American Mineralogist, Volume 82, pages 1001±1006, 1997

Sinoite (Si2N2O): Crystallization from EL chondrite impact melts

ALAN E. RUBIN Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, U.S.A.

ABSTRACT

Sinoite (Si2N2O) was previously observed only in EL6 chondrites and recently modeled as having formed over geologic time scales at metamorphic temperatures of ϳ950 ЊC. I found several ϳ10±210 ␮m-sized subhedral and euhedral grains of twinned, optically zoned sinoite associated with euhedral enstatite and euhedral graphite within impact-melted portions of QUE94368, the ®rst EL4 chondrite. The presence of sinoite within a type 4 chondrite mitigates against the metamorphic model of sinoite formation; it seems more

likely that sinoite crystallized from a liquid. During impact melting of EL material, N2 may have been released from lattice defects in sul®des whereupon it reacted with reduced Si dissolved in the metallic Fe-Ni melt and with ®ne-grained or molten silica derived from the silicate fraction of the EL assemblage. The N that formed the sinoite was derived from the silicate melt or from temporary, melt-®lled cavities constructed from unmelted EL material in which the nitrogen fugacity may have reached ϳ40 to 130 bars (0.004 to 0.013 GPa). Sinoite in EL6 chondrites may have formed either metamorphically, as previously proposed, or by means of crystallization from an impact melt, as in QUE94368. In the latter case, sinoite-bearing EL6 chondrites would be annealed impact-melt breccias.

