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First U.S. Occurrence of Wodginite from Powhatan County, Virginia

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First U.S. Occurrence of Wodginite from Powhatan County, Virginia American Mineralogist, Volume 69, pages 807-809, 1984 First U.S. occurrenceof wodginitefrom PowhatanCounty, Virginia MrcHeel A. WIse eNo Pnrn er,nNf Department of Earth Sciences, University of Manitoba Winnipeg, Manitoba R3T 2N2 Canada Abstract Wodginite occurs as black subhedral grains in the Be, Nb-Ta, Sn bearing Herbb #2 pegmatite in Powhatan County, Virginia. The wodginite is associatedwith amazonite and topaz in the intermediate zone of the pegmatite. Other Nb-Ta-bearing minerals found associated with wodginite are cassiterite, in part as inclusions in wodginite, and par- tially ordered man6ianocolumbite.The Herbb wodginite is compositionally similar to previ- ously studied wodginitesand has an averageformula (Mnz.aoFe?.1sb+.or(Snz.oqTirrcTaost Fd.14)x.8e(Ta7.ozNbos8)>8.0oOrz.The average unit cell dimensions ate a 9.471(2), b tt.43t(2),c 5.108(l)A, p90"47'(t),v 553.1(l)43. Introduction ite), quartz and muscovite are the dominant minerals present. Beryl, topaz, columbite-tantalite,wodginite, A black mineral, at first believed to be columbite- cassiterite,and minor spessartinealso occur in the peg- tantalite, was collected by the first author from the Herbb matite, mainly in the clayey decomposition products. #2 pegmatite in Powhatan County, central Virginia. Wodginite is found scatteredthroughout the kaolinized Griffitts et al. (1953) noted "small lustrous tabular to feldsparof the intermediatezones. Veinlets of wodginite equant crystals of columbite-tantalite"in albitized por- also occur near perthitic amazonitebodies and to a lesser tions of this pegmatite,a speciesalso quotedby Jahnset eitent are also associatedwith topaz. al. (1952).Recent examination of Nb,Ta-bearingminerals from this locality has led to the identification of the rare Physical properties Mn,Sn,Ta-oxidemineral, wodginite. Wodginite occurs as black anhedral to subhedral crys- Wodginite, (Mn,Fe)a(Sn,Ta,Ti)a(Ta,Nb)sOrz,was first tals, typically 0.5 cm to 6 cm in length.The largestcrystal describedby Nickel et al. (1963)from the type localities found at the locality to date was nearly 13cm in length (D. Wodgina, Australia and Bernic Lake, Manitoba. Since its Richardson,pers. comm.). In transmittedlight, the wod- discovery, 18 wodginite localities have been cited in the ginite is translucent, has a reddish brown color along the literature. To date this is the first occurrenceof wodginite edges of thin splinters and is grayish-white in reflected in the U.S., along with wodginite from the Peerless light. The streak is light brown. A specific gravity deter- pegmatiteat Keystone, South Dakota (Cernf et al., in mination was not attempted due to the presence of review). This paper describesthe occurrence,chemistry cassiteriteinclusions. and properties of the Virginian wodginite and associated Nb,Ta-bearing minerals. X-ray crystallography X-ray powder-diffraction data were obtained on a Phil- Occurrence ips diffractometer using Ni-filtered CuKa radiation (I : The Herbb #2 pegmatitelies within a NE-trending belt l.54l8A) and scanningspeed l/2' ZPzlmin.X-ray powder- of pegmatitesapproximately 5.75 km NE of Flat Rock in diffraction data were calibrated with annealed CaF2 PowhatanCounty, Virginia (Fig. l). The arealies within @ : 5.a6204).Unit cell dimensionswere calculatedby the Piedmont province of the central Appalachians in the crrnpn computer program on both natural and heated central Virginia. The pegmatite is enclosed within a samplesof wodginite. The unit cell parametersare given feldspathic biotite schist. The pegmatite is lenticular in in Table 1. The results show good agreementwith previ- shape, strikes N20"E and dips steeply to the east. It is ously publishedwodginite cell dimensions(e.g., Graham approximately 75 meters long and 2 to 15 meters thick in and Thornber. 