American Mineralogist, Volume 70, pages 1044-1049, 1985 Wodginite- and associatedoxide rninerals from the Peerlesspegmatite' PenningtonCountY, South Dakota P. eeRNf '"'*'wi::{;'il},fii#f ,'ty#:;'xl,"ty;nitoba W. L. Rosnnrs Departmentof Geology South Dakota School of Mines and Technology Rapid City, South Dakota 57701 T. S. Encn ^elip R. CHePIu,c,x Department of Earth Sciences,Unhsersity of Manitoba Winnipeg, Msnitoba, Canada R3T 2N2 Abstract Wodginite occursin one of the intermediatealbite-rich units of the Peerlesspegmatite, near Keystone, Pennington County, South Dakota. It is associatedwith cassiteritecontaining exsolvedtantalite and tapiolite, and with microlite. The chemicalcomposition of wodginite varies from (Mnr.nrFeo.uo)(Sn3.ruTa6.55Fef.!.Tio.ou)(Tau.g6Nb1.1a)O.r to Mn3.ee(Sn3.31 Tao.orFef.loTio.or)(Tar.rrNbo.rr)O.2. With decreasingtotal iron, the color of wodginite changesfrom black to pale cinnamon brown. The markedly monoclinic unit cell dimensions (q 9.533-9.552,b ll.47l-11.476, c 5.116-5.123A,p g}.8-.gl.2", v 559.4-561.5A3)suggest a well-orderedstructure. Wodginite and associatedmicrolite seemto be the final precipitatesin a sequence ferrocolumbite+ manganocolumbite+ manganotantalite- wodginite+ microlite. Fractionation progressesin this sequencefrom 0.07 to >0.93 in Tal[a + Nb) and from 0.39to >0.99 in Mn/(Mn * Fe). Introduction morphology and internal structureis provided by Sheridan (1957). To date, wodginite has been reported from at least 30 et al. pegmatite major sourceof feldspars, worldwide localities,but until recentlythis mineral has not The Peerless was a during the been recorded in the United States. The first U.S. oc- scrapmica, beryl and amblygonite-montebrasite sheet mica, curren@s were discoveredin the Black Hills of South first half of the 20th century. Punch and byproducts. Dakota in 1979,as describedin the presentpaper, and in columbite-tantaliteand cassiteritewere minor rare exampleof the Herbb no. 2 pegmatite,Virginia in 1981 (Wise and Petrologically,the Peerlessrepresents a pegmatitewhich contains all Li eernf, 1984). a phosphorus-richlithium bound in primary phosphates(particularly as huge quan- The parent pegmatite tities of amblygonite-montebrasite)to the virtual exclusion The Peerless pegmatite near Keystone, Pennington of anhydrous Li-aluminosilicates(cf. Burt and Londonn County, South Dakota is one of the more fractionatedand 1982for the phaserelationships). internally complex pegmatitesof the northern margin of A number of different minerals has been reported from the pegmatitefield of the southern Black Hills. According this pegmatite: plagioclase, qvafiz, muscovite varieties, to Sheridan et al. (1957),the pegmatite has an anticlinal microcline-perthiteand microcline are the most abundant shape,with a subhorizontalNW-trending crest flanked by rock-forming minerals; beryl, tourmaline, triphylite, SW- and NE-dipping limbs. The outcrop is about amblygonite-montebrasite,apatite, columbite-tantaliteand 185 x 110 m in size.Three major keels and severalminor cassiteriteare subordinate to a@essoryphases; minor to rolls complicate the apparently fracture-controlledunder- rare mineral speciesinclude garnet, biotite, zircon, spodu- side of the pegmatite,whereas the crestis relatively simple, mene,uraninite, chrysoberyl,tantalian rutile, opal, pyrolu- consistingessentially of two subparallelbulges. Internally, site, goethite,triploidite(?), carbonate-apatite, vivianite', fer- the pegmatiteconsists of 7 zones,2 replacementunits and 2 risicklerite, heterosite,libethenite, pseudomalachite, goya- types of fracture-filling units. A detailed descriptionof the zite, autunite, torbernite, siderite,malachite, azurite, loell- 0003-o04x/85/09r0-1044$02.00 t044 ABnni ET AL.: woDcINITE FR,M sourH DAKyTA 1(M5 ingit(?), pyrite, marcasite,stannite and kesterite.During Table 1. The examinedassociations of Nb, Ta, Sn-bearingmin- the present study, wodginite, microlite and tapiolite have erals from the PeerlessPegnratite beenadded to this list. The Peerlesspegmatite is a Na, p, F-enriched rare- Association A: platy columbite (l-6 mmthick, 3-80 cm am$T studdedwith zircon, in cleavelandite: elementpegmatite, with Li, Be and subordinateNb, Ta, Sn samplesA1(ct) to 45(ct). mineralization,and modest enrichmentin Rb and Cs. The -------60-Iffi-inAssociation B: thick tabular columbite- tantalite (5 to K/Rb and K/Cs ratios rcach 25.5 and 74Orespectively in maximum di mensi on ) i n coarse-orai ned a I bi te and cleavelandite, with subordinate nuicovite; the K-feldsparsof intermediatezones, 18 and 420 in mus- samplesBl(ct) to 84(ct). covite of intermediate zones, and 13.2 and 3E5 in late, -T20-mlAssociation C: coarse-grained anhedral cassiterite (5 to slightly Li-enriched muscovite of the central parts of the with exsolved columbite-tantalite and taDio- pegmatite Iite, in cleavelandite+ muscoviteadjacent to (0.35 wt.