INTRODUCTION chondrite found at 84Њ35Ј58.4482ЉS, 162Њ11Ј16.6142Љ W in the Queen Alexandra Range of Victoria Land, Antarc- Natural silicon oxynitride (Si2N2O) was ®rst observed but not identi®ed by Lacroix (1905) in two EL6 chon- tica, in 1994. The meteorite was classi®ed initially by drites. It was identi®ed by Andersen et al. (1964) and Keil McBride and Mason (1996) as an E5 chondrite, but, as I and Andersen (1965a, 1965b) in several EL6 chondrites show below, QUE94368 is the ®rst EL4 chondrite. It is of weathering class C (McBride and Mason 1996), im- where it occurs as Յ200 ␮m-sized euhedral, lath-like grains associated with metallic Fe-Ni and enstatite. The plying severe surface rustiness and pervasive terrestrial new orthorhombic mineral was named sinoite, an acro- oxidation of metal particles. nym of its chemical formula. ANALYTICAL PROCEDURES The origin of sinoite is controversial. Herndon and Suess (1976) and Sears (1980) suggested that sinoite is a Polished thin section QUE94368,4, obtained from the nebular phase, formed by condensation at high tempera- NASA Johnson Space Center in Houston, was studied tures and pressures from a gas of solar composition. microscopically using transmitted and re¯ected light. However, more recent thermodynamic calculations (e.g., Minerals were analyzed with the UCLA automated Ca- Larimer and Bartholomay 1979; Fegley 1983) disputed meca Camebax-microbeam electron microprobe using this. For example, Fegley (1983) found that the thermo- crystal spectrometers, a sample current of ϳ12 nA at 15 dynamic activities of silicon oxynitride were reduced sig- kV and analysis times of 20 s. PAP corrections were ap- ni®cantly by the prior condensation of more stable Si- plied to the data. A lead stearate crystal was used to scan bearing species, making it unlikely that sinoite could form for O and N peaks in sinoite. For O, 400 points were as an equilibrium product from a gas of solar composi- scanned at a sample current of 2.2 nA at 15 kV; for N, tion. Petaev and Khodakovsky (1986) and Fogel et al. 20 points were scanned at 1.2 nA and 15 kV. (1989) proposed that sinoite formed metamorphically at RESULTS temperatures appreciably below that of the laboratory synthesis of silicon oxynitride (1450 ЊC; Brosset and Id- Petrologic characteristics of QUE94368 restedt 1964). Muenow et al. (1992) suggested that sinoite QUE94368 consists of enstatite, olivine, sinoite, ka- formed at EL6 metamorphic temperatures (i.e., ϳ950 ЊC; macite, schreibersite, graphite, troilite, rare grains of fer- Muenow et al. 1992; Wasson et al. 1994) over geologic roan alabandite, and goethite (formed by terrestrial time scales under conditions wherein Si-bearing metallic weathering of kamacite). Olivine constitutes ϳ0.2 vol% Fe-Ni acted as a catalyst. of QUE94368,4; it is rare but de®nitely present in the Below, I report the ®rst occurrence of sinoite outside Smithsonian library section of QUE94368 (T.J. McCoy, EL6 chondrites. It occurs in QUE94368, a 1.2 g enstatite personal communication). Schreibersite occurs typically 0003±004X/97/0910±1001$05.00 1001 1002 RUBIN: SINOITE IN CHONDRITE IMPACT MELTS as 2±5 ␮m-sized patches at kamacite-silicate grain Enclosed within these sinoite grains are smaller (typi- boundaries. cally 6 ϫ 20 ␮m) euhedral, lath-like sinoite crystals that The rock contains moderately recrystallized chondrules appear to have crystallized at earlier stages from the melt (Fig. 1a) with ®ne-grained, silicic-feldspathic mesostases. (Fig. 1e). Their continued observability presumably in- Chondrules range in apparent diameter from 220 to 1100 dicates some discontinuity in sinoite crystallization his- ␮m and average ϳ520 ␮m(nϭ15). One 1020 ϫ 1180 tory and might re¯ect slight differences in composition ␮m-sized porphyritic olivine-pyroxene chondrule is pres- (e.g., because of the incorporation of impurities). A few ent; this chondrule contains 20±100 ␮m-sized forsterite of the optically zoned sinoite grains have congruent bands grains (Fa 0.19 Ϯ 0.05) poikilitically enclosed in enstatite phe- of differing birefringence immediately beneath the grain nocrysts (Fig. 1b). edge (Fig. 1f ); whereas these bands also may indicate Approximately 10±15 vol% of the rock consists of discontinuities in crystallization, it is possible that they regions containing numerous grains of euhedral enstatite are an artifact of thin section preparation. Although op- (0.5 ϫ 7 ␮m±30 ϫ 170 ␮m) surrounded by kamacite tically observable twinning is rare in orthorhombic crys- (Fig. 1c) and goethite. The euhedral enstatite grains are tals (aragonite is a notable exception), many of the sinoite moderately more ferroan than most enstatite phenocrysts grains in QUE94368 are twinned (Fig. 1d±1f ). Although in intact chondrules (Fs0.57 vs. Fs0.34; Table 1). Whereas the twinning probably results from late-stage mutual in- most enstatite grains in chondrules contain curvilinear terference between growing crystals, post-formation trails of tiny kamacite blebs, such trails are essentially shock-induced twinning cannot be ruled out. absent in the euhedral enstatite grains. Rubin and Scott Wavelength scans using a lead stearate crystal across (1997) showed that very similar euhedral enstatite grains the theoretical peaks for O and N on the largest sinoite are abundant in Abee and other EH chondrite impact-melt grains (Fig. 2) indicate the presence of abundant O and breccias and virtually absent in unmelted EH chondrites. N in the mineral. Along with abundant Si determined In Abee some of the euhedral enstatite grains have nu- with a TAP crystal, these peaks identify the mineral in cleated on the surfaces of partly resorbed chondrules. QUE94368 as sinoite. Also present in the same regions of QUE94368 that contain euhedral enstatite grains are abundant euhedral DISCUSSION laths of graphite (4 ϫ 34 ␮m±16 ϫ 150 ␮m; Fig. 1c), a few of which possess pyramidal terminations. The euhed- Classi®cation of QUE94368 ral graphite grains are very similar to those in the Abee, QUE94368 is not a type 3 chondrite because it lacks Y-791790, and Y-791810 EH impact-melt breccias (Rub- very sharply de®ned chondrules, does not contain glassy in and Scott 1997) as well as those in the weakly shocked chondrule mesostases, and has relatively homogeneous ureilites ALHA78019 and Nova 001 (Berkley and Jones Ca-poor pyroxene compositions (Table 1). Its moderately 1982; Treiman and Berkley 1994). Similar primary ig- distinct chondritic structure is characteristic of a type 4 neous graphite occurs as a rare phase in terrestrial ultra- chondrite (Fig. 1a). Approximately 0.2 vol% olivine is ma®c xenoliths within alkali basalt (e.g., Figs. 5±7 of present. Because olivine is absent in some type 4 and all Kornprobst et al. 1987). type 5 and type 6 enstatite chondrites (Binns 1967; Rubin Graphite also occurs in QUE94368 as ϳ15 ␮m-diam- et al. 1997), I classify QUE94368 as type 4. eter aggregates within kamacite; whereas most of the Characteristics indicating that QUE94368 is an EL graphite in these aggregates is ®ne-grained, 10±40% of it occurs as smooth, relatively homogeneous patches that chondrite include: (1) The presense of kamacite with rel- appear to have partly recrystallized. atively low Si (0.5±0.7 wt%; McBride and Mason 1996), more similar to that in EL chondrites (ϳ1 wt%) than that Sinoite in EH chondrites (ϳ3 wt%; Keil 1968); (2) The absence Sinoite occurs as highly birefringent grains within the of niningerite and the occurrence of rare grains of ferroan euhedral-enstatite- and euhedral-graphite-bearing portions alabandite (this study), {EL chondrites of all petrologic of thin section QUE94368,4; it is also present in the types contain ferroan alabandite [(Mn,Fe)S] and lack ni- Smithsonian library section (T.J. McCoy, personal com- ningerite [(Mg,Fe)S], whereas EH3-5 chondrites contain munication). In section QUE94368,4, one 50 ␮m anhed- niningerite and lack ferroan alabandite (e.g., Keil 1968; ral sinoite grain is adjacent to one end of a 27 ϫ 140 ␮m Prinz et al. 1984; Lin et al. 1991; El Goresy et al. 1992)}; euhedral enstatite grain; a smaller (6 ϫ 12 ␮m) subhedral and (3) The presence of chondrules with an average ap- sinoite grain is adjacent to the other end of the same parent diameter of ϳ520 ␮m (this study). This average is euhedral enstatite grain. Most of the sinoite in this section very close to the mean apparent diameter of chondrules occurs in a multi-grain cluster of 70±210 ␮m-sized, main- in the ALH85119, MAC88136, and PCA91020 EL3 ly rectangular, euhedral grains with distinct optical zoning chondrites (ϳ550 ␮m; A.E. Rubin, unpublished data) and (Fig. 1d±1f ). Although separated, some of the grains in appreciably greater than that of EH3 chondrules (ϳ220 the cluster appear as if they could ®t together. The sinoite ␮m; Rubin and Grossman 1987). Thus, QUE94368 is the cluster is adjacent to euhedral enstatite grains and is sur- ®rst known EL4 chondrite, completing the EL3-EL6 rounded by kamacite and goethite (Fig. 1d). metamorphic sequence (cf. Rubin et al. 1997). RUBIN: SINOITE IN CHONDRITE IMPACT MELTS 1003

