American Mineralogist, Volume 72, pages 137-147, 1987

An unusual pyroxene, melilite, and iron oxide mineralassemblage in 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. HooprnDepartment of Geology, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54701, U.S.A'

Pnrr-rp E. RosnunnncDepartment 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 andpartially fused a 20-m-thick overlying sequence of sandstone, siltstone, and shale produc-

ing a multi-colored vesicular rock resembling slag (buchite). This coal-fire buchite is highlyheterogeneous and consists largely of clinopyroxenes, melilites, and oxides, with minorglass. The pyroxenes are unusually rich in Al and Fe3*, and the Fe3*-rich melilites andoxides 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 fromDiroFTs,rCaTs, to DirFTsrrCaTs.n. Compositions approaching the FATs endmember havea maximum tetrahedral Fe3+ content of 0.17 atoms per formula unit (8.50/0 tetrahedraloccupancy). In the melilites, extensive coupled substitution of the type Fe3+ * Al3+ :

Si4+ + R2+ produces compositional variation away from the gehlenite-ferrigehlenite join,

R3+(AlrSiO?)-R3+(Fel+SiOr), and toward the hypothetical endmember R2+ R3+(R3*O').The substitution of Fe3* and Al for Si and R2* appears to involve both the four and theeight-coordinated sites and results in an Fe3* content higher than in endmember ferrigeh-lenite. Both hematite and magnesioferrite show extensive [l - pe:+ substitution resultingin a solid-solution series from magnesioferrite, ps:+(Fe3+,Mg)Oo, along the spinel-mag-nesioferrite join to SproMfro and along the magnesioferrite-hematite join to MfuoHmoo.Most compositions lie in the Al-rich field and have formulas that are slightly cation de-ficient. The chemical data suggest the presence of a miscibility gap between Sp'oo andSp*oMfro and a solvus dome between hematite and magnesioferrite solid solutions.

INrnooucrroN

Coal-frre buchites (partially fused rocks) produced bythe in situ, near-surface natural combustion of coal seamsare common geologic features in the western United States.Despite their conspicuous presence and utilization as asource of road metal, little is known of their mineralogy.A buchite that crops out east of Buffalo, Wyoming (Fig.l), contains an unusual silicate and oxide mineral assem-blage, which is the subject ofthis paper.

Gnor,ocrc sETTTNG

Many of the buttes and divides east of Buffalo, Wyo-ming, are capped with clinker produced by the near-sur-face combustion of the Walters and Healy coal seamscontained in the Wasatch Formation (Dobrovolny, 198 l).The buchite sample examined in this study was takenfrom a clinker outcrop exposed along Interstate 90 ap-proximately 4 km east of Buffalo, Wyoming. Combustionof the Healy seam, which locally has a thickness of ap-

0003-o04x/87/0 l 02-o I 37$02.00

proximately 5 m (Mapel, 1959), thermally altered a 20-m-thick sequence of shales, siltstones, and sandstones.The zone of alteration is predominantly red owing to theabundance offinely disseminated iron oxides, but in sev-eral places the overlying strata underwent partial fusion,resulting in a black vesicular rock assembling slag (buch-ite). The largely greenish-black buchite is in direct contactwith a reddish-brown siliceous shale that shows no evi-dence of partial fusion. The protolith for the buchite wasprobably a calcareous siltstone or shale, both of whichstratigraphically overlie the Healy seam.

Mnrrrons oF ANALYSF

The buchite sample chosen for detailed examination is mot-tled in hues oforange-red, pale yellow-green, and black and re-sembles a highly vesiculated smelter slag. The sample was sec-tioned across the macroscopically colorful zones, and coverlessthin sections were prepared for petrogaphic and electron-micro-probe analysis. Small fragments of mineralogically interestingzones were ground to 200 mesh and separated, using magnetic

r37

,a+

"t""r4o^

\3o

138 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

Table 1. Chemical analyses of the Buffalo,Wyoming, coal-fire buchite

Bulk Glasschemistry. chemistfi

Oxide (wt%) (wt%)

sio,Al203KrONa.OCaOMgoFer03.'Tio,MnOP.ou

Total

47.2420.461.321.369.802 1 9

14.870.800.121.84

100.00

54.s922.8010.826.110.640.253.410.550.04n.d.

99.21

Fig. 1. Map showing the location of the buchite along Inter-state 90 near Buffalo. Wyoming.

and specifrc-gravity methods, into fractions for detailed powder-diffraction analysis. Precision lattice parameters for two of themineral separates (pyroxene and melilite) were obtained by least-squares refinement of CuKa dffiactometer data, which were in-ternally calibrated with NaCl.

The whole-rock chemistry of the buchite was determined bymixing a representative 3.5-g sample ground to 200 mesh with7 g of lithiurn tetraborate and fusing it into a bead (Hooper andAtkins, 1969), which was subsequently analyzed using a PhilipsP.W. 1410 X-ray spectrograph.

MrNrn-llocy, cHEMrsrRy, AND pETRocRApHy

Petrographic examination and X-ray powder-diffrac-tion analysis ofthe reddish-brown siliceous shale in con-tact with the buchite revealed the presence of mullite,cristobalite, tridymite, hematite, and relic detrital qvartz,a mineral assemblage similar to that observed in the ther-mally altered shales at Kemmerer, Wyoming (Hooper,1982).

The various hues present in the vesiculated buchite arethe product of fine-scale variations in the minerals pres-ent; the green portions are rich in pyroxene, the blackpofrions are rich in iron oxides, and the orange-red por-tions are rich in melilite and silicate glass. The gas vesi-cles are lined with druzy orange silicate glass or with afine matte of feldspar, pyroxene, or iron oxide crystals.Macroscopic examination of broken surfaces reveals acolor and mineral zoning concentric about the gas vesi-cles in many places.

The bulk sample and associated glass (Table l) are sig-nificantly lower in silica and higher in alumina and ferriciron than buchites described from the Kemmerer coalburn (Hooper,1982; Hooper et al., 1986), which may, inpart, explain its unusual mineral assemblage.

In thin section, the buchite consists of an intimate mix-ture (in decreasing order ofabundance) ofanorthite, cli-nopyroxene, iron oxides (magnesioferrite and hematite),melilite, and silicate glass. Most of the crystalline phasesare euhedral with crystal sizes varying from several mi-crometers to several millimeters. In some places, string-

* X-ray fluorescence analysis using the method ofHooper and Atkins (1969) normalized to 100 wt%.

'- Owing to the highly oxidizing conditions underwhich this rock formed, all Fe reported as F€F*.

f Number of point analyses equal to 20.

ers of melilite and magnesioferrite crystals contained ina pyroxene groundmass loop around large pyroxene crys-tals as well as around gas vesicles, suggestive of viscousflow of a crystalline mush. In other areas, micrometer-sized melilite riddles both the large pyroxene crystals aswell as the fine-grained anorthite and glass groundmass.Mineral zoning around gas vesicles is also evident in thinsection. One particularly well-developed example con-sists ofa black oxide vesicle lining, surrounded by a layerof reddish-brown melilite, which in turn is encased in amass of yellow-green pyroxene.