% LirO). Beryl also shows increased massivequartz; samples contentsof LirO (< 1.10wt.o/o) and CsrO (< 1.05wt.%). Cl(c) with exsolvedCl (ct) C2(c) with exsolvedC2(ct) Paragenetic relationships C3(c) with exsolvedC3(t) of wodginite C4(c) with exsolvedC4(t) Wodginite was found in the cleavelanditefringe of an Association D: wodginite (divergent columnaraggregate albitic unit exposedin the northeastern,partly overhanging 50 mmlong) with inclusions and/or exsolutionsof cassiterite,'landite microlite and tapiolite, in cleave- wall of the main pegmatitequarry, very close to the E-8, + muscovite adjacent to massivequartz; sectionline ofSheridan et al. (l957,plate 4). The cleavelan- samDI es dite fringe which containsintergrown Dl(w) black-brownwith overgrowthof Dl(t), muscoviteis located and D](c) with exsolvedD](ct) along the contact of the albitic unit with an underllng D2(w) brown D3(w) pale brownw'ith massof qvaftz. SectionE-E' in Sheridanet al. (1957,plate D3(c) and D3(m) 6) indicates that the present exposure correspondsto a A, B, C, D - mineral associations;l, 2, 3,.., - samDles segmentof quartz zone 6b overlain within eachmineral association;ct - columbite-tantalite. by the 3rd intermediate c.- cassiterite, w - wodginite, t - tapiolite, m - micro- cleavelandite+ quartz zone 5, as designatedby theseau- n f,e. thors. Becauseof the relative abundanceof columbite-tantalite and cassiteritein the wodginite-bearingalbitic unit, repre- procedure; becauseof the lack of material, it was X-rayed in a sentativespecimens of theseminerals were collected over Gandolfi camera and the film data were corrected for shrinkage. the accessiblewidth of this unit, and examinedin order to In all cases,the program cenrr (Appleman and Evans, 1973)was establishthe generalparagenetic and geochemicalenviron- usedfor unit cell refinement. ment in which the wodginite occurs.The different mineral In the heating experiments, separatefragments 0.5 to 2 mm in associationsare listed in Table 1, letter-codedin the se- size were extracted from the samples and were heated in air at 1000'C for 16 hours. Heating quence of their occurrencefrom inside the albitic unit relatively coarse-grainedmaterial toward the massivequartz zone. minimizesoxidation of Fe2* to sucha degreethat unit cell dimen- sions of columbites-tantalitesheated in air and in controlled at- Experimental methods mosphere (CO1CO2: 2.51D are statistically identical (unpubl. data of eernf and Turnock). All chemicalanalyses were done with an MAC 5 electronmi- croprobeoperating in both wavelength-dispersive(WD) and Mineralogy energy-dispersive(ED) modes.Standards used were cassiterite (SnLc), chromite (FeKa), sphene (TiKc), manganotantaliteWodginite (MnKc, TaLa, Md), BarNaNbrO* and CaNbrO" (NbLa). Con- The divergentcluster of subhedraland somewhatskele- ditions of analysis were: acceleratingpotentials of 20, 15 kV, samplecurrents of 40 nA (WD) and 5 nA (ED) measuredon brass tal wodginite crystals is black at the point of spreading, (WD) and ZnS (ED), count times of l0 s (WD) and 200 live grading into medium-brown and turning pale cinnamon- seconds(ED). Wavelengthdispersive spectral data were reduced brown at the diverging crystal terminations. The black with the program EMnlDR vrr (Rucklidge Gasparrini, 1969). point of the cluster contains cassiteritegrains with micro- Energy-dispersivespectra were collected with a xsvEx Model 7000 scopic tantalite inclusions, and microgranular tapiolite ED spectrometer and were reduced with rEvE ( software utilizing along contacts of cassiteritewith wodginite. Intermediate the program MAcrc v (Colby, 1980).Peak overlap problems en- parts of the wodginite clusterare not associatedwith other countered during analysis of ED spectra were resolved by strip- phases;however, the divergingwodginite terminationscon- ping techniquesinvolving library spectra. tain exsolvedcassiterite, and euhedralcrystals of pale gray Most unit cell dimensionswere calculatedfrom X-ray powder microlite along wodginitecontacts with albite. diffraction data obtained with Philips X-ray diffractometers using In general, the wodginite Ni-filtered or graphite-monochromatedCuKa radiation (,l': composition is close to the ideal formula MnSnTarOr, particularly 1.541SA)and scanningspoeds of U4 to U2" 2llmin. A minimum in the diverging of 10 and a maximum of 33 reflections measuredbetween 10. and pale-brown terminations (associationD, Table 1, Fig. 1). 7O"20 were used in least-squaresrefinement of the data. Annealed AnalysesDlw and D3w are among the most Sn-rich com- CaF, (a : 5.46204)was used as an internal calibration standard. positions found in wodginites(unpubl. data of T.S.E. and Wodginite