALOGIST

& emN9, P. (1982):The paragenesisof sirnpsonite. Frank-Kamenetskyi,ed.). Nauka, Leningrad, Russia GeoI.Assoc. Can. - Mineral. Assoc.Can, ProgramAbstr. (25-30;in Russ.). 7.48. & -(1974b): Experimentalexamination of HewnronNs, F.C. & McCeurraoN,C.A. polymorphic and isomorphic relationshipsin the system (1992): The wodginite group. tr. Crystal chemistry. Can. FeM2O6 - FeTqO6 - MnTa2O6- MnNb2O6./z Crystal Mineral.30,613-631. Chemistry and Structural Mineralogy (V.A. Frank- Kamenetskyi, ed.). Nauka, Lening&d, Russia (82-93; Ewtttc, R.C. (1976): Metamict columbite re-examined. in Russ.). Mineral.Mag. 40, 898-899. VoN KNoRRtrrG,O. & Fannr, A. (1981):On the mineralogy FELrB{, E.J. (1967): Composition-dependentorder - disorder and geochemistry of niobium and tantalum in some phenornenain CoM2-*TqO6 and NiNbl*TqO6, Mater. granite pegmatitesand alkali glanites of Africa, Bull. Res.Bull. 2, 13-24. Minlral. 104.496-5M.

Gor,oscnrraror, V.M. (L926): Geochemische Sasave, T.G. & Lffrrn.IEN,M. (1969): Ferroan Verteilungsgesetze der Elemente. VI. Uber die wodginite from Ankole, south-westUganda. GeoI. Soc. I(rystallstrukturenvom Rutiltypus mit Bermerkungenzur Finl.qnd,Bull. 41, 65-69. Geochemie Zweiwertiger und Vierwertiger Elemente. VidenslcAkad- i OsIoI, Math-Nat. K., Skr.1,5-21. Larm, S.L, JoIrANsoN,B. & VmrurBN, M. (1983): Contributions to the chemistry of tapiolite - mangano- Gruce,J.D., FB.cusoN, R.B. & HAwr{oRNts,F. (1976): The tapiolite, a new mineral. Geol. Soc, Finland, Bull, 55, crystal structuresof tantalite, ixiolite and wodginite from 101-109. Bemic l,ake, Manitoba. I. Tantalitg and wodginite. Can. Mineral.1].540-549. Lavnnrremo, L.F., RozaNov, K.I. & Roanmnnc, D.S. (1971): First find of tapiolite in the Llkraine.DoH. Acad. Gnonr, L.A., Encrr, T.S., Punus, A., Hewrnonue, F.C. & Sci. USSR,Earth Sci.Sect. 197, t26-1'n. cAINFs, R. (1992): Stadngite discredited. Geol. Assoc. Can.- Mircral. Assoc.Can., Progran Abstr, 17, M. LnovcHENKo, E.I. (1966): Accessory tapiolile from pegmatitesof the Ukranian crystalline massrf.Mineral. VoN IIrTDE{srAM,O. (1968): Neutron and X-ray ditfraction SbomilqLvov 2.0, 1 13-1 15 (in Russ.). studies on tapiolite and some synthetic substancesof trirutile structure. Ark Kemi 2.8^375-387. MoREAU,I. & TneMasuns,G. (1965): Contribution a l'6tude dess6ries columbite - tartalite et tapiolite - mossite.Ana Hu'rcm.lsoN, R.W. (1955): Preliminary report on investiga- Soc. G6oLBelgi4ue 88, 301-390. tions of minerals of columbium and tantalum and of certainassociated minerals. Ara Mineral. &,432452. NoRDENSKOID,A.E. (1863):Beskrinfning dfver d.ei Finland furaa Mineralier. Helsingfors(Helsinki), Finland. HurroN, C.O. (1958): Notes on tapiotte with special (1955): les niobotantalatesde I'Anti- reference to tapiolite from southem Westland, New hRMD,lcEAr,F. Sur . Bull, Soc. MiniraL Z,ealafi,. Arn M irc ral. 43, I 12-| 19. Atlas, Maroc: tapiolite et columbite fr. Cristallo gr. 78, 123-156. (1959): Manganomossiterestudied. Am, Mineral. (1945): the northern Brazilian u.9-18. Poucu, F.H. Simpsoniteand pegmatiteregion. Geol, Soc,ArtL, BtdL 56, 505-514. Jutz, D, (1986): ErznineralparagencseSn-Nb--Ta-filhrender Pegmatite, Finnis River Area. Northem Territories/ Pnvce,M. & Cnrsmn, J. (1978):Minerals of the Greenbushes Australien M.Sc. thesis,Inst. Mineral. Lagerstiittenlehre, ttnfief,d.Mineral. Rec, 9. 8l-M. Rhein-Westfalische Technische Hochschule. Aachen. Germany. QLENSEL,P. (1957): The paragenesis of the Varutrlisk pegmatiteincluding a review of its mineral assembl,age. KgvosrovA, V.A. & AnruarcEL'sKAyA,V.N. (1971): A find Arlc Mineral. Geol, 2. 9-125. of a manganesespecies of tapiolite. Dokl, Acad.. Sci. USS&Earth Sci.Sect.194,128-129. RAMDouR, P. (1955): Die Erzmineralien und ihre Verwachstmg en Akademie-Verlag,Berlin, Germany' KoNtr