FIGURE 1. Photomicrographs of QUE94368. (a) Whole thin multi-grain cluster of sinoite grains (medium gray) with visible section view of QUE94368,4 showing discernible but moderately twin boundaries (medium gray). Light gray material surrounding recrystallized chondrules. A large goethite-rich weathering vein the enstatite and sinoite is goethite formed by the terrestrial (black) cuts across the section mainly from north to south. Trans- weathering of kamacite. Re¯ected light. (e) Optical zoning in the mitted light. (b) Porphyritic olivine-pyroxene chondrule contain- cluster of sinoite grains. Some grains contain internal lath-like ing 20±100 ␮m-sized forsterite grains (white; ol, arrow) poikil- zones that represent the boundaries of sinoite grains that crys- itically enclosed in enstatite phenocrysts (gray). X-nicols. (c) tallized from the melt at early stages. X-nicols. (f) Same view as Impact-melted region of the rock containing numerous euhedral in (d) showing sinoite grain cluster with prominent twinning and enstatite laths (medium gray) as well as a few graphite laths (light congruent bands of differing birefringence beneath the grain edge gray; gr, arrows) surrounded by kamacite (white). Re¯ected (arrow). X-nicols. light. (d) Euhedral enstatite lath (dark gray; en) adjacent to a 1004 RUBIN: SINOITE IN CHONDRITE IMPACT MELTS

TABLE 1. Mean compositions and standard deviations (wt%) of enstatite and olivine in QUE94368

Euhedral Enstatite in FeO-enriched Olivine enst grains chondrules enst in a chd in a chd

No. of grains 8 8 1 2

SiO2 60.9 Ϯ 0.4 60.9 Ϯ 0.3 59.8 41.6

Al2O3 0.11 Ϯ 0.02 0.15 Ϯ 0.04 0.13 Ͻ0.04

Cr2O3 Ͻ0.04 Ͻ0.04 Ͻ0.04 Ͻ0.04 FeO* 0.40 Ϯ 0.12 0.24 Ϯ 0.08 1.2 0.20 MnO Ͻ0.04 Ͻ0.04 0.04 Ͻ0.04 MgO 39.0 Ϯ 0.2 38.9 Ϯ 0.4 37.8 58.2 CaO 0.56 Ϯ 0.09 0.57 Ϯ 0.07 0.64 Ͻ0.04 Total 101.0 100.8 99.6 100.0

End-member Fs0.57Wo1.0 Fs0.34Wo1.0 Fs1.7Wo1.2 Fa0.19 Note: enst ϭ enstatite; chd ϭ chondrule. * Total Fe as FeO.

