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An Unusual Pyroxene, Melilite, and Iron Oxide Mineral Assemblage in A

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An Unusual Pyroxene, Melilite, and Iron Oxide Mineral Assemblage in A American Mineralogist, Volume 72, pages 137-147, 1987 An unusual pyroxene,melilite, and iron oxide mineral assemblagein a coal-fire buchite from Buffalo, Wyoming FuNxr-rn F. Forr, Jn. Department of Geology, Washington State University, Pullman, Washington 99164, U.S.A. Ronnnr L. Hooprn Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701, U.S.A' Pnrr-rp E. Rosnunnnc Department of Geology, Washington State University, Pullman, Washington 99164' U.S.A. Ansrn-lcr Near-surface combustion of the Healy coal seam near Buffalo, Wyoming, baked and partially fused a 20-m-thick overlying sequenceof sandstone,siltstone, and shale produc- ing a multi-colored vesicular rock resemblingslag (buchite). This coal-fire buchite is highly heterogeneousand consists largely of clinopyroxenes, melilites, and oxides, with minor glass.The pyroxenes are unusually rich in Al and Fe3*, and the Fe3*-rich melilites and oxides have chemical compositions heretofore unreported in nature. Clinopyroxene compositions straddle the diopside (Diferri-aluminum Tschermaks (FATs) join in the system Di-CaTs (CaAlrSiOuFFTs (CaFel"SiOu) and range from DiroFTs,rCaTs,to DirFTsrrCaTs.n.Compositions approachingthe FATs endmemberhave a maximum tetrahedral Fe3+content of 0.17 atoms per formula unit (8.50/0tetrahedral occupancy).In the melilites, extensive coupled substitution of the type Fe3+ * Al3+ : Si4++ R2+ producescompositional variation away from the gehlenite-ferrigehlenitejoin, R3+(AlrSiO?)-R3+(Fel+SiOr),and toward the hypotheticalendmember R2+ R3+(R3*O'). The substitution of Fe3* and Al for Si and R2* appearsto involve both the four and the eight-coordinatedsites and results in an Fe3* content higher than in endmember ferrigeh- lenite. Both hematite and magnesioferriteshow extensive[l - pe:+ substitution resulting in a solid-solution series from magnesioferrite,ps:+(Fe3+,Mg)Oo, along the spinel-mag- nesioferrite join to SproMfroand along the magnesioferrite-hematitejoin to MfuoHmoo. Most compositions lie in the Al-rich field and have formulas that are slightly cation de- ficient. The chemical data suggestthe presenceof a miscibility gap between Sp'ooand Sp*oMfroand a solvus dome between hematite and magnesioferritesolid solutions. INrnooucrroN proximately 5 m (Mapel, 1959), thermally altered a 20- sequenceof shales, siltstones, and sandstones. Coal-frre buchites (partially fused rocks) produced by m-thick The zone of alteration is predominantly red owing to the the in situ, near-surfacenatural combustion of coal seams abundanceoffinely disseminatediron oxides,but in sev- are common geologicfeatures in the westernUnited States. placesthe overlying strata underwent partial fusion, Despite their conspicuouspresence and utilization as a eral resulting in a black vesicular rock assemblingslag (buch- sourceof road metal, little is known of their mineralogy. The largelygreenish-black buchite is in direct contact A buchite that crops out east of Buffalo, Wyoming (Fig. ite). with a reddish-brown siliceous shale that shows no evi- l), contains an unusual silicate and oxide mineral assem- of partial fusion. The protolith for the buchite was blage,which is the subject ofthis paper. dence probably a calcareoussiltstone or shale, both of which Gnor,ocrc sETTTNG stratigraphically overlie the Healy seam. Many of the buttes and divides east of Buffalo, Wyo- Mnrrrons oF ANALYSF ming, are capped with clinker produced by the near-sur- The buchitesample chosen for detailedexamination is mot- of the Walters and Healy coal seams face combustion tled in huesoforange-red, pale yellow-green, and blackand re- containedin the WasatchFormation (Dobrovolny, 198l). semblesa highlyvesiculated smelter slag. The samplewas sec- The buchite sample examined in this study was taken tionedacross the macroscopicallycolorful zones, and coverless from a clinker outcrop exposed along Interstate 90 ap- thin sectionswere prepared for petrogaphicand electron-micro- proximately 4 km eastof Buffalo, Wyoming. Combustion probeanalysis. Small fragmentsof mineralogicallyinteresting of the Healy seam, which locally has a thickness of ap- zoneswere ground to 200 meshand separated,using magnetic 0003-o04x/87/0l 02-o I 37$02.00 r37 138 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE ,a+ Table 1. Chemicalanalyses of the Buffalo, 4o^ Wyoming,coal-fire buchite Bulk Glass \3o "t""r chemistry. chemistfi Oxide (wt%) (wt%) sio, 47.24 54.s9 Al203 20.46 22.80 KrO 1.32 10.82 Na.O 1.36 6.11 CaO 9.80 0.64 Mgo 219 0.25 Fer03.' 