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 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 , 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 . 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 "-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

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, rhodonite for Mn, fenosilite for Fe, uvarovite for Cr, for Ti, and almandine for Al and Si. Counting times were set to a maximum of 30 s for most major elementsand proved sufficient to obtain statistically sig- nificant results. All raw data were corrected for atomic number (Z), absorption (l), and fluorescence(.F) using Cameca software. Becauseof the quenched textures, it was helpful to obtain imagesusing backscattered-electron (nsn)imaging in order to isolate mineral grains for quan- titative analysis.This technique was especiallyuseful for distinguishingbetween intergrowths ofpyroxene and dor- nte. In addition to chemical analysesobtained from pol- ished thin sections,the single crystal used to obtain unit- cell parameterswas also analyzed(Table l). The crystal was carbon coated and a specialsample holder was used that allowed placement of the crystal, still attached to a Fig. 4. Backscattered-electronphotograph of barred Ba_rich glassfiber, directly under the electron beam. The crystal feldspar(Bfs), wollastonite (wo), magnetite(mt), and apatite (ap) was then rotated until a flat surfacewas obtained, and an formed within an interstitial pocket ofquenched liquid. Scalebar analysis was performed. Becauseof the potential errors : l0 lm. associatedwith this technique,such as a variable take-off COSCAET AL.: DORRITE r443

TABLE1. Microprobeanalyses of dorrite -^bcdefghl

12.00 10.53 10.44 sio, 11.16 11.19 11.02 11.05 11.93 11.38 12.41 11.35 8.62 13.33 AIrO3 24.85 2231 22.77 13.56 13.56 0.92 0.99 0.90 0.90 1.04 0.41 Tio, 0.56 0.59 061 0.01 0.07 0.00 0.00 0.04 0.02 0.01 CrrO. 0.05 0.26 59.06 55.50 43.39 45.J I 52.03 51.14 53.60 52.57 Fe.O. 41 65 2.71 3.42 3.00 2.74 3.30 3.3s z-ou 3.31 FeO 2.77 0.70 0.22 0.27 0.25 0.45 0.44 0.52 0.40 MnO 019 5.16 4.31 4.59 Mgo 5.57 5.44 5.44 4.88 5.38 5.16 13.13 13.38 13.45 13.50 13.29 12.08 13.63 13.29 13.39 0.23 0.06 0.05 0.08 0.09 0.11 0.10 0.15 Naro o.02 0.01 0.03 0.02 002 0.02 0.02 0.03 0.o2 0.01 K"O 99.32 100.43 100.26 Sum 100.47 99.82 9993 99.42 100.29 100.19 't.73'l 1.596 1.603 1.690 1.798 1.844 1.643 5l 1.599 1.632 1.584 2.403 4.196 3.835 3.903 2 445 2.408 2.225 2.055 AI 2.044 2.101 2.773 2.001 Fe3* 0.205 0.533 0.494 1.865 1.794 4.125 4.007 4.093 3.978 4.159 4.386 Fe3* 4.286 4.229 4.275 0.046 0.065 0.067 0.105 0.112 0.103 0.104 0.122 Ti 0.060 0.001 0.000 0.006 0.029 0.00e 0.000 0.000 0.005 0.003 Cr 1.181 1.002 1.048 M9 1.189 1.183 1.180 1.112 1.209 1.169 0.422 o.422 0.333 o.425 0.354 0.437 t-e'- 0.332 0.366 0.333 0.028 0.031 0.058 0.057 0.067 0.052 0.007 Mn 0.023 0.033 0.269 0.055 0.104 0.096 0.106 0.177 0.193 0.229 0.256 1.952 1.926 1.980 1.981 1.974 1.969 1.964 1.967 Ca 1.989 0.045 0.068 0.007 0.016 0.015 0.023 0.026 0.033 0.031 Na 0.002 0.003 0.006 K 0.004 0.004 0.004 0.003 0.005 0.003 distinct zones enriched (A-A,, Fig 3) approximately3.mm in length.,.acrossa siarp boundary separatingmodally Note; Analysesare from a traverse forthi cell-parameterrefinement (i)'Formulae inctinopyroxene (a-D ano meririie 1g-n); r1d.si. nirgrno*n'[ di-el1y11o1j::[n^': gt:,i'^r*d orilnJ nasisot t4 t-ations;""9FeO-and Fe,O. are recalculated from normalized formula. "rJl"fijAtj6

