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Dorrite Icar(Mgrfe;+Xal4sir)Orol.- a New Member of the Aenigmatite

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Dorrite Icar(Mgrfe;+Xal4sir)Orol.- a New Member of the Aenigmatite American Mineralogist, Volume 73, pages 1440-1448, 1988 Dorrite ICar(MgrFe;+XAl4Sir)Orol.-a new member of the aenigmatitegroup from a pyrometamorphicmelt-rockx Mrcnl.nr, A. Cosclo Ror,,tNoR. Rousn,Enrc J. Essnrqn Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan, 48109-1063, U.S.A. Ansrru.ct Dorrite, a new mineral of the aenigmatitegroup, has been found in a paralava from the Powder River Basin, Wyoming. The type specimen used for X-ray analysis has been quantitatively analyzed,yielding the formula (Ca,rrNa"orllo,XCaoorMg,orMnoorFefrjoTioor- Fellr)(FelSoAl,noSi, uo)Oro; the end-member formula is inferred to be Car(MgrFel*)(AloSir)- Oro. The chemical compositions of individual dorrite grains within the same thin section display wide variations in tetrahedral AllFe3+, but contain dominant Fe3+in the octahe- dral sites and little Tia+. As is typical of aenigmatite-goup minerals, dorrite is triclinic and twinned, producing a pseudomonoclinicsymmetry. The spacegroup is Pl or PI with refined parametersa : 10.505(3),b : 10.897(3),c : 9.019(l) A, o : 106.26(2),p : 95.16(2)',and 7 : 124.75(2);V: 772.5(4)A' (Z:2) andp"^,": 3.959g,/cm3. Dorrite has very strong absorption and high relief with approximate refractive indices of a : 1.82, P: 1.84,and 7: 1.86 (all +0.01). Dorrite is nearly opaquein thin section,whereas in ultrathinned areasit displays the following pleochroic formula: X: red-orangeto brown, I: yellowish brown, and Z : greenishbrown. It occurs as anhedral to prismatic grains up to 0. I mm in length and in closeassociation with esseneiteor titanian andradite. Other minerals coexistingwith dorrite include plagioclase,gehlenite-akermanite, and magnetite- magnesioferrite-spinelsolid solutions, with lesserBa-rich feldspar, wollastonite, ulvcispi- nel, nepheline, apatite, ferroan sahamalite,and secondarybarite and calcite. The mineral is named for J. A. Dorr, Jr., late professorat the University of Michigan. A series of mineral reactions relate dorrite + magnetite * clinopyroxene, rhdnite + magnetite * olivine * clinopyroxene,and aenigmatite + pyroxene * olivine assemblages that are widespreadin nature. The conditions necessaryfor crystallizing theseassemblages appear restricted to low pressuresand high temperatures.Dorrite is stable in strongly oxidizing, high-temperature,low-pressure environments. INrnooucrrou spar, apatite, ulv6spinel, ferroan sahamalite,and second- ary barite, and calcite. During a general investigation of pyrometamorphic The mineral that we now call dorrite has apparently melt-rock (paralava) in the Powder River Basin, Wyo- been described twice previously. The first report was from ming (Cosca et al., 1989), an aenigmatite-like mineral a basaltJimestonecontact on R6union Island (Havette et was identified based on chemical, optical, and X-ray al., 1982). There it forms small, nearly opaque grains powder-diffraction observations. Normalization of mi- coexisting with melilite and titaniferous fassaite.It was croprobe analyses results in the idealized formula labeled mineral X, and was found in association with CarMgrFel+(Alosir)Or0,although somegrains revealed up mineral X, (probably a melanite garnet). The secondre- to 75o/o replacement of tetrahedrally coordinated Al by port is from an investigation of a paralavafrom the Pow- Fe3*. This phase is inferred to be isostructural with the der River Basin by Foit et al. (1987).They identified a aenigmatite group of minerals (aenigmatite, rh6nite, mineral as Fe3+-richmelilite, but the chemistry and op- serendibite, welshite, and krinovite plus synthetic "bai- tical properties of this "iron-melilite" are quite diferent kovite"), but is chemically most similar to rhdnite from those of the melilite group and are similar to those [CarMg,Ti(Alrsio)Or0]. The lack of Ti and presenceof of the rhcinite-like phase described here. It appearsthat dominant Fe3+in octahedralcoordination support its sta- a rhdniteJike phase was analyzedand mistakenly asso- tus as a new mineral. Phenocrystsor quenched crystals ciated with a melilite X-ray pattern. In paper coexisting with this phaseinclude plagioclase,gehlenite- this we report data on the physical, chemical, and structural akermanite, magnetite-magnesioferrite-spinelsolid so- properties of a new rhdniteJike mineral and discussits lutions, esseneite,nepheline, wollastonite, Ba-rich feld- relationship to previously describedrhdnites. * Contributionno. 