QUE94368 as an impact-melt breccia Numerous euhedral enstatite grains, very similar to those that occur in Abee and other EH impact-melt brec- cias (Rubin and Scott 1997), are concentrated in regions constituting 10±15 vol% of QUE94368. It seems likely that the euhedral enstatite grains in QUE94368 formed in the same manner as those in Abee, and that QUE94368, like Abee, is an impact-melt breccia. The euhedral shapes of the enstatite grains in these rocks were acquired through primary crystallization from a melt, consistent with the high surface energy and high surface-energy an- isotropy of orthopyroxene (e.g., Spry 1969). The higher

FeO contents of the euhedral enstatite grains in FIGURE 2. Wavelength spectrometer scans of the large si- QUE94368 in comparison with most enstatite pheno- noite grain (shown at bottom right of Fig. 1e) using a lead ste- crysts in chondrules in this rock probably re¯ect crystal- arate crystal. Counts are plotted against sin ␪ϫ105.(a) O (raw lization from a melt slightly enriched in FeO. This en- data). In these units, the theoretical O peak is at sin ␪ϭ23874. richment most likely resulted from the impact melting of (b) N (smoothed curve). In these units, the theoretical N peak is rare chondrules containing relatively FeO-rich, Ca-poor at sin ␪ϭ31939. In conjunction with the determination with a pyroxene grains (cf. Table 1); for example, a few Ca-poor TAP crystal of abundant Si in the grain, these peaks con®rm the pyroxene grains in EL3 chondrites contain up to 12 mol% identi®cation of the mineral as sinoite. Fs (Score and Mason 1987). The absence of curvilinear trails of kamacite blebs in (Table 4.4 of Dodd 1981), was probably not heated ap- the euhedral enstatite grains is consistent with crystalli- preciably above 700 ЊC (except in its impact-melted zation of euhedral enstatite after the shock event that part- regions), argues against the metamorphic model for si- ly melted QUE94368 and caused extensive silicate dark- noite formation. In contrast, the presence of twinned, op- ening in the unmelted portions (cf. Rubin 1992). The tically zoned, euhedral grains of sinoite within impact- sinoite crystals in QUE94368 also lack curvilinear trails melted portions of QUE94368 indicate that sinoite in this of kamacite blebs and, hence, also appear to have crys- rock crystallized from a melt. tallized after the main shock event. N in enstatite chondrites probably occurs mainly within The euhedral graphite grains in QUE94368 occur in lattice defects in sul®de phases (Muenow et al. 1992). the same regions as the euhedral enstatite grains. The graphite grains are very similar to those in Abee and are Dynamic high-temperature mass-spectrometric analysis of EH and EL chondrites indicates thatNϩ is released not mainly associated with kamacite. Outside these 2 regions, graphite occurs as aggregates within kamacite between ϳ950 and 1080 ЊC (Muenow et al. 1992); this just as in unmelted EH chondrites such as Indarch. This interval corresponds approximately to the temperature of occurrence suggests that during the impact events that the metallic-Fe-Ni±sul®de cotectic. During the impact melted QUE94368 and Abee, graphite-bearing metal was melting of EL material, N2 would be released from sul- melted and euhedral graphite crystallized as a primary ®de. Some of the N would dissolve in the silicate melt phase separate from the metal. (which can accommodate up to 0.5 wt% N at 1500 ЊC; Fogel 1994), substituting for O (e.g., Baur 1972).

Origin of sinoite Although some N2 probably escaped during the impact The presence of sinoite within an olivine-bearing type event, it seems likely that suf®cient amounts were re- 4 chondrite, which, by analogy to ordinary chondrites tained for the formation of sinoite because another RUBIN: SINOITE IN CHONDRITE IMPACT MELTS 1005