as Car(MgrFel+)- angle, the results of the single-crystalanalysis presented end-member composition of dorrite grains (Table l) are in Table I must be viewed with some caution. Our best (Al4sir)Oro.However, some Fe-rich formula Car- estimate of the accuracy of this analysis is +40loof the bettei represented by the ideal in chemical com- amount presentfor Fe, Mg, Al, and Si and perhapstwice (Fel+Fei+XFe;*Sir)Or0.The variation (Fig' that uncertainty for the other elements- position is due mainly to tetrahedral substitutions display a The analysesin Table I were used to define an ideal, 5a). where the inferred tetrahedral occupancies

Trsue2, Clinopyroxeneand garnet analyses Clinopyroxene Garnet gh 30.67 29.79 30.74 29.79 27.24 26.88 30.21 sio, 2982 4.46 3.30 Alro3 16.81 17.60 14.01 12.54 11.58 9.58 1.54 1.97 2.35 2.39 6.02 5.14 Tio, 1.22 098 0.04 0.06 0.01 0.01 0.00 0.02 0.00 0.03 23.68 25.95 Fe,Oo 24.85 24.24 24.69 27.81 31.88 34.65 1.74 1.55 0.97 0.76 0.00 0.00 FeO 0.86 0.77 0.20 0.15 0.14 o.23 0.19 0.31 0.19 MnO 0.19 0.51 0.56 Mgo 265 z.I o 2.90 2.90 2.O2 1.92 23.20 23.00 22.73 22.59 32.99 32.54 CaO 23.31 23.30 0.01 0.23 0.18 0.29 0.21 0.17 0.17 0.02 Naro 0.02 0.00 0.00 0.00 0.01 0.01 0.00 0.02 KrO 98.10 98.81 Sum 99.97 99.83 99.27 100.02 99.13 99.29 2.602 1.198 1.195 1.251 1.218 1.143 1.139 2.568 o.447 0.330 0.796 0.831 0.672 0.604 0.573 o.478 AI 0.385 0.328 Ti 0.037 0.030 0.047 0.061 0.o74 o.077 0.000 0.000 0.000 0.001 0.000 0.000 Cr 0.001 0.002 1.660 0.732 0.755 0.856 1.007 1.105 1.513 Fe3t 0.751 0.06s 0.071 Mg 0.159 0.165 0 176 0.177 0.126 0.121 0.027 0.000 0.000 Fe.- 0.029 0.026 0.059 0.053 0.034 0.005 0.008 0.007 0.011 0.013 0.013 Mn 0.007 0.005 2.994 1.001 1.011 1.007 1.O22 1.026 3.005 Ca 1.004 0.002 0.013 0.023 0.016 0.014 0.014 0.002 Na 0.018 0.002 0.000 K 0.000 0.000 0.001 0.000 0.000 0.001 1. Formulaeare calculatedon the basis ol cations Note..Letter labelsof cotumnsdenote grains that coexist with correspondingdorrites from Table (pyroxene:4;garnet:8)FeOandFerO"arerecalculatedfromnormalizedformula' t444 COSCAET AL.: DORRITE

MgtFe

Si l-e Fig' 5' (a) Tetrahedral occupancyofdorrites compared to rhdnites in the literature. Dorrites are from this study (filled circles), Havette et al. (1982) (squares), and Foit et al. (1987) (triangles).Rhcinite analyses(open circles)contain <1.0 wto/oNa,O and weie takenfromthefollowing:Soellner, 1907;Babkine, 1964; Cameron etal.,D76;Fuchs, 1971;GriinhagenandSeck,1972;Kyleand, Price, 1975;Magothier and Velde, 1976;olsson, 1983;and Nishio et at., tsts. (b) octahedralo".op-.y of dorritescompared to rhrinites in the literature [sourcesof data and symbols are the same as in (a)].