The iron oxides range in size from tiny euhedral crys-tals to large irregular masses and stringers hundreds ofmicrometers long. Virtually all of the oxide masses andeuhedra, regardless of size, have components with high(hematite) and low (magnesioferrite) reflectivity. Thesmaller masses and euhedra frequently have a high-re-flectivity core and a lower-reflectivity rim, whereas thelarger masses may consist of highly reflective lamellae ornumerous geometrically arranged blebs encased in a low-er-reflectivity host reminiscent of the Fe-Ti oxide inter-growths described by Haggarty (1976).

The clinopyroxene is moderately pleochroic in hues ofpale green (a) and bright straw yellow (0 and 7). It dis-plays a relatively high birefringence (0.025-0.030) andunlike most Fe-rich clinopyroxenes is optically negativewith 2V*: 65-7 5. The most striking optical property ofthe small (-0.5 mm) tabular melilite crystals is theirintense color and very high birefringence (>0.040). TheFe-rich melilites are moderately pleochroic from dark red-dish-brown to orange-brown, whereas the more alumi-nous melilites have a yellowish-brown (e) to olive green(c.r) pleochroism. No gradation in color corresponding tocompositional intermediates was observed. The melilitesappear to be optically negative over the entire range ofobserved composition. The optical properties of the iso-tropic magnesioferrite crystals are equally variable. TheFe-rich magnesioferrites are generally opaque in thin sec-tion, although very thin crystals display a deep blood-red

FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

DiCaMsSi206

o 8

139

1 8

t 6

l 4

a 1 2r

< l o

0 6

CATsCaAl2si06

FATscaFe3tAtsi06

FTScaFe;'si06

Fig. 2. The compositions of clinopyroxenes from the I-90buchite (O : I point analysis per crystal and tr : multiple pointanalyses per crystal) and fassaites and fassaitic augites (r) takenfrom the literature by Huckenholz et al. (1974). The hexagonand filled square represent buchite clinopyroxene compositionsreported by Cosca and Essene (1985) and Cosca et al. (1985).The diagram is a modification of Fig. 19 in Huckenholz et al.(1974). Dashed lines define the area ofclinopyroxene solid so-lutions at low pressure (see Huckenholz et al., 197 4, for details).

color. The more aluminous magnesioferrites are trans-parent and range in color from orange to colorless. Theunusual optical properties ofthe clinopyroxene and mel-ilite, suggestive of an interesting chemistry, provided theimpetus for this investigation.

Cr,rNopvnoxnNB

Mineral chemistry

The variations in clinopyroxene optical properties ob-served in thin section are correlated to variations in Feand Al content that are observed over hundreds or inplaces tens of micrometers. In general, the deeper shadesofgreen and the more marked pleochroism are associatedwith higher Fe contents. The clinopyroxene microprobedala are presented assuming Fe in the trivalent state.This is consistent with the extremely oxidizing conditionsof formation and calculated structural formulas with ex-tremely small to nonexistent cation deficiencies (Caw-thorn and Collerson, 1974). Ca is nearly stoichiometric,0.92-1.00 cations per formula unit (pfu), and Ti and Naare present in only minor amounts, 0.02-0.04 cationspfu.

Microchemical analysis of approximately 70 crystalsrevealed a wide range of chemical variability from Fe3+-and Al-poor, Si- and Mg-rich compositions (crystal 94,Table 2; Fig. 2) approaching endmember diopside (Di,CaMgSirOu) to Fe3+- and Al-rich, Si- and Mg-poor com-positions (crystal 65, Table 2; Fie. 2) approaching ferri-

r 1 1 3 ' s ' t " , l i r n "

2 3 2 5

Fig. 3. The variation of Al + Fe3* with Si + Mg for the I-90pyroxene (O : 1 point analysis per crystal and ! : multiplepoint analyses per crystal). Numbers of ions based on six oxy-gens.

aluminum Tschermaks component (FATs; CaFeAlSiOu).The observed compositional variability (Fig. 2) is re-stricted to the area ofclinopyroxene solid solution at lowpressure that straddles the Di-FATs join in the Di-CaTs(CaAl,SiOuFFTs (CaFe3*SiOr) system (Huckenholz et al.,1974). Thus, the compositional variability of these cli-nopyroxenes largely conforms to the coupled substitutionSio* + Mgz+ : Fe3+ * Al3* (Fig. 3). Only crystals withcompositions approaching FATs exhibited measurablechemical zoning as is reflected in the standard deviationsof oxide weight percentages obtained for crystal 52 (Table2).

Many of the clinopyroxene crystals have higher Fe3*and Al contents and lower Si contents than any naturalpyroxene (e.g., fassaites) previously reported (Majmun-dar, 1971 Mason, 1974 Devine and Sigurdsson, 1980;Fig. 2) including two from another coal-fire buchite in thePowder River Basin of Wyoming (Cosca and Essene, 1985;Cosca et al., 1985). Although Cosca et al. (1985) reportedcompositions ranging from diopside to "near end mem-ber ferri-aluminum Tschermaks," the chemical compo-sitions (see also Cosca and Essene, 1985) are displacedfrom this join toward CATS (Fig. 2). A complete rangeof substitution between Di and FATs in coal-fire buchitesis to be expected since the experimental studies of Hiji-kata and Onuma (1969) and Huckenholz et al. (1974)reveal complete solid solution between Di and FATs andbetween FTsrrCaTs, and FTsorCaTss2, respectively, above1100'C at I atm.

Ifcation occupancies are assigned assuming that Si de-ficiencies in the tetrahedral site are compensated by Aland that any remaining deficiency is compensated by Fe3*(Hartman, 1969; Ohashi and Hariya, 1973; Ghose et al.,1975; Huggins et al., 1977), then for most of the crystals,virtually all of the available Al and generally a smallamount of Fe3+ (<0. 17 cations pfu) are in tetrahedralcoordination. The preference of the tetrahedral site forAl over Fe3+ is reflected in the trends in the chemical data.Crystals with higher Fe3+/Al atom ratios (> 1.5) have moreSi (less Al) in the tetrahedral site (Fig. 4). Conversely, in

ih

F"

r . $1 'l3.5Lr-'"-ill::t l

140 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

Table 2. Electron-microprobe analyses of Fe3+-richclinopyroxene in the buchite

Crystal'52

Crystal Crystal Composite of65 94 74 crystals

sio,Alro3Tio,Fe.O.t*MgoMnOCaONaroK"O

Total

32.28(4.3s)14.13(3.24)0.91(22)

25.21(4.60)4.4s(2.61)0.1 3(8)

23.06(29)0.32(15)0.02(3)

100.51(63)

1 .29

0.020.76o.26

0.980.02

Weight percent27.72(1.15) 26.70 47 .1215.14(1.08) 15.21 3.961.04(23) 0.74 0.54

31.86(2.47) 32.35 9.981.92(471 1.44 14.220.08(4) 0.0s 0.25

22.64\241 22.73 22.980.24(14) 0.16 0.440.00 0.00 0.01

100.67(54) 99.38 99.50

Cations per six oxygen1 . 1 3 1 . 1 ' t 1 . 7 70.73 0.74 0.180.03 0.02 0.020.98 1.01 0.280.12 0.09 0.80