14.87 3.41 Tio, 0.80 0.55 MnO 0.12 0.04 P.ou 1.84 n.d. Total 100.00 99.21 * X-ray fluorescenceanalysis using the method of Hooperand Atkins(1969) normalized to 100 wt%. '- Owing to the highly oxidizingconditions under Fig. 1. Map showing the location of the buchite along Inter- which this rock formed, all Fe reported as F€F*. state90 near Buffalo.Wyoming. f Number of point analysesequal to 20. and specifrc-gravitymethods, into fractions for detailed powder- diffraction analysis. Precision lattice parametersfor two of the ers of melilite and magnesioferritecrystals contained in mineral separates(pyroxene and melilite) were obtained by least- a pyroxenegroundmass loop around largepyroxene crys- squaresrefinement of CuKa dffiactometer data, which were in- tals as well as around gas vesicles,suggestive of viscous ternally calibrated with NaCl. flow of a crystalline mush. In other areas, micrometer- The whole-rock chemistry of the buchite was determined by mixing a representative3.5-g sample ground to 200 mesh with sized melilite riddles both the large pyroxene crystals as 7 g of lithiurn tetraborateand fusing it into a bead (Hooper and well as the fine-grained anorthite and glassgroundmass. Atkins, 1969),which was subsequentlyanalyzed using a Philips Mineral zoning around gasvesicles is also evident in thin P.W. 1410X-ray spectrograph. section. One particularly well-developed example con- sistsofa black oxide vesicle lining, surroundedby a layer of reddish-brown melilite, which in turn is encasedin a pETRocRApHy MrNrn-llocy, cHEMrsrRy, AND mass of yellow-greenpyroxene. Petrographic examination and X-ray powder-diffrac- The iron oxides range in size from tiny euhedral crys- tion analysis ofthe reddish-brown siliceous shale in con- tals to large irregular massesand stringers hundreds of tact with the buchite revealed the presence of mullite, micrometers long. Virtually all of the oxide massesand cristobalite, tridymite, hematite, and relic detrital qvartz, euhedra, regardlessof size, have components with high a mineral assemblage similar to that observed in the ther- (hematite) and low (magnesioferrite) reflectivity. The mally altered shales at Kemmerer, Wyoming (Hooper, smaller massesand euhedra frequently have a high-re- 1982). flectivity core and a lower-reflectivity rim, whereas the The various huespresent in the vesiculatedbuchite are larger massesmay consist of highly reflective lamellae or the product of fine-scalevariations in the minerals pres- numerous geometrically arrangedblebs encasedin a low- ent; the green portions are rich in pyroxene, the black er-reflectivity host reminiscent of the Fe-Ti oxide inter- pofrions are rich in iron oxides, and the orange-redpor- growths describedby Haggarty (1976). tions are rich in melilite and silicate glass.The gas vesi- The clinopyroxene is moderately pleochroic in hues of cles are lined with druzy orange silicate glass or with a pale green (a) and bright straw yellow (0 and 7). It dis- fine matte of feldspar, pyroxene, or iron oxide crystals. plays a relatively high birefringence (0.025-0.030) and Macroscopic examination of broken surfacesreveals a unlike most Fe-rich clinopyroxenesis optically negative color and mineral zoning concentric about the gas vesi- with 2V*: 65-75. The most striking opticalproperty of cles in many places. the small (-0.5 mm) tabular melilite crystals is their The bulk sample and associatedglass (Table l) are sig- intense color and very high birefringence (>0.040). The nificantly lower in silica and higher in alumina and ferric Fe-rich melilites aremoderately pleochroic from dark red- iron than buchites described from the Kemmerer coal dish-brown to orange-brown, whereas the more alumi- burn (Hooper,1982;Hooper et al., 1986),which may, in nous melilites have a yellowish-brown (e) to olive green part, explain its unusual mineral assemblage. (c.r)pleochroism. No gradation in color correspondingto In thin section,the buchite consistsof an intimate mix- compositional intermediateswas observed.The melilites ture (in decreasingorder ofabundance) ofanorthite, cli- appear to be optically negative over the entire range of nopyroxene,iron oxides (magnesioferriteand hematite), observedcomposition. The optical properties of the iso- melilite, and silicate glass.Most of the crystalline phases tropic magnesioferritecrystals are equally variable. The are euhedral with crystal sizesvarying from several mi- Fe-rich magnesioferritesare generallyopaque in thin sec- crometers to several millimeters. In some places,string- tion, although very thin crystalsdisplay a deepblood-red FOIT ET AL.: UNUSUAL ASSEMBLAGEIN A COAL-FIREBUCHITE 139 Di CaMsSi206 18 t6 i h l4 F" a 12 r < lo o8 06 ' r1 13 's 't", 23 25 r.$1 lirn " '"-ill::l3.5Lr- Fig. 3. The variation of Al + Fe3* with Si + Mg for the I-90 pyroxene (O : 1 point analysis per crystal and ! : multiple tl point analysesper crystal). Numbers of ions based on six oxy- gens. CATs FATs FTS CaAl2si06 caFe3tAtsi06 caFe;'si06 aluminum Tschermakscomponent (FATs; CaFeAlSiOu).
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