SHANNoN,R.D. (1976): Revised effective ionic radii and & Srvor"t, J. (1967): Sukulaite- TqSn2OT- and systematic studies of interatomic distances in halides wodginite as inclusions in cassiterite in the granite and chalcogenides.Acta Crysnllogr. A32, 7 51,-767. pegmatitein Sukula Tammelain SW Finland. C.R. Soc. G6ol. Finlande 39, 173-187. SrNrpso\E.S. (1917):On tapiolitein the Pilbaragoldfield, W'esternAustraliu Mineral. Mag 18, 107-121. Wrrrzw, H. (1976): Kristallstrukturverfeinerungvon Wolframiten und Columbiten. Z. Kristallogr. lM, Torzar

Tomsca& P.B., WrsE M.A., eERNf, P. & TRT.TBMAN,D.L. WISE,M.A. (I987)t Geochemistryand Crystal Chemistryof (1994): Reconnaissancestudies of four pegmatite Nb,Ta and Sn Minerals from thz YellowlatifePegmatite populationsin the NorthwestTerritoies. GeoI.Surv. Can , FieA N.W.T, Ph.D. thesis, Univ. Manitoba. Winnipeg, Bull.475.3l-62. Manitoba-

Turuqocr, A.C. (1965): Fe-Ta oxides: phase relations at Lenoux,M., almN9,P. & TnRNocK,A.C. (1988): 1200"C.,f.Am. CeramicSoc. 4, 258-262. The FeM2O6-TiO2 and FeTa2O6TiO2 systems:phase relationships at 1 afin. pressure. Geol. Assoc. Can - (1966): Syntheticwodgrnite, tapiolite and tantalite. Mincral. Assoc.Can., Program Abstr. 13, 136. Catt Mineral. 8, 461470. Zre,o, Bnl, Lr, WH-XrAN& Cer, YueN-Ir (1977): Conditions VoIosItrN, A.V. (1983): Evolution and reaction series of of formation of , cassiterite, columbite, taltalum minerals.In New Ideas in GeneticMineralogy. microlite and tapiolite and experimentalstudies on the Nauka,Leningra4 Russia(21-26; in Russ.). variation of Nb and Ta in wolfoamiteand cassiterite.fi ChiuHau Hsueh2,1,23-1,35(in Chinese). - & ParnoMovsKyl,YA.A. (1983):Mineral phasesof aluminum - tantalum in rare-elementWgmlattf,s. fup. Vses.Mineral. Obshchest.ll2, 67-76 (in Russ.).

VoRMA,A. (1965):Notes on columbiteand tapiolite from the pegmatitearea of Tarnmelaand Someroin SW Finland. Received February 9, 1993, revised manuscript accepted C.R.,Soc. G6ol. Finlande 37, L69-176. December20. 1995,