range in Fe3+/(Fe3++ Al) from 0.2 to 0.7. This chemical of aenigmatite is known (Cannillo et al., 197l), and as- suming this mineral is isostructural with dorrite (as in- dicated by our data), similar crystal-chemicalresults may apply. Cannillo et al. (1971) refined the tetrahedraloc- cupanciesof aenigmatite; their results indicate a strong preferenceof Si for two of the six tetrahedral sites.Small amounts of Fe3+ are subequally distributed in the re- maining four tetrahedral sites.Based on the regular vari- The calculated octahedral occupanciesof dorrite dis- ation oftetrahedral occupancyinferred from our micro- play very little variation, ranging between 68 and.75o/o probe data, similar ordering of Si may be expectedin the Fe3+(Fig. 5b). Similar results are obtained from the data tetrahedral sitesofdorrite. Cannillo et al. (1971) also pro- of Havette et al. (1982) and Foit et al. (19g7). These vided evidencethat Ti is largely ordered in the smallest greatly values exceedthose of rhdnites, which exhibit a of the seven octahedral sites, with lesserordering of Ti rangefrom 0 to 20o/o;a 300/ooctahedral-site occupancy of and Fe2+in the remaining ones. Although generally re- Fe3+is calculated from a rhdnite presentedby Johnston stricted to eightfold coordination, small amounts of Ca and Stout (1985), although their analysis indicates solid were inferred to occupy the octahedral sites in aenigma- solution toward aenigmatite. tite (Cannillo et al., 197l). This is consistentwith similar Comparison of rhdnite and dorrite compositions taken assignmentsinferred from microprobe analysesof dorrite from the literature to those obtained in this study (Fig. (Table 1). The small amount of sixfold-coordinated Ca 5) reveal patterns possibly indicating a solvus between may reflect distortions (expansion)ofthe octahedral site these two minerals. This could explain the restricted associatedwith the high-temperature environments in compositional variations thus far shown for thesechem- which these minerals form. ically and structurally similar phases.A dorrite-rhOnite Dorrite is chemically related to rhdnite by the follow- solvus may also be indicated by the observationsof Hav- ing coupled substitutions: ette et al. (1982) ofa closeassociation between rhdnite and their mineral X,. r6JR2+r41Si: r6tR3+r4tR3+ (l) Becausethe structuresof dorrite and rhonite have not and yet beendetermined, the degreeofcation ordering (ifany) in phases these remains unknown. However, the srructure r6tR2+r6tTi : r6lR3+r6lR3+ (2) COSCAET AL.: DORRITE t445 where R2+ is Fe2+or Mg and R3* is Al or Fe3+.These ('si)(n') . (n1(h") exchangesare shown schematicallyin Figure 6, projected in terms of theoretical end members.The two previously co.tn'z.nt'ttnt si.to.o cortnt.nto' )tnt* si)o.o describeddorrites are shown in Figure 6 and include min- X, from R6union Island (Havette et al., 1982) and eral .*" the "iron melilite" of Foit et al. (1987).If solid solution ideot dorrite) toward aenigmatite is small (< 1.0 wto/oNarO), this pro- jection is useful to distinguish rhdnite from dorrite and :- ou indicates the types of solid solutions that may be antici- o +1 pated. Rhdnites containing significant Na (e.g.,Kyle and s* Price, I 975; Johnston and Stout, I 985) are excludedfrom greater ii this diagram. Compositions with than one Ti cat- s ion per formula unit are not properly representedon this diagram becausethey require an additional exchangesuch \\ as NaTi : CaR3*, or CaTi : MgSi for the most Ti-rich rhiinite from the Allende meteorite (Fuchs, l97l); a low- o ooo er-valencestate for Ti (Fuchs,1978; Beckett et al., 1986) and hencea different substitutional schemeare also pos- sible for the Allende rhdnite. LOztK5 ttr\tl2 5t4ru2o co.tn]'nl-ri ltn'.'si)o.o rh6nite Prrvsrcar, AND oPTrcAL PRoPERTTES Fig. 6. Compositionalvariation ofdorrites and rhOnitesob- Dorrite forms small prismatic crystals up to 0.1 mm tained