0.010.99 1.01 0.930.02 0.01 0.03

Formulas assuming a tetrahedral-sitepreference Si > Al > Fe3*

1 4 0

Fe"

A I

o 8 0

SiAITiFe3*MgMn

NaK

1 3 0 1 5 0

Crystal 52: (Cao sNao ojXFqi.Mgo,rTioo"Nao o,)(Sir r3Alo ?3F414)O6Crystal 65: (Ca1 ooxFeS.i6MgoosTioo2Naoo,CaoolL]o01XSi1 1rAlo74F415)O6Crystal 94: (CaoesNa€ GMgoo5XMgo?5F€j3Tio orXSi, ??Alo 16Fqt5)O6Composite: (CaorsNaoor(F4joMgoruTioornoor)(Si1 2sAlosFqi6)O6

. Average of 13 point analyses on the same crystal.'* Fe assumed to be in the trivalent state because (1) of the extremely

oxidizing conditions ol clinopyroxene formation and (2) cation totals areor are very nearly stoichiometric for all of the crystals analyzed.

crystals having lower Fe3*/Al atom ratios (< 1.5), the tet-rahedral site is usually more deficient in Si, allowing theinclusion of more Al. This is not to imply that at highFe3+/Al atom ratios there is no IVFe3+. In almost all ofthe crystals there is insufficient Si and Al to fill the tet-rahedral sites. A high positive correlation, r: 0.83, be-tween the tetrahedral site deficiency and the Fe3+/Al atomratio (Fig. 5) suggests that in crystals having the highestratio, a small amount of Fe3+ (<0. 17 atoms pfu) is intetrahedral coordination. The crystals lying above thedashed line in Figure 5 have the highest Si and Mg con-tents (Si + Mg > 2.0 atoms pfu). The high Si content ofthese crystals results in a smaller than expected tetrahe-dral deficiency (I"tr) and, therefore, a smaller than ex-pected amount of Fe3+ substitution in the tetrahedral sitesdespite their very high Fe3+/Al ratios.

Although X-ray structural studies of natural Fe3+- andAl-rich clinopyroxenes (Peacor, 1967;Hazen and Finger,1977; Cosca et al., 1985) have not revealed the presenceof ''Fe3*, crystal-chemical considerations (Hartman, 1969)and X-ray structure and 57Fe Mdssbauer studies indicatethat some Fe3+ substitution in clinopyroxene tetrahedralsites is possible. A Mdssbauer spectral analysis of silica-deficient clinopyroxenes on the join CaFe3+AlSiO6-CaTiAlrOu (Akasaka, 1983) and an X-ray structure anal-ysis of synthetic clinopyroxene of FATs composition,CaFe3+AlSiOu, (Ghose et al., 1975), revealed that 4.5-

Esi

Fig. 4. The variaticn of Si with the Fe3+/Al atom ratio (O :I point analysis per crystal and tr : multiple point analyses percrystal). Numbers of ions based on six oxygens.

9.00/o ofthe tetrahedral sites are occupied by Fe3+, a rangecomparable to the 0-8.50/o observed for the buchite cli-nopyroxenes from Buffalo, Wyoming. Likewise, Kurepinet al. (1981) reported that only 2/z of the octahedral sitesin a FATs plroxene (CaFe3+AlSiOu) synthesized at 1200'Care occupied by Fe3+, implying thal l7o/o of the tetrahe-dral sites are occupied by Fe3+. A silica-deficient, Al- andFe3*-rich clinopyroxene identified in iron ore sinters (Dy-son and Juckes, I 972) requires the presence of Fe3+ in upto 160/o ofthe tetrahedral sites.

It is not surprising in light of our data that the ferri-aluminum clinopyroxene, (Car o, Nao or )(Feo urMgo rr-Alo,oTio o.Feo or)(Al0 ?esir,,)Ou, from a Wyoming coal-firebuchite examined by Cosca et al. (1985) did not containdetectable IvFe3+ The Fe3*/Al atom ratio in this clino-pyroxene is 0.73, which is in the range of ratios for crys-tals in which apparently no IVFes+ is present (Fig. 5), andthis crystal contains sufficient Si and Al to completely fillthe tetrahedral site. The amount of IvFe3+ is very low(<90/o of the tetrahedral sites) even in the clinopyroxenesthat have relatively high Fe3*/Al atom ratios (1.0-1.7).

X-ray data

Enough ferri-aluminum clinopyroxene for an X-raypowder-diffraction analysis internally calibrated with NaClwas obtained through heavyJiquid and magnetic sepa-ration techniques. The indexed pattern is presented inTable 3. Least-squares refinement of the unit-cell param-eters in space group C2/c (Cosca et al., 1985) using the18 most intense reflections yielded a -- 9.793(3) A, b :

8.835(4) A, c : 5.326(4) A, P : 105.84(4f and cell vol-ume : 443.3(3) A'. These parameters and cell volume(Table 3) are close to those obtained for a synthetic Fe3+-and Al-rich clinopyroxene, Ca(Feo rMg.oTio'XAlo ?Sil 3)06(Akasaka and Onuma, 1980), having a similar composi-tion but are significantly larger than those of syntheticFATs, CaFe3+AlSiO6, (Huckenholz et al., 1974). The

-%. '

' t t8 .

. r . |

. //. /

//

/ t a

" . t l

" 1 .9 : .^ 6

I

FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE t4l

Table 3. X-ray powder-diffraction pattern and least-squaresrefined cell parameters of the Fe3*-rich pyroxene and the

synthetic Fe3*-rich pyroxene of Akasaka andOnuma (1980)

Fe3*- and Al-rich clinopyroxene Fassaite*'

d* (A) ilh d-. (A) d* (A) ilh

Fe-AI

4oc

11 7

10033273146

421 21 1

8o

1 91 11 41 41 5o5

1 1

o7

1 41 2

c

3o

1 51 01 0

c

55

1002020

40

T 5

1 5

2220

5

1 03224

1 7

o 1 2

1 1 0200o201 1 12202213 1 03 1 1i312020022213 1 1112222o223303314210414022021325 1 0150I F l

313042223531440600602

6.434.714.43

3.2155.3.0007"2.9589-29022'2.5731.

2.5603.

2.5238-2.3174-

2.1460-2.1290'2.1128.