N-bearing phase (i.e., osbornite, TiN) was reported in oth- breccias (e.g., Hvittis and Blith®eld; Rubin 1983a, 1984; er samples of impact-melted enstatite chondrite material. Rubin et al. 1997); others contain clasts or large opaque Kinsey et al. (1995) found 15 ␮m-sized osbornite grains veins for which an impact origin seems probable (e.g., within an impact-melt rock clast in EL6 Hvittis (although Atlanta, Eagle and Khairpur; Rubin 1983b; Olsen et al. it is absent from the Hvittis matrix). McCoy et al. (1995) 1988; Rubin et al. 1997). Jajh deh Kot Lalu contains a reported three 10±20 ␮m-sized osbornite grains in the ϳ2 ϫ 16-mm chondrule-free, oldhamite-rich vein that Ilafegh 009 EL impact-melt rock and one 15 ϫ 30 ␮m probably formed by an impact process (Rubin et al. grain in impact-melted regions of the Happy Canyon EL 1997). Because all known EL6 chondrites are shock stage impact-melt breccia. In all three cases, it seems very like- S2 (even though some have been signi®cantly altered by ly that osbornite crystallized from the melt. impacts), it seems likely that proto-EL6 material was Sinoite in QUE94368 probably formed in a manner shocked to S3-S5 levels (and some was impact melted) somewhat analogous to the laboratory synthesis of silicon before peak metamorphism (Rubin et al. 1997); annealing oxynitride (Brosset and Idrestedt 1964): N2 reacted with erased many of the shock features in these samples. The reduced Si dissolved in the metallic Fe-Ni melt and with rocks were shocked again to stage S2 after metamorphism. ®ne-grained (or molten) silica derived from the silicate Although it is possible that sinoite in EL6 chondrites fraction of the EL chondrite assemblage. At 1400±1500 formed metamorphically over geologic time scales as ЊC, the following reaction, suggested by Ryall and Muan suggested by Muenow et al. (1992), a plausible alterna- (1969), may have occurred: tive is that sinoite in EL6 chondrites formed in the same manner as sinoite in QUE94368, i.e., by crystallization SiO ϩ 3Si ϩ 2N (g) ϭ 2Si N O 2 2 2 2 from an EL chondrite impact melt. This formation is con- 0 for which⌬Gf , the free energy of reaction, ranges from sistent with the euhedral shapes of many of the EL6 si- approximately Ϫ530 to Ϫ550 kJ/mol (Fegley 1981). It noite grains and with the inferred shock history of some seems unlikely that sinoite could have formed by a re- EL6 chondrites. This alternative leaves open the possi- action between silica and silicon nitride (i.e., SiO2 ϩ bility that the rare euhedral enstatite laths in some sinoite-

Si3N4 ϭ 2Si2N2O) because Si3N4 in enstatite chondrites bearing EL6 chondrites also crystallized from impact occurs as very rare Յ2 ␮m-sized grains (Alexander et al. melts before metamorphism. 1994; Lee et al. 1995). The bulk N contents of enstatite chondrites were mea-

It is unclear if the N2 that formed the sinoite was de- sured by several teams of investigators and found to vary rived from the silicate melt or from temporary, melt-®lled from ϳ100 to 1000 ␮g/g (Moore et al. 1969; Kung and cavities constructed from unmelted EL material in which Clayton 1978; Thiemens and Clayton 1983; Grady et al. relatively high N2 partial pressures were achieved. The 1986; Muenow et al. 1992). Although Grady et al. (1986) nitrogen fugacity (in atm) in equilibrium with sinoite can found no systematic tendency for N content to vary with be determined from the following equation after Alex- petrologic type, the data of Moore et al. (1969) indicate ander et al. (1994): that bulk N may be higher in sinoite-bearing EL6 chon- log (f ) ϭ (Ϫ15011/T) ϩ 8.245 Ϫ [1.5 log (X )] drites (ϳ650±780 ␮g/g) than in EL6 chondrites that lack NSi2 sinoite (ϳ50 ␮g/g) or in EH chondrites (none of which Ϫ[(17450/T) Ϫ 9.45] (X ) Si contain sinoite) (ϳ180±430 ␮g/g). If this is the case, si- where XSi is the mole fraction of Si in kamacite and T is noite formation may be restricted to impact-melted ensta- the temperature in kelvins. From the average composition tite chondrites that initially possessed abundant bulk N. of kamacite in EL chondrites (Table 5 of Keil 1968), XSi can be determined to be ϳ0.026. For temperatures of ACKNOWLEDGMENTS I thank K. Keil and W.A. Dollase for discussions, F.T. Kyte, A.L. Sailer, 1400 ЊC and 1500 ЊC, the log(fN2) values are ϩ1.6 and ϩ2.1, respectively. These values correspond tof values and J.T. Wasson for technical assistance, T.J. McCoy for examining the N2 Smithsonian library section of QUE94368, and the Antarctic Meteorite of ϳ40 bars and ϳ130 bars, respectively. Working Group for providing thin section QUE94368,4. Helpful reviews were provided by M.K. Weisberg and K. Lodders. This work was sup