from this study and taken from the literature plotted in long. Crystalsare dark red-brown to dark brown in color, termsof hypotheticalend members (sources of dataand symbols have a submetallic luster, and are nearly opaque except arethe sameas in Fig. 5a). in very thin crystals(< l5 rlm). Some grains exhibit good cleavagesthat are assumedparallel to {010} and {001} by analogy to the other members of the aenigmatite group. thin section displayed a large, negative 2V of approxi- The Mohs hardness,determined by crushinghand-picked mately 90o,consistent with the very approximate refrac- grainsbetween polished slabsof apatite, is approximately tive indices.Pleochroism is only observedin ultrathinned 5. The scratcheswere confirmed in the apatite with the thin sectionsor small grains and is very strong with the aid of a binocular microscope. Grains are brittle when formula X : red-orange brown, y : yellowish brown, crushed, have an irregular , and leave a gray . and Z : greenishbrown. Though probably present, dis- The volume directly obtained from the crystal used in the persion could not be observed owing to the strong ab- unit-cell refinementis 772.5(4)A'. Basedon the chemical so{ptron. composition of this grain (Table l), the density was cal- culatedas 3.959 g1cm3.Density wasnot measuredbecause Cnvsr.q'I-r,ocRAPHY most of the grains contain inclusions of magnetite that The unit cell and spacegroup ofdorrite were obtained preclude an accurate measurement.The volume of the using the precessionand cone-axismethods and a Syntex end-member dorrite is 769 A'. This was estimated by P2, single-crystaldifractometer. A set of four precession extrapolatingthe volume of the dorrite solid solution with levels showedwhat appearedto be a monoclinic intensity small amounts of isostructural pyroxenes to obtain an distribution and a C-centeredcell of dimensionsa : 9.98, end-member formula Car(MgrFe!*)(Al4sir)Oro.Similarly, D : 5.08,c: 5.24A, and A :99.9'. However,a cone- a volume for the Fe-rich composition of dorrite axis photograph with a as the precessingaxis revealed [Car(Fe3*Fei*XFei+Si,)O,o]has beencalculated as 816 A'. additional very weak levels requiring a doubling of a. A The calculation assumesthat variations in the molar vol- photogaph of the first superstructurelevel parallel to the ume of dorrite are well representedby changesin equiv- b*c* plane showed that there were two additional lattice alent amounts of pyroxenes,that volumes of clinopyrox- points per cell, requiring that the b and c parametersalso ene vary linearly, and that no significant excessmolar be doubled. Comparison of the dorrite reciprocal lattice volumes occur. with the reciprocal lattice diagrams of twinned and un- The optical properties of dorrite have been determined twinned krinovite (Merlino, 1972) proved conclusively in oils by using separatedmineral grains. These proper- that dorrite is triclinic-pseudomonoclinic and twinned by ties are difficult to measure becauseof the extreme ab- a twofold rotation about the pseudomonoclinic b axis. sorption exhibited by this mineral even under an intense Such twinning is the rule in minerals of the aenigmatite (white) light source.Consequently the optic figures were group. Merlino (1972) also tabulated the parameters of usually diffirse, although they were sometimes adequate the three alternative pseudomonoclinic unit cells that have to determine the optical orientation. The refractive in- been used by various workers for aenigmatite,krinovite, diceswere measured as the following:a : 1.82,0: 1.84, and rhdnite and also the parameters of the true, un- and y : 1.86 (all +0.01). The dorrite from an ultrathin twinned triclinic cell of the Delauney type. The pseudo- t446 COSCA ET AL.: DORRITE