2.0321-

1.9956

1.8421.1.7372',

1 .67

1 .641.6248.1 . 6 1 1 1 '1 .561.5394'

6.4444 6.4714.7104 4.7104.4174 4.4533.6888 3.6893.2222 3.2362.9909 3.0022.9589 2.9612.9065 2.9092.5536 2.5682.5632 2.5612.5616 2.5582.5234 2.5282.3171 2.316

2.2392.2202.218

2-1481 2.1572.1278 2.1372.1111 2 .115

2.0422.0321 2.0322.0298 2.0271.9751 1.98091.8427 1 .84311.7367 1.7502

1.68241 .6777 1.67941.6728 1 .67691.6411 1 .64131.6254 1 .62951 . 6 1 1 1 1 . 6 1 7 81.5701 1.56991 .5389 1.5391

*E

Fig. 5. The variation of inferred tetrahedral site deficiency('"tr) with the Fe3+/Al atom ratio (O : 1 point analysis percrystal and tr : multiple point analyses per crystal). tvg : f .Q -(Si + Al) with numbers of ions based on six oxygens per formulaunit. Crystals lying above the dashed line have a high Si + Mgcontent (Si > 1.50 atoms pfu, Mg > 0.49 atoms pfu) whereasthose below have a low Si + Mg content (Si < 1.50 atoms pfuand Mg < 0.45 atoms pfu). X: Fe3+/Al ratio in FATs.

smaller c unit-cell parameter of the ferri-aluminum cli-nopyroxene compared to FATs clinopyroxene is due tothe replacement of Al3+ by the smaller Si4+ in the tetra-hedral chain, whereas the larger b and c parameters aredue to the replacement of Fe3+ and Al3+ in the Ml siteby the larger Mg2+ cation. The angle B is largely un-changed because of the complete occupancy of the M2site by Ca in all three of these clinopyroxenes (Hucken-holz et al., 19'14).

Mnr,rr,rrnMineral chernistry

Most melilites are solid solutions of [kermanite-Ca,(MgSirO,), gehlenite-Car(AlrSiOr), Fe ikermanite-Car(Fer*SirOr), and Na melilite-NaCa(AlSirO,) and oc-cur in undersaturated igneous rocks, thermally metamor-phosed impure carbonates, and blast furnace slags (Deeret al., 1963). Although melilites close in composition tothese ideal endmembers are rare in nature, all of the end-members (Akermanite, Osborn and Schairer, l94l; geh-lenite, Yoder, 1950; Fe lkermanite, Bowen et al., 1933;Na melilite, Nurse and Midgley, 1953) have apparentlybeen synthesized. Seifert (1974) has suggested that Fe Ak-ermanite is not stable and that a maximum solid solutionof 800/o Fe Akermanite in lkermanite is achieved at I 170'Cand I atm. Moreover, Huckenholz and Ott (1978) havereported that Fe3+ + Al3+ substitution in gehlenite reach-es a maximum of 37.5 wto/o of the ferrigehlenite compo-nent (CarFel+SiO,) at 1237"C and I atm and that Fe geh-lenite (CarFe3+AlSiOi) is not stable at any temperaturein the pressure range I atm to 7 kbar.

Fe3*- and Synthetic Fe3*-Al-rich and Al-rich

clinopyroxene clinopyroxenet Synthetic FATS+

9.771(2)8.843(2)5.334(1)

106.01(2)443.0(2)

* Reflections used in least-squares refinement of unit-cell parameters.'* Fassaite data from Borg and Smith (1969; p.250-2521' Reflections

with //o < 5 that do not correspond to observed Fe3*-rich clinopyroxenereflections have been omitted.

t Having composition Ca(FeosMgo4Tio,)(Si,3Alo7)06 (Akasaka and On-uma, 1980).

+ Huckenholz et al., 1974.

The buchite melilite is unusual in that many of thecrystals are richer in Fe3+ (FerO, averages 46. 14 wto/o witha maximum of 60.09 wto/0, Table 4) and poorer in Si andCa than the endmember ferrigehlenite (FerO, : 48.11wto/o); this is a result of the coupled substitution Fe3+ *Al3* : Si4+ + R2*, which displaces compositions fromthe "gehlenite-ferrigehlenite" join, R3+(AI,SiO?)R3+-(Fel*SiOr), toward the hypothetical endmemberR'z+R3+(R;+O') (Fig. 6). The major compositional vari-ability in these melilites involves Fe3+ - Al3+ substitu-tion with very little variability in Si, Mg, or Ca, which isreflected in the small standard deviations of the analyses

411 7

1 7

1 7

a (A)b (A)c (A)p f )v(A1

9.7e3(3)8.835(4)5.326(3)

105.84(3)443.3(3)

9.751(9)8.781(8)5.360(8)

105.87(8)441.4(7)

R- R"1R; o?)

t42 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

Table 4. Electron-microprobe analyses of Fe3*-rich melilites

Composite

Single 36 36crystal- crystalst. crystals

High HighAl F€F*

melilite melilite

sio,Alro3Tio,FerO"Cr"O"FeOMnONioMgoCaONaroKrO

Total

1 1 .85(21)1 7 30(50)0.42(31

s1 .31(90)n.d.

0.21(6)n.o6.14(25)

12.74(s)0.31(6)0.03(3)

100.31(50)

0 6 11.050.021.99

0.01

0.470.700.03tr.

Wdght percent11 .73(1 .27) 11 .73 12.55 9.3621.09(5.03) 21.09 29.56 11.850.91(48) 0.91 1 .60 0.57

46.14(6.221 42.01 36.97 60.090.05(3) 0.0s o.02 0.00

3.720.23(10) O.23 0.14 0.350.02(2) o.02 0.02 0.026.96(1.16) 6.96 6.74 s.91

12 95(34) 't2.95 13.47 12.330.31(1 1) 0.31 n.d n.d.0.02(21 0.O2 n.d. n.d.

100.41(71) 100 00 101.07 100.50

Cations per seven oxygensn2rtr"!-sio2 n!1r"3*ersiorl nftrrrsiorl

Fig. 6. The compositions of melilite from the I-90 buchite.Melilite compositions plotted on the basis of assumed tetrahe-dral-site occupancies (O: I point analysis per crystal and !:multiple point analyses per crystal). R3*(Al,SiO?) : "gehlenite,"CarFes+AlSiO, : "Fe gehlenite," R?+(Fe1+SiOr) : "ferri-geh-lenite" where pz+ : principally Ca.

(Table 4) and the limited range in Si + Mg and Fe3+ +Al cation totals (Fig. 7). This is in sharp contrast to thewide variation in Si, Mg, Al, and Fe3+ in the clinopyrox-enes due to Fe3* + Al3+ + Sia+ + Mgr+ coupling alongthe Di-FATs join. The absence of melilite compositionsthat approach endmember ferrigehlenite or Fe gehlenitesupports the contention of Huckenholz and Ott (1978)that these phases are not stable at I atm. There is verylittle compositional zoning in these melilite crystals, asdemonstrated by the small standard deviations of oxideweight percentages of random point analyses of a singlecrystal (Table 4).

1 2

t " . t

Al+ Fet '

Fig. 7. The variation of Fe3+ * Al with Si + Mg in melilitesfrom the I-90 buchite (O : I point analysis per crystal and tr :multiple point analyses per crystal).

Single crystal: (Cao 7oFe8:sMgo4?no,,Nao*Tio*Mno o,XFqi4All qSio61)O?Composite 1 : (Cao,oFefjrMgo $lo 11Nao @Tio osMno orXF4l6Al1 2ESio se)O?Composite 2:

(Cao7rFe8l?Feo i6Mgoslo 06Nao oJio@Mno o')(F414Al' 265io60)07High-Al meli l i te: (Cao6eFe8loMgo4s!o l6Tio06Mno olXF4j3Al, 6,Sio60)07High-Fe melil i te: (GaolFeflrMgoorlo 11Tio@Mnoor)(Fqt5Alo,ssios)O,

Note. Values in oarentheses are the standard deviations. Total Fe re-ported as Fe3*.