19741. outcrop. After heating in air at 1000"Cfor 16 hours, a slight shift This zoned pegmatite is mineralogically complex, and in the cell dimensionswas noticed.Sample VA-3 showed is similar to the Morefield and Rutherford pegmatitesof a significant decreasein the a, c and B dimensions. This the Amelia pegmatite district, except that it is deeply decreasein cell dimensionupon heatingmay be indicative weathered. Blocky, green perthitic microcline (amazon- ofan increasein ordering. The significanceofsuch a shift m03-004x/84/070E-0E07$02.00 E07 808 WISE AND CN,RNT: WODGINITE Fejjo"r.rrlTat.ozNbosE)>s.g6Orz (cf. Ferguson et al., 1976 for preferencesin site occupancies). Cassiterite A secondphase in the X-ray powder diffraction pattern of samplesVA-4 and VA-6 was identified as cassiterite. Unit cell dimensionsare given in Table l. The cassiterite occurs as elongated inclusions within the wodginite but shows no apparent orientation with respect to the wod- ginite structure. Under reflected light it is grayish-white like the wodginite; however, it is distinguishablefrom wodginite under crossednicols by its granular texture. Chemicalcomposition of a cassiteriteinclusion is given ih Table l. The Ta2O5,NbzOs and FeO contentsare consid- erably higher than those normally found in cassiterites, but correlate well with compositions of cassiteritesfrom Nb,Ta-enriched pegmatites, particularly wodginite-bear- ing ones(Cerni et al. l98lb). Manganocolumbite Manganocolumbitewas found as isolated euhedral crystalsin the pegmatitedumps. It is indistinguishablein hand specimenfrom wodginite and was identified by X- ray powder diffraction. In its natural state, the samplecan be described as a partially ordered columbite. This sam- ple becamefully ordered upon heating in air at 1000"Cfor 16 hours (cf. Komkov 1970,eernf and Turnock l97l for order-disorder relationships in the columbite-tantalite series). The chemical composition of manganocolumbite(Table O rKM l) showsa TalNb ratio of 0.40 and a Fe/Mn ratio of 0.90. Fig. l. Location of the Herbb#2 andneighboring pegmatites Table l. Chemicalcomposition of wodginite,mangano- in thePowhatan area. columbiteand cassiterite Sample nunbe r Ta205 t{b205 Ti02 Sn02 Fe203 Mno Feo tla20 Total in wodginiteis not known; it may possiblybe relatedto vA-z(w) 65.9 5.7 3.6 12.8 0.5 8.1 3.4 0,02 100.l2 reduction VA-3(w) 66.7 6.3 4 0 ll.3 0.5 8.0 3.5 0.04 100.34 of mosaicityor improvedordering of the struc- vA-4(w) 58.3 4.0 3.5 12,6 0.0 8.0 3.2 C.0 99.6 ture. VA-6(w) 67.8 s.0 3.5 12.5 0.7 8.4 3.1 c.0 101.0 VA-7(cb) 31.2 46.8 l 9 0.4 0.0 9.4 8.6 0.02 98.32 vA-68(c) 10.6 0.6 0.8 88.8 0.0 0.2 I .5 0.0 102.6 Chemical composition Tar* Nbl' Ti** snq* FeJ* ilnt* Fez* flalt Total ',|.18 Chemical analyses were performed on the MAC 5 vA-2(w) 7.42 1.07 | . t2 z.tl 0.19 2.84 0.02 15.95 vA-3(s) 7.46 1.17 | .24 I .85 0.l6 2,79 1.20 0.03 t5.90 election microprobein the wavelengthdispersive mode. vA-4({) 7.85 0.76 l.ll 2 t2 000 2.85 t.l3 0.00 15.83 VA-6(w) 7.62 0.93 1.09 2.06 0.22 2.94 1,07 0.00 15.93 The following standardswere used for the analyses: vA-7( cb) 2.20 5.49 0.37 0.04 0.00 2.07 t.87 0.00 12.05 vA-68( manganotantalite(Mn,Ta), chromite (Fe), stibiotantalite c) 0.14 0.01 0.03 1.74 0.00 0.01 0.07 c,00 2,w liaturaI t-R v, A- (Nb), cassiterite(Sn), SrTiO3 Gi), and microlite (Na,Ca). sR e,R gol+e(r)' The specimen vA-2(w) 9.469(2't -t-1.427(2\1r.433(2) s.lo8(r) s52.e(r) current was 0.(X mA at 15 kV. Data vA-3(w) 9.472(11 s.il0(r) e0:4s(r), ss3.o(l) reduction was made using a modified version vA-4(w) 9.469(3) 11.433(2) s.r09(r) 90:4e(r)' ss3.'l(r) of the vA-5(w) 9.474(2't 11.436(3) 5.106(r) 90"47(r), 5s3.2(r) EMrADRvrr computerprogram of Rucklidgeand Gaspar- vA-7(cb) 14.322(3) s.73e(l) 5.085(2) 4r8.0(r) vA-68(c) 4.735(I ) 3,r74(r) 7r.2l(3) rini (1969). Table I gives the results of the electron Heated microprobeanalyses. VA-2(w) e.46e(3) il.430(4) s.099(r) solsq(z)' s5r.9(2) The Herbb wodginites fall within the compositional VA-3(w) 9.457(2') r't.42713') 5.098(2) eo:55(2)' 5s0.8(2) vA-4(w) 9.467(3) 't't.424(2,il.425(4) 5.r00(2) eo:ss(2)' s5r.6(2) limits of other wodginites cited in the literature. The VA-6(w) 9.470(3) s.r00(2) 90"s7(2)' 5sr.5( 2) average chemical formula calculated on the basis of 32 VA-7(cb) r4.335(3) 5.738(r) s.07r(2) 417.r0) oxygens,and with Fe > 4.00A-cations (w) - wodginite, (cb) - mnganocolurblte, (c) - casslterite allocatedas Fe3* * based on 32 orygens for wodginite, l2 oxygens for nanganocolulblte, and to the C-site, is (Mn2.s6Fe?js)>aor(Snz.6aTi1.1aTaa.57 2 oxygens for casslterite WISE AND CTNNT: WODGINITE E(x) (xb.Taloe.s rating Ra+ into the minerals of the columbite-tantalite group.renrulato, andp2+ a ps+ into rutile-typephases. Figure 2 also shows the preference of tetragonal rutile- type structures for Fe and Ta, and the Mn and Ta enrichment of wodginite relative to the manganocolum- (Fe,Mn) (Nb,Ta )2 06 bite.
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