TmLe3. X-raypowder diffraction data for dorrite tern are presentedin Table 3. Reflections were indexed by least-squaresrefinement of the observed d values to- /* 0oe gether with the independently determined unit-cell data. 20 8.1 8.21 001 5 2.257 2.271 241 An attempt was made to refine the triclinic cell parame- 8 19 100 2.269 223 ters ofdorrite from the powder data alone, but the results I 19 010 2.264 243 20 7.5 7.51 0i1 2.256 143 were unsatisfactoryowing to the small number of reflec- 5 6.4 6 53 111 2.245 223 tions that could be unambiguously indexed. 10 4.87 4.89 111 5 2.205 2.209 310 4 89 011 2.209 130 DrscussroN 5 4.39 4.28 221 2.207 041 of rhdnite and aenigmatite are generally 5 4.22 4.25 102 2.206 441 Occurrences 424 201 2.202 132 restricted to silica-undersaturated,mafic to intermediate, 10 3.46 3.47 212 2.202 432 extrusive and intrusive alkalic rocks (Deer et al., 1978). 100 2.971 2.992 120 60 2.125 2.140 442 From these reports emergesa pattern of a near-ubiqui- 2.992 210 2.134 411 tous associationof either rhdnite or aenigmatite in nat- 2.985 013 2j30 251 (or 2.975 031 2.125 204 ural paragenesescoexisting with olivine, magnetite 2.954 231 2.099 034 spinel solid solutions),and clinopyroxene.Nearly all pub- 2.918 11 3 2.096 043 lished descriptions of rhOnite contain coexisting olivine 2.886 103 2.091 550 (usually intermediate to fayalitic), clinopyroxene (usually 2.882 312 2.093 323 titanian augite or fassaite),and a spinel solid solution 5 2 815 2.816 122 2.092 530 2.705 2.719 241 20 2 035 2.041 540 (often Ti-rich magnetite)(e.g., LaCroix, 1909; Y agi, 1953; 2.706 203 2.041 450 Ficke, 196l; Babkine et al., 1964:'Cameron et al., 1970; 2.704 113 2.031 423 Fuchs, l97l; Brooks eI al., 1979 Havette eI al., 1982 2.701 1842 302 5 Olsson, 1983;Johnston and Stout, 1985).Similar asso- 80 2 558 2.560 213 20 1.737 2.560 113 10 1.695 ciations are describedoccurring with aenigmatite,but in 80 2.515 2.527 202 15 1.626 thesedescriptions the clinopyroxeneis usually aegirineor 2.521 211 20 1.613 a titanian, sodic augite (e.g., Carmichael, 1962 Abbott, 5 2453 2.458 221 20 1.544 1967;Lindsley et al., l97|,Plattand Woolley,1986; Stolz, 2.452 431 30 1.511 A associationnow exists 2.446 222 30 1 482 1986;Jorgensen, 1987). similar 2.446 022 5 1.456 for the mineral dorrite (Havette et al., 1982; Foit et al., 10 2349 2.361 140 5 1.432 1987; Coscaet al., 1989).The repeatedobservation of 2.361 31i associatedphases suggests similarities in the conditions 2.356 440 required for generatingaenigmatite-, rhiinite-, and dorr- 2345 422 2.338 303 ite-bearingassemblages. 2334 312 Abbott (1967) suggestedthat the following qualitative necessaryfor the formation of aenigmatite: Nofe;Data were obtained with a 114.6-mm-Gandolfi camera, Mnjiltered conditions are FeKd radiation,and a Si internalstandard. Intensities were visuallyesti- (l) relatively high concentrationsof TiO,, (2) peralkalini- mated ty, and (3) low oxygenfugacity. Lindsley (1971) placed quantitative limits on the stability of aenigmatite and re- stricted its occurrence to a temperature of >750 "C, a monocliniccell of dorrite having a : 19.97 , b : 30.17, c pressureof <0.5 kbar, and oxygen fugacities below the : 10.48A, and A:99.9' foundin this studyis the "OF Ni-NiO oxygenbuffer. Although aenigmatitemay be syn- (Olsen-Fuchs)cell:' in Merlino's terminology. thesizedat the above temperatureand pressureat the Ni- Using the above information and the diffractometer NiO oxygen buffer (Lindsley, 1971), with time (five data of the samecrystal, the true (untwinned) cell of dorr- months in his experiments)when subjectedto the above ite was found to be triclinic, Pl or PI, with refined pa- conditions, the aenigmatite breaks down completely to rametersa : 10.505(3),b : 10.897(3),c:9.019(l) A, a acmite + Ti-rich magnetite + qtrarlz. These reaction : 106.26(2),P:95.16(2)',andy : 124.75(2)'.The unit- products are probably the result of an oxidation-reduc- -- cell volume for this crystal Q 2) is 772.5(4) Ar; the tion equilibrium such as the following reaction proposed axial ratio a:b:c is 0.9640:l:0.8277 . The transformation by Marsh (1975): matrices between the triclinic cell and the various pseu- 3NarFer2*TiSiuOro+ 20, : 611u".3*Si2O6+ 6SiO, domonoclinic cells are given by Merlino (1972). + 3FerTiOo* Fe2+Fel+Oo. X-ray powder-diftaction data from approximately 12 (3) hand-pickedgrains ofdorrite wereobtained using a I 14.6- mm-diameter Gandolfi camera, Mn-filtered FeKa radia- Thus, aenigmatiteappears sensitive to changesin.f)r and tion, and elemental Si as an internal standard. The sam- is restricted to oxygen fugacities below that of the Ni- ple used for the Gandolfi photograph was polycrystalline, NiO buffer (Lindsley, l97l). as individual crystalswere too small to give a usablepho- Although the occurrence of aenigmatite in natural tograph. Indexed reflectionsand intensities from this pat- paragenesesis generally assumedto be the result of an COSCA ET AL.: DORRITE 1447 oxidation reaction involving and a sodic liquid The Atrlfor Reaction 7 has been calculated as 14 cm3, (e.g.,Nicholls and Carmichael,1969), a seriesof simple confirming that dorrite is the phasefavored by low pres- solid-solid reactionsmay also be written relating the sta- sure. This result is in agreementwith the physical envi- bility of aenigmatite,dorrite, and rhdnite relative to py- ronment containing dorrite and also with textures ob- roxenes,olivines, spinels, and/or oxides and appearsto servedin paralavas,where dorrites appearto be replacing especiallywell representrh6nite and dorrite occurrences magnetite-spinel solid solutions. The most Fe3+-rich in nature. For example, aenigmatite may form from a dorrites in paralava may be producedwhen accompanied pyroxene component and olivine by the reaction by increasedmagnetite solid solution as representedby the reaction 2NaFefrjTiorsiro6+ 2Fe,SiOo: NarFe!*TiSi6Oro.(4) 2CaFel*Siou* 2Fe2+Fel*Oo may viewed Rh0nite and dorrite be as Ca analoguesto : Car(Fel+Fel*)(Fel*Sir)Oro. (8) the aenigmatitein Reaction 4. The stability of end-mem- ber rhdnite CarMg,Ti(Al,Sio)O20is limited by the follow- The AZfor this reaction is estimated to be 19 cm3,again ing reactions: indicating that the aenigmatite-groupphase is stable at low pressures. + CaMgSi,O6] + Mg,TiOo * Mg,SiOo [CaAlSiAlOu Where rhdnite has been reported with aenigmatite : CarMgrTi(Alrsi4)Oro (5) (Yagi, 1953), rhdnite forms cores surrounded by discon- and tinuously zoned aenigmatite and perhaps reflecls varia- tions in temperture and oxidation statewith time. A sim- + caMgsi,oul + 2Mg,Sioo lcaAlTiAlo. ilar phenomenon may be representedby the coexistence : Ca,MgrTi(Alrsio)Oro. (6) of rhdnite and dorrite (Havette et al., 1982) in separate zonesof the basaltJimestonecontact on R6union Island. If thermodynamic data were available for rhdnite and Dorrite, rhiinite, and aenigmatite all form at very low adequatemixing parameterswere available for pyroxene, pressuresand at high temperatures in rocks of alkaline olivine, and rhdnite, such reactions could be used to de- bulk composition. In paralava, dorrite appearsto be fa- fine the stability limits of rhdnite in paragenesesinvolv- vored over rhiinite formation becauseof the high oxi- ing theseminerals. Unfortunately, no thermodynamicdata dation state and by the generalabsence ofTi in the sed- and only very lirnited experimental data (Boivin, 1980; imentary rocks melting to form patalava. Kunzmann et al., 1986) exist for rhtinite that could be used to constrain its stability. Preliminary experimental Note added in proof. The of an Fe3*- data of Kunzmann et al. (1986) for an olivine nephelinite rich syntheticdorrite and its relation to aenigmatite-group bulk composition suggestthat rhiinite forms at supersol- minerals have recently beendescribed by Mumme (1988, idus conditions at very low pressure.These conditions Neues Jahrbuch {iir Mineralogie Monatshefte, 359-366). are consistentwith most reports of rh6nite, which usually Thesecrystal-structure results compare favorably with our describeit as a late-stagecrystallization product of alka- data and indicate that small variations in the unit-cell line lavas(e.g., Soellner, 1907 ; LaCroix, 1909; Ficke, I 96 I ; parameters of dorrite occur with increased Fe3* substi- Brooks et al., 19'79;Kyle and Price, 1975;Olsson, 1983; tution. The composition reportedby Mumme ( I 988) plots Johnston and Stout, 1985) and intrusive rocks (Tomita, close to the upper right-hand corner ofFigure 6. 