. Values represent the mean of I point analyses on this crystal.t* Values represent the mean of 36 point analyses on 36 different crys-

tals.

Assignment of the available (Si,Al) and Ca atoms tothe tetrahedral and eight-coordinated sites, respectively,in compliance with the structural data of Louisnathan(1971) and Kimata and Ii (1982), leaves these sites slight-ly cation deficient. Although Mg is normally found intetrahedral coordination in melilites (6kermanite; Kimataand Ii, l98l), it was assigned to the eight-coordinatedsite because it is appreciably larger than Fe3+, leaving theremaining tetrahedral sites to be filled by Fe:+ (Table 4).This is consistent with the site assignments for compo-sitions synthesized on the gehlenite-ferrigehlenite join(Huckenholz and Ott, 1978). Even if all the available Si,Al, and Mg were placed in tetrahedral coordination, asignificant Fe3+ occupancy ofthe tetrahedral sites wouldstill be required. Assigning the cations according to thisscheme leads to nearly a constant ratio of Ca: Mg: Fe3+in the eight-coordinated site and a variable Fe3+ : Al ratioin the tetrahedral sites.

A significant cation deficiency is common to all of themelilite structural formulas. Partitionine of the total Fe

SiAITiFe3*C13*Fe2*MnN iMg

NaK

0.s91.250.031 a R

tr.

0.01tr.0.530.700.03tr.

Formulas

0.60 0.60 0.501.26 1.67 0.750.03 0.06 0.021.61 1 .33 2 .42tr. tr. tr.

0 .160.01 0.01 0.02tr. tr. tr.

0.53 0.48 0.470.71 0.69 0.710.03tr.

t 30

U'

3 1 63 0 4

FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE t43

Table 5. X-ray powder-diftraction patterns and least-squaresrefined unit-cell parameters of Fe3+-rich melilite and

gehlenite

Hemat i teFe2O"

Ab% MsAr2q Mso

Fig. 8. The compositions of the oxides in the buchite [O :I point analysis per crystal and a : (Alo,Feflj)(MgAl,Felj)Oo;Bacon and Welch, 19541. Numbered points correspond to anal-yses in Table 7.

lenite join. The larger c/a ratio (0.661, Table 5) in thebuchite melilite may reflect Fe3+ substitution in both theeight- and four-coordinated sites, which is required bycompositional variation extending from the gehlenite-fer-rigehlenite join toward pz+ft:+(Rl+Or), rather than in justthe four-coordinated sites as required by compositionalvariation along the join. A better understanding ofhowthe melilite structure accommodates the extensive sub-stitution of Fe3+ in sites of different coordination is thefocus of X-ray structural studies currently underway.

InoN oxrnns

Mineral chemistry

Three types of oxides were observed in this sample;abundant Al- and Mg-rich hematites and aluminousmagnesioferrites and minor pseudobrookite.

The hematite occurs both as discrete crystals and asintergrowths with the magnesioferrite. The hematitechemistry is variable (Fig. 8); every crystal analyzed con-tains Al and Mg and many also contain minor amountsof Na, Ca, Si, Mn, Ni, Cr, and Ti (Table 6). A maximumsubstitution of 15 molo/o AlrO, and 6 molo/o MgO wasobserved in crystal 82 (Table 6). Although aluminous he-matites have been reported in soils (Schwertmann et al.,1977; Bigham et al., 1978) and bauxite deposits (Solymarand Jonas, l97l), these coal-burn hematites represent themost Al-rich natural hematites reported to date.

Numerous experimental investigations of Al substitu-tion in hematite have been conducted over a wide rangeof temperatures using diverse starting materials and prep-aration techniques. The l5 molo/o maximum substitutionobserved in crystal 82 compares favorably with the 15-18 molo/o solubility limit observed by Muan and Gee

Fes*-rich meliliteThis study

4"" (A) tth d"* (A)

GehleniteLouisnathan (1971)

d.." (A) lh 4". (A)

1 1 00011011 1 12102012112200023 1 01022211123013 1 1202212.321400..4 1 0222330.',302312

3.7160

3.07912.8577

2.4530

2 40692 .31 16

1.9785

1 .7619

c.4cco

5.09934 2541

18 372543.4504

24 3.0765100 2.8577

2.72782.5497

24 2.43982.4209

21 2.40s324 2.3099

2.29632.20092.12712.0506

14 1.97041.92881 .87131.86271 .81851 .8107

27 1 7628

5 5.4575 5.087I 4.247

28 37212 3.451

27 3.074100 2.85615 2.72915 2.54332 2.44112 2.41530 2.4054 2.305

19 2.2963 2.2004 2.125

10 2.0474 1.973

55 1.929I 18724 1.860

55 1 .8197 1.809

45 1.761

5.4495.0814.2313.7113.4753 0662.8482.7382.5472.4372.4212.4082.3142.2912.2062.1172.0381.9701.9211.8761.8651 .8181 8121.768

GehleniteLouisnathan

(1 971 )

Fe3*-richgehlenite

Huckenholzand Ott Fe3*-rich melilite(1978) This studyf

a (a)c (4)v(A")cla

7.706(5)s.06s(7)

301.00.658

7.7125.081

302.20 659

7.715(9)5.0e9(6)

303.6(5)0.661

- This reflection was listed as 122 by Louisnathan (1971).. These reflections were masked by those of the Fe3*-rich pyroxene

that was also present in the sample.t All observed reflections were used in the least-squares refinement

into the two valence states in such a way as to reduce thecomposite (36 crystals) analytical total to 100 wto/o resultsin a Fe3+ : Fe2* ratio of l0 and a significantly reducedcation deficiency in the calculated formulas (Table 4).1[is srrgBests that a small and probably variable amountof Fe2+ is present in these melilites.

X-ray data

The X-ray powder-diffraction pattern of the buchitemelilite is very similar to that of gehlenite (Table 5). Theleast-squares refined unit-cell dimensions and volume aresomewhat larger than those of endmember gehlenite(Louisnathan, l97l) and the most Fe3+-rich gehlenite(37.5 molo/o ferrigehlenite, FerO, = 18 wto/o) synthesizedby Huckenholz and Ott (1978) at 1400'C and I atm.Huckenholz and Ott (1978) observed an increase ofap-proximately 20/o in cell volume and nearly constant c/alattice parameter ratios (0.658 + 0.001) over the com-positional range from 0 to 37.5 molo/o ferrigehlenite forsamples synthesized at I atm on the gehlenite-ferrigeh-

144 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

o

oa o . 1 2

o

o

I O . O Gd

o

o

oF .r..f

Hemat i teFe2Oa

SpinelMsAl2O4

Peric laseMgo

o.oo r-1 . 9 0 2 . 3 02 1 0 2 . 2 0

Fe3++ lt + Ti

Fig. 9. Variation of the tetrahedral- and octahedral-site va-cancies with Fe3+ + Al + Tia+ substitution in the magnesiofer-rites (O : I point analysis per crystal).