454 from the MineralogicalLaboratory, The sampledescribed in this study (DR-8) comesfrom Departmentof GeologicalSciences, University of Michigan. the Durham Ranch,located just eastof Highway 59, about 0003404x/88/rll2-r440502.00 t440 COSCAET AL.:DORRITE t44l photographof dorrite forming Fig. 1. Backscattered-electronphotograph of dorrite(dr), cli- Fig. 2. Backscattered-electron asin Fig' l ' Scalebar : nopyroxene(cpx), magnetite (mt), and melilite(mel). Note tex- arounda vesicle.Symbols are the same tural disequilibriumofthe magnetiteinclusions. Scale bar : 100 100pm. pm. 13 km northeast of Reno Junction arrd 25 km south of Gillette, Wyoming. Rocks from this and other nearby localities form mesasthat are separableinto two distinct horizons. The lowest horizon is composed of unaltered sedimentary rocks, generally medium- to fine-grained siltstones, sandstones,and conglomerates.This horizon generallyfound in closeassociation with the is overlain by a thick (l-30 m) capping of baked and (Annr),but is and magnetite. Clusters of reddened sedimentary rocks. Between the two horizons us"-blug. clinopyroxene grains develop around vesicles (Fig' is a small zone of incompletely combustedcoal and coal- dorrite sometimes ash<ontaining paralava. The paralava is a vesicular, 2). mainly crystalline domains of dif- slaglike rock that contains a variety of minerals with a A number of lobate, variety offlow and quenchedtextures (Cosca et al., I 989)' The new mineral was named dorrite after Dr. John (Jack)A. Dorr, Jr., late professorof stratigraphy,paleon- tology, and regionalgeology at the University of Michigan' Dr. Dorr and his students conducted much of their re- searchin Wyoming, and early in our study' he provided display evidence of reaction, us with some locations of outcrops containing pyrometa- contacts between domains boundaries between two or morphic rock. The new mineral and mineral name were possibly representing local (+ crystals)' The most extreme approved, prior to publication, by the IMA, Commission more different liquids these domains occur in the on New Minerals and Mineral Names. Type material is modal variations between and melilite, and dorrite is located in the Smithsonian Museum (NMNH 163357), percentagesof clinopyroxene of highest melilite concentration' and mineral grains used in the X-ray studies are housed not found in regions does occur with melilite, clino- in the mineral collections at the University of Michigan' However, where dorrite pyroxene is not found, arrd a phase with an empirical PsrnocRAPtrY iormula consistent with titanian andradite [Caroo- ( Fe Si O In thin section,most dorrite grains display an intimate ( Fe fr{,Mn oo, Mg oo, Fe Iio Ti 610) 3i".Al0.36 2.56) ,2l (and nearby clinopyroxenes)oc- relationship with an esseneite-richclinopyroxene (Ca- appeaisinsieaa. Dorrites with the garnet tend to be the most Fe3*AlSiOu)that containssmall amounts of diopside (Di)' curing in association described as X2, associat€dw'ith Ca Tschermak's component (CaTs), and/or Fe Tscher- f'e riitr. The mineral X, (dorrite) by Havette et al. (1982) mak's component (FTs) (Dirr-rrCaTso-rrFTso-'n).Dorrite rhdnite and mineral R6union Island, may also be a occurs most often within grains of clinopyroxene and in from a contact zone on possible mineral X, has a sphene-type many casesforms pseudomorphsafter clinopyroxene. In garnet. It also that totals of Havette et al' (1982) turn, many of the dorrite grains contain inclusions of pri- Jtructure.The low analytical data and may indicate the presence marily magnetite (Mt) that contains significant amounts are also similar to our OH substituting for O' With the of magnesioferrite (Mf) and spinel (Sp) (Mtr'-u'Mf"-r- of small amounts of 1442 COSCAET AL.: DORRITE ;:*itr;3 ;t' :?., ..: iit j.!'t' r : . .,':' "+ r:#':1,ffi l: ;:i. -*1; ;-- Fig. 3. Thin section, showing the distinct mineralogic zonesreferred to in the text. A-A' showsthe approximate location of the traverseacross which the data in Tables r and,2 were collected.Scale bar : 2 mm. exception of very small amounts p, of no additional ele_ site (Jb) solid solutions (Mr8,Jb,8)Gig. a). Barite and cal- ments were observedduring a wavelength-dispersivescan cite appear in fractures and vesicle fillings but are inter- of the Durham Ranch garnet. preted to be secondary.In addition, small, highly reflective Dorrite and associatedphases in the type specimen spheresof Fe-bearingsahamalite occur in some melilite. (paralava DR-8) are preserved with quenched rextures such as skeletal and hopper grains and often contain or Crrnlrrsrny Chemical analysesof mineral grains were obtained from polished thin sections using a Cameca cAMEBAXmicro- probe, and these data are presentedin Tables 1 and,2. Standard operating conditions consistedofan accelerat- ing voltage of 15 kV and a sample current of l0 pA. A l-pm-diameter electron beam was used for all analyses. (Wonn), nepheline (Ner.Ks,,), and magnetite (Mt)-jacob_ The following well-characteized, natural and synthetic phaseswere used as standardsfor quantitative analysis: hedenbergitefor Ca,jadeite for Na, hornblende for K and Mg,
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