1934;Cameron et al., 1970).In occulrenceswhere tex- tural evidencesupports rhtjnite as a near-liquidus phase, AcxNowr,pncMENTS it is rimmed by an unidentified opaque mineral (Cam- W. M. Butler of the Chemistry Department, University of eron et al., 1970)or by aenigmatite(Yagi, 1953),possibly We thank Michigan, for collectingdata with the four-circle X-ray diffractometer.D. indicating that rhdnite may have a somewhathigher ther- R Peacoris thanked for helpful discussions,and for reviewing an earlier mal stability than aenigmatite. manuscript. C. H. Henderson is acknowledgedfor maintaining the elec- In all rhdnite analysesexcept those from the Allende tron microscopein excellentworking order. A review by F. F. Foit, Jr', meteorite (Fuchs, l97l; 1978)significant Fe3+is present, and comments by J. A. Mandarino on behalf of the IMA are both $ate- fully acknowledged.The electron microprobe used in this study was ac- suggestingthat rh6nite is stableunder relatively oxidizing quired under NSF grant EAR-82-12764-Pafts ofthis researchhave been conditions. Dorrite, on the other hand, contains even supportedby NSF grant EAR-84-08 I 68 to E.J.E. greaterproportions ofFe3* and possibly indicates stabil- ity limits of higher oxygen fugacity. This inference is RnrnnnNcBscrrno consistentwith previous estimatesof oxygen fugacity for Abbott, M.J. (1967) Aenigmatite from the groundmassof a peralkaline Wyoming paralava to be near the hematite-magnetite trach)4e.American Mineralogist, 52, 1896-190 L buffer (Coscaand Peacor, 1987). The two geologicenvi- Babkine,J., Conqu6re,F., Vilminot, J.C., and Duong, P.K. (1964) Sur un gisement rhiinite (Monistrol-d'Allier, Haute Loire). Comptes ronments from which dorrite has been describedattest to nouveau de Rendus Hebdomadairesdes Seancesde I'Academie des Sciences,258, the relatively rare conditions necessaryfor its crystalli- s479-5481. zation. End-member dorrite may be limited by a reaction Beckett, J.R., Grossman, L., and Haggerty, S.E. (1986) Origin of Ti3+- such as the following: bearingrhdnite in Ca-, Al-rich inclusions:An experimentalstudy. Me- teoritics.Zl. 332-333. 2CaFei*SiO. + 2MgAl,O4 : Car(MgrF{*XAloSir)Oro(7) Boivin, P (1980) Donn6es exp6rimentalespreliminaires sur la stabilit6 t448 COSCAET AL.:DORRITE

de la rhdnite i I atmosphere:Application aux gisementsnaturels. Bul- Kyle, P R., and Price, R C. (1975) Occurrencesofrhtinite in alkalic lavas letin de Min6ralogie,103, 491-502 of the McMurdo Volcanic Group, Antarctica, and Dunedin Volcano, Brooks,C.K., Pedersen,A.K., and Rex, D.C (1979)The petrologyand New Zealand.American Mrneralogist, 60, 722-7 25 ageofalkaline mafic lavasfrom the nunatak zoneofcentral EastGreen- LaCroix, M A (1909) Note sur la rhonite du Puy de Bameire i Saint- land.Gronlands Geologiske Undersogelse Bulletin, 133, l-28. Sandoux Bulletin de Soci6t6Frangaise de Min6ralogre et de Cnstal- Cameron,K L, Carman,M.F., and Butler,J.C. (1970)Rhdnite from Big r^oraohie.32. 325-331. BendNational Park,Texas. American Mineralogist, 55,864-874. Lindsley, D H. (1971) Synthesisand preliminary results on the stability Cannillo,E.,Mazzi, P.D , Fang,J H , Robinson,P D., andOhya, Y. (1971) of aenigmatite (Na,FerTiSi.O,o) Carnegie Institution of Washington The crystal structureof aenigmatite.American Mineralogist, 56,427- Year Book 70. 