(1956), von Steinwehr (1967), Schwertmann etal. (1979),Fysh and Clark (1982), and Barron et al. (1984) for alu-minous hematites prepared by firing ferrihydrates andoxide-hydroxide mixtures in the temperature range 40G-1000'C. AIrO3 (3-5 molo/o) appears to "strengthen" thehematite structure by "strain relief through small amountsof smaller Al cations in the octahedral lattice sites" (De-Grave et al., 1985; Schwertmann et al., 1979).

In contrast to the hematites, the magnesioferrites ex-hibit a considerable range of compositional variability

Table 6. Electron-microprobe analyses ofhematites

Crystal Crystal Composite of82 80 21 crystals

Weight percent0.78 0.06 0.10(1 5)9.70 1.22 2.87(1.78)2.31 0.15 2.08(1.40)

83.02 96.60 92.81(2 95)0.00 0.00 0.04(5)0 20 0.40 0.20(9)0.00 0.00 0.02(3)1 .15 0.2s 0.93(s7)0 .1s 0 .04 0 .19(31)o.oo 0 09 0.03(4)0.05 0 00 0.01(1)

97.36 98.80 99 28(75)

Cations per three oxygens0.02 tr tr.0.29 0.040.04 tr.

0 0 90.04

CorundumAt2o3

Fig. 10. Compositions of coexisting hematite and magnesi-oferrite solid solutions [O : 1 point analysis per crystal and A :(Alo rFeo rXMgAlo rFeo r)Oo; Bacon and Welch, I 9541.

(Fig. 8), extending from (Alo,rFefli'troo')(Alon,M&,n'Fe3.j, Mnoo,Cr'o,)Oo (composition 91, Table 7) beyondthe synthetic phase (AlorFe3jXMeAL'Feli)Oo (Baconand Welch, 1954) to (Fefli'Aloo'f]oo')(MgonoFefl.!,'Alo,oMno*)Oo (composition 79, Table 7), thence tocomposition Fe]j,Mgo orAlo ,,Mno orTio o,Ca" orlo ,r)Oo(composition 70, Table 7) approximately midway alongthe hematite-magrresioferrite join. The total concentra-tion of other elements, notably Ti, Ni, and Cr in thesesamples rarely exceeds 2 wto/0.

Most crystals are Al rich, slightly defective, and do notlie on their respective joins (Fig. 8) owing to substitutionof the type 2(Al3+,Fe3*) + tr:3Mg'*. This is clearlydemonstrated by the high linear correlation (r : 0.99)between total (Al * Fe3+ + Tio*) and the total tetrahedraland octahedral site vacancy (Fig. 9). Crystals having com-positions midway between hematite and magnesioferrite(Fig. 8) have the largest cation deficiencies, 0. I 4-O .17 ca|-ions pfu (Fig. 9).

The site occupancies of the intermediate phase(Alo,Fefrj)(MgAlorFefrj)Oo, determined by Bacon andWelch (1954) suggest that substitution of Al3+ for Fe3+takes place in both the octahedral and tetrahedral sites ofthe inverse spinel structure. Since MgAlrO4 is a normalspinel, a miscibility gap on the spinel-magnesioferrite join

is to be expected; it may correspond to the compositionalbreak between Sp,oo and SproMfro Fig. 8).

Several of the oxide masses consist of intergrowths ofhematite and magnesioferrite solid solutions. The generalassociation of the more aluminous hematites with themore aluminous magnesioferrites (Fig. l0) and their tex-tures suggest high-temperature oxidation-driven "exso-lution" analogous to that observed in the Fe-Ti oxides(Haggerty, 1976). The magnesioferrite limb of the solvus

sio,Al,o3Tio,FerO"CrrO3MnONioMgoCaONaroKrO

Total

0.04Ir.

tr.

SiAITiFe3*CrMnNiMg

NaK

1.94 1 .82tr

0.01 0.02tr.

0.01 0.04tr. 0.01tr. tr.

FormulasCrystal 82: (Fe?:eAlorrMgo@Tio@Sioo2!oo2)O3Crystal 80: (Fe?.t4Alo04Mgo01Mno01)O3Composite: (FqirAlo orMgo MTio o4Mno o2Cao 01)O3

1 .59

tr.

FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL.FIRE BUCHITE t45

dome appears to extend all the way to SproMfro (Fig. 10).Crystals having compositions approximately midway be-tween FerO, und

"":+1Fe3*Mg)Oo Gig. 8) and having the

greatest cation deficiency do not show any visible signsof crystalline inhomogeneity at 400 x magnification.

The only Ti-rich oxide observed in this sample was aslightly Fe-deficient, Al- and Mg-bearing pseudobrookiteof very limited compositional variability (Table 7). It oc-curs principally as rare, translucent euhedral crystalsimbedded in the silicate phases. Nowhere was pseudo-brookite observed in contact with other oxides. The ex-tremely small size (approximately I pm x l0 pm) andlathlike habit of the crystals is probably responsible fortheir low analytical totals and the presence of minor Mg,Ca, and Si in their electron-microprobe analyses.

Suvrlrlny AND coNcLUsroNS

Detailed electron-microprobe microanalysis and X-raypowder-diffraction analysis of a sample of coal-fire buch-ite revealed the presence of Fe3"-rich clinopyroxenes,melilites, and oxides. The melilites and iron oxides havechemical compositions heretofore unreported in nature.

Clinopyroxene compositions straddle the diopside (Dilferri-aluminum Tschermaks (FATs) join and range fromDiroFTs,rCaTs, to DinFTsrrCaTsrr. Although a completerange of compositions on this join has been synthesized,the most Fe3*-rich natural pyroxene previously reported,also from a coal-fire buchite (Cosca and Essene, 1985),had the approximate formula Di,rFTsrrCaTsor. Cationassignments based on accepted conventions suggest amaximum substitution of 0. l7 Fe3+ atoms pfu in the tet-rahedral sites of compositions approaching endmemberFATs. This represents the greatest amount of Fe3* sub-stitution reported in the tetrahedral site of a natural py-roxene.

The most unusual silicates observed in this study arethe extremely Fe'*- and Al-rich melilites. No melilitecompositions even remotely approaching these have beenreported from experimental studies or as natural samples.These compositions reflect extensive coupled substitu-tion of the type Fe3+ * AI3* : Sio* + R2+ away fromthe "gehlenite-ferrigehlenite" join, R3*(AlrSiO?).R3*(FerSiOr), and toward the hypothetical endmemberR'z+R3+(R3+Or); the crystals contain 25-50 molo/o of thelatter endmember. Unlike the substitution of Fe3+ fortetrahedral Al along the gehlenite-ferrigehlenite join, con-comitant substitution of Fe3+ and Al for Si and R2* ap-pears to involve both the four- and eight-coordinated sitesin the melilite structure and results in a higher Fe3+ con-tent than in endmember ferrigehlenite. The extremely hrghFe3+ content and restricted range of compositional vari-ation (lack ofextensive substitution toward the gehlenite-ferrigehlenite join) suggest that the coal-burn melilite maynot be isostructural with gehlenite.