188-190 446 Lindsley,D.H., Smith, D., and Haggerty,S.E. (1971) Petrology and min- Carmichael,I.S E. (1962) Pantelleriticliquids and their phenocrysts.Min- eral chemistry of a differentiated flow of Picture Gorge basalt near eralogicalMagazine, 33, 86-l 13. Spray,Oregon CarnegieInstitution ofWashington Year Book 70, 264- Cosca,M.A., and Peacor,D.R. (1987) Chemistry and structureof essene- 285. ite (CaFe3*AlSiO6),a new pyroxene produced by pyrometamorphism. Magonthier,M.C., and Velde, D (1976) Mineralogyand petrologyof American Mineralogist, 72, 148-l 56. some Tertiary leucite-rhiinite basanitesfrom central France Mineral- Cosca,M A., Essene,E.J., Geissman, J G., Simmons,W B., and Coates, ogicalMagazine, 40, 817-826. D A (1989) Pyrometamorphicrocks associatedwith naturally bumed Marsh, J.S. (1975) Aenigmatite stability in silica-undersaturatedrocks. coal beds,Powder River Basin, Wyoming. American Mineralogist, 74, Contributionsto Mineralogyand Petrology,50, 135-144. rn press. Merlino, S. (1972) X-ray crystallographyof krinovite. Zeitschrift fiir Kris- Deer, W A., Howie, R.A., and Zussman,J. (1978) Rock-forming minerals tallographie,1 36, 8 l-88 (2nd edition), vol. 2A, p. 640-658 Wiley, New York. Nicholls,J., and Carmichael,I.S.E. (1969) Peralkalineacid liquids: A Ficke, B. (1961) PetrologischeUntersuchungen an Tertiaren bsaltischen petrologicalstudy. Contributionsto Mineralogyand Petrology,20,268- bis phonolithischenVulkaniten der Rhtin. TschermaksMineralogische 294. und PetrographischeMitteilungen,7 , 337436. Nishio, F., Katsushima,T., and Ohmae, H. (1985) Volcanic ash layersin Foit, F F., Hooper,R.L, and Rosenberg,P.E. (1987) An unusualpyrox- bare ice areasnear the the Yamato Mountains, Dronning Maud Land ene, melilite, and iron assemblagein a coal-fire buchite and rhe Allan Hills, Victoria Land, Antarctica. Annals of Glaciology, from Buffalo, Wyoming. American Mineralogist, 72, 137-147. 7, 3441. Fuchs, L.H (1971) Occurrenceofwollastonite, rhdnite, and andradite in Olsson,H.B. (1983)Rhdnite from Skdne(Scania), southem Sweden. Geo- the Allende meterorite American Mineralogist, 56, 2053-2068. logiska Fdreningensi Stockholm Fdrhandlingar, 105, 281-286 -(1978) The mineralogy of a rhtinite-bearing calcium-aluminum- Platt, R G., and Woolley,A R (1986)The mafic mineralogyof the per- rich inclusionin the Allendemeteorite. Meteoritics. 13.73-87. alkaline syenitesand granitesof the Mulanje complex, Malawi. Min- Grtinhagen,H., and Seck,H.A. (1972) Rhonit aus einem Melaphonolith eralogicalMagazine, 50, 85-99 vom Puy de Saint-Sandoux(Auvergne) Tschermaks Mineralogische Soellner,J. (1907) Uber Rhiinite, ein neues2inigmatitiihnliches Mineral und PetrographischeMitteilungen, I 8, I 7-38. und iiber die verbreitung desselbeninbasaltischen Gesteinen. Neues Havette, A., Clocchiatti, R , Nativel, P., and Montaggioni, L. (1982) Une Jahrbuch fiir Mineralogie Abhandlungen,24, 475-547. paragendseinhabituelle e fassaite,m6lilite et rhtinite dans un basalte Stolz, A J (1986)Mineralogy ofthe Nandewar volcano,northeastem New alcalin contamin6 au contact d'un r6cif corallien (Saint-Leu, lle de la South Wales,Australia. Mineralogical Magazine, 50, 241-255. R6union). Bulletin de Min6ralogie, lO5, 364-375. Tomita, T. (1934) On kaersutitefrom Dogo, Oki Islands,Japan, and its Johnston,A.D., and Stout,J.H. (1985)Compositional variation of nat- magmatic alteration and resorption Journal ofthe ShanghaiScientific urally occurring rhoenite. American Mineralogist, 70, 121l-1216 Institute,1,99-136 Jorgenson,K.A. (l 987) Mineralogy and petrology ofalkaline granophyric Yagi, K. (1953) Petrochemicalstudies of the alkalic rocks of the Morotu xenolithsfrom the Thorsmork ignimbrite, southem Iceland.Lithos, 20, district, Sakhalin.Geological Society of America Bulletin, 64, 7 69-809 l 53-l 68. Kunzmann,T., Spicker,G., and Huckenholz,H.G (1986)Stabilitiit von Rhiinit in natiirlichen und synthetischenParagenesen (abs.) Fort- MeNuscnrrr RECETVEDJeNulrv 14, 1988 schritte der Mineralogie, 64, 92 MnNuscnrsr AccEPTEDJurv 29. 1988