The oxides are represented by hematite and magnesi-oferrite, both of which show extensive Al3+ - Fe3+ sub-stitution. Substitution extends from magnesioferrite,Fe3* (Fe3+Mg)Oo, along the spinel-magnesioferrite join to

Table 7. Electron-microprobe analyses of magnesioferritesand Pseudobrookite

Crystal Crystal Crystal Pseudc91. 79' 70 brookite-.

Weight percentsio,Al,o"Tio,Fe.O.CrrO3MnONioMgo

NaroKrO

Total

0 . 1 455.38o.24

21.860.510.280.07

20.560.070 0 00 0 0

99.11

0.004.740.00

73.650.001.260.00

19.1 I0.060.070.00

98.96

0.11 1.40(62)2.84 1.27(15)0.21 33.10(35)

86.85 58.02(67)0.00 0.03(1)1.90 0.36(2)0 .12 0 .01(1)8.46 1.13(13)o.28 1.17(29)0.00 n.d.0.00 n.d.

100.76 96.49

0.060.071.001.77

0.01

0.070.05

1.69

0.430.010.01

0 7 9

J I

AITiFe3*

MnN iMgCaNA

Cations per four oxygens

0 .18

1.83

0.04

0.94

FormulasCrystal 91 : (Alo ?rFe8t,tro oTXAlo e?Mgo reF€rMno olCro or)O1Crystal 79: (FeS6AloostroolXMgosF4SrAloloMnod)O4Crystal 70: (Fetll Mgo 4Alo'1 Mno osTio o,Cao o1!o 1s)O4Pseudobrookite: (Feit?Aloo?Mgo oTSioo.Ca6 65Mn6o,)TiOu

* Cations in aluminous magnesioferrites assigned on the basis of (1) allMg in six-coordination, (2) equal distribution of Fe in four- and six-coor-dination, and (3) all other first-transition elements in six-coordination' Theseassignments conform to those tor (Alo5FeosXMgAlo,Feos)O4 reported byBacon and Welch (1954).

.'Average of several point analyses on three difierent crystals'

SproMfro and along the magnesioferrite-hematite join toMfuoHmoo with most compositions lying in the Al-richfield and characterized by a slight defect. The break incomposition between Sp,ooMfo and SproMfro probablycorresponds to the miscibility gap between the normaland the inverse (magnesioferrite) spinel structure. Micro-chemical analysis of the hematite-magnesioferrite inter-growths suggests the presence of a solvus dome, one limbof which extends from magnesioferrite to SproMto.

The coal-fire buchite from Buffalo, Wyoming, is ex-tremely heterogeneous on a scale oftens to hundreds ofmicrometers, consisting ofchemical domains within whicheach phase has an approximately uniform composition.Thus, while the bulk sample represents extreme disequi-librium, local equilibrium may exist within domains. Theheterogeneity of this sample, despite the probability thatthe protolith was of uniform composition, is thought tobe due to the effects of extreme and highly transient tem-perature gradients that resulted in partial fractional fu-sion and recrystallization. A detailed investigation ofparagenesis and the interrelationships of phase compo-sitions is now underway.

Inasmuch as coal burns represent an unusual and rel-atively unexplored natural environment of mineral for-mation, it is not surprising that new mineral composi-

0 . 1 10.012.21

0.05

0.430 0 1

146 FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE

tions have been discovered. Further studies will, no doubt,reveal the existence of heretofore unexpected mineralcompositions and perhaps new minerals. Thus, coal burnsshould provide a fertile field for future mineralogicalstudies.

AcxNowr-nocMENTs

We thank Subrata Ghose and H. G. Huckenholz for theirthoughtful reviews of our manuscript.

RnrnnnNcns

Akasaka, Masahida. (1983)'Fe M<issbauer study of clinopyrox-enes in the join CaFe3*AlSiOu-CaTiAl.Ou. Physics and Chem-istry of Minerals, 9, 205-21 1.

Akasaka, Masahida, and Onuma, Kosuke. (1980) The joinCaMgSi,Ou-CaFeAlSiOu-CaTiAl,Ou and its bearing on the Ti-rich fassaitic pyroxenes. Contributions to Mineralogy and Pe-trology, 7 l, 30l-312.

Bacon, G.E., and Welch, A.J.E. (1954) An X-ray examinationof the spinel-type mixed oxide MgFeAlO.. Acta Crystallogra-phica,7,36l-363.

Barron, V., Rendon, J.L., Torrent, J., and Cordoba, C.J. (198a)Relation of infrared, crystallochemical, and morphologicalproperties of aluminum substituted hematites. Clays and ClayMinerals, 32, 4'l 5-479.

Bigham, J.M., Golden, D.C., Bowen, L.H., Buol, S.W., andWeed,S.B. (1978) Iron oxide mineralogy of well-drained ultisols andoxisols: l. Characteization of iron oxides in soil clays byMdssbauer spectroscopy, X-ray diffractometry and selectedchemical techniques. Journal of the American Soil ScienceSociety, 42, 816-825.

Bowen, N.L., Schairer, J.F., and Posnjak, E. (1933) The systemCaO-FeO-SiO,. American Journal of Science, 26, 193-284.

Cawthorn, R.G., and Collerson, K.D. (1974) The recalculationof pyroxene end member parameters and the estimation offerrous and ferri-iron content from electron microprobe anal-yses. American Mineralogist, 59, 1203-1208.

Cosca, M.A., and Essene, E.J. (1985) Paralava chemistry andconditions of formation, Powder River Basin, Wyoming (abs.).EOS. 66. 396.

Cosca, M.A., Peacor, D.R., and Essene, E.J. (1985) The chem-istry and structure of a natural ferri-aluminum Tschermakiticpyroxene produced by pyrometamorphism. Geological Soci-ety of America Abstracts with Programs, 17, 553.

Deer, W.A., Howie, R.A., and Zussman, J. (1963) Rock-formingminerals, 1: Ortho and ring silicates. Longman, London.

- (1978) Rock-forming minerals, 24: Single chain silicates.Longman, London.

DeGrave, Eddy, Verbeeck, 4.E., and Chambaere, D.G. (1985)Influence of small aluminum substitutions in the hematite lat-tice. Physics Letters A, l07A(4), l8l-184.

Devine, J.D., and Sigurdsson, H. (1980) Garnet-fassaite calc-silicate nodule from La Soufri0re, St. Vincent. American Min-eralogist, 65, 302-305.

Dobrovolny, Ernest. (1981) Geologic map of the Buffalo quad-rangle, Johnson County, Wyoming. U.S. Geological SurveyGeologic Quadrangle Map GQ-1552.

Dyson, D.J., and Juckes, L.M. (1972) A silica deficient pyroxenein iron ore sinters. Mineralogical Magazine,38,872-877.

Fysh, S.A., and Clark, P.E. (1982) Aluminous hematite: A Mdss-bauer study. Physics and Chemistry of Minerals, 8, 257-267 .

Ghose, Subrata, Wan, Che-ng, Okamura, F.P., and Weidner,J.R. (1975) Site preference and crystal chemistry of transitionmetal ions in pyroxenes and olivines (abs.). Acta Crystallogra-phica, sec. A, 31, 576.

Ginzburg, I.V. (1969) Immiscibility of the natural pyroxenesdiopside and fassaite and the criterion for it. Doklady Acad-emy ofSciences USSR, Earth Science Section, 186, 106-109.

Haggerty, S.E. (1976) Oxidation of opaque mineral oxides inbasalts. Mineralogical Society of America Reviews in Miner-alogy, Hg-l-Hg-100.

Hartman, Piet. (1969) Can Tia+ replace Sia* in silicates? Min-eralogical Magazine, 37 , 366-369.

Hazen, R.M., and Finger, L.W. (1977) Crystal structure andcompositional variation of Angra dos Reis fassaite. Earth andPlanetary Science ktters, 35, 357-362.

Hieke, O. (1945) I giacimenti di contatto del Monte Costone(Adamello menidionale). Memoric dell'Instituto Geologicodella Universita di Padova. 15, l-46.

Hijikata, Ken-ichi. (1968) Unit-cell dimensions of the clinopy-roxenes along the join CaMgSirOu-CaFe3*AlSiOu. Faculty ofScience Journal, Hokkaido University, Series 4, Geology andMineralogy 4, 149-157.

Hijikata, Ken-ichi, and Onuma, Kosuke. (1969) Phase equilibriaof the system CaMgSirOu-CaFe3+AlSiOu in air. Japanese As-sociation of Mineralogists, Petrologists, and Economic Geol-ogists Journal, 62, 209-217 .

Hooper, R.L. (1982) Mineralogy of a coal burn near Kemmerer,Wyoming. M.S. thesis, Washington State University, Pull-man.

Hooper, P.R., and Atkins, L. (1969) The preparation of fusedsamples in X-ray fluorescence analysis. Mineralogical Maga-zine, 37 , 409-413.

Huckenholz, H.G., and Ott, W.D. (1978) Synthesis, stability,and aluminum-iron substitution in gehlenite-ferrigehlenite solidsolutions. Neues Jahrbuch fiir Mineralogie Monatshefte, 521-536.

Huckenholz, H.G., Lindhuber, W., and Springer, J. (197a) Thejoin CaSiOr-AlrO,-Fe'O, of the CaO-AlrOr-FerO.-SiO' qua-ternary system and its bearing on the formation ofgranditicgarnets and fassaitic pyroxenes. Neues Jahrbuch liir Mineral-ogie Abhandlungen 121, 160-207.

Huggins, F.8., Virgo, D., and Huckenholz, H.G. (1977) Tita-nium-containing silicate garnets. l. The distribution of Al,Fe3+, Tia+ between octahedral and tetrahedral sites. AmericanMineralogist, 62, 4'1 5-490.

Kimata, M., and Ii, N. (1981) The crysral structure of syntheticakermanite, CarMgSirOr. Neues Jahrbuch liir MineralogieMonatshefte. H.I.. l-10.

-(1982) The structural property of synthetic gehlenite,CarAlrSiOr. Neues Jahrbuch fiir Mineralogie Abhandlungen,r44 ,254-267.

Knopf, Adolph, and Lee, D.E. (1957) Fassaite from near Helena,Montana. American Mineralogist, 42, 7 3-7 7.

Kurepin, V.A., Polshin, E.V., and Alibekov, G.I. (1981) Intra-crystalline distribution of iron (3+) and aluminum cations inclinopyroxene (CaFe3*AlSiOu). Mineralogicheskii Zhurnal, 3,83-88.

Louisnathan, S.J. (1971) Refinement of the crystal structure ofa natural gehlenite, CarAl(A1,Si)rOr. Canadian Mineralogist,r0,822-837.

Majmundar, H.H. (1971) Fassaite from Madagascar. CanadianMineralogist, 10, 899-903.

Mapel, W.J. (1959) Geology and coal resources of the Buffalo-Lake DeSmet area, Johnson and Sheridan Counties, Wyo-ming. U.S. Geological Survey Bulletin 1078, 148 p.

Mason, Brian. (l 974) Aluminum-titanium-rich pyroxenes withspecial reference to the Allende meteorite. American Miner-alogist, 59, 1198-1202.

Muan, Arnulf, and Gee, C.L. (1956) Phase equilibrium studiesin the system iron oxidel-AlrO, in air at 1 atm O. pressure.American Ceramic Society Journal, 39, 207 -21 4.

Nurse, R.W., and Mid8ley, H.G. (1953) Studies in the melilitesofid solutions. Iron and Steel Institute Journal, 17 4, I2l-l3l .

Ohashi, Haruo, and Hariya, Yu. (1973) Order-disorder of ferriciron and aluminum in the system CaMgSirOu-CaFeAlSiOu athigh pressure. Japanese Association Mineralogists, Petrolo-gists, and Economic Geologists Journal, 68,230-233.

Osborn, 8.F., and Schairer, J.F. (1941) The ternary system pseu-

FOIT ET AL.: UNUSUAL ASSEMBLAGE IN A COAL-FIRE BUCHITE t47

dowollastonite-Akermanite-gehlenite. American Journal ofScience, 239,715-763.

Peacor, D.R. (1967) Refinement of the crystal structure of a py-roxene of formula MrMr(Sir5Alo5)06. American Mineralo-ggsI, 52,3l-41.

Rao, A.T., and Rao, M.V. (1970) Fassaite from a calc-silicateskarn vein near Gondivalosa, Orissa, India. American Min-eralogist, 55, 975-980.

Schwertmann, U., Fitzpatrick, R.W., and Le Roux, J./1977) Alsubstitution and differential disorder in soil hematites. Claysand Clay Minerals, 25,373-3'14.

Schwertmann, U., Fitzpatrick, R.W., Taylor, R.M., and Lewis,D.G. (1979) The influence of Al on iron oxides. Part II. Prep-aration and properties of Al-substituted hematites. Clays andClay Minerals, 27, 105-112.

Seifert, F. ( 1 9 74) The j oin Akermanite-ferroAkermanite. CarnegieInstitution of Washington Year Book 73,436-440.

Solymar, Karoly, and Jonas, Klara. (1971) A goethite racsaba

beepult Al-tartalom vizsgalata is jelentosege a magyar bauxi-tokban. Kohasz Lapok, 104, 226-235.

Tilley, C.E. (1938) Aluminous pyroxenes in metamorphosedlimestones. Geological Magazine, 75, 8 l-86.

Tomasi, L. (1940) Fassaite di val di solda e sua paragenesi. StudiTrentini di Scienze Naturali. 21. 85-l11

von Steinwehr, H.E. (1967) Gitterkonstanten im systema-(Al,Fe,CrLO, und ihr Abweichen von der Vegardregal. Znit-schrift fiir Kristallographie Mineralogie, 125, 37 7 -403.

Willemse, J., and Bensch, J.J. (1964) Inclusions of original car-bonate rocks in gabbro and norite of the eastern part of theBushveld complex. Geological Society of South Africa Trans-actions. 6'1. l-87.

Yoder, H.S. (1950) Stability relations ofgrossularite. Journal ofGeology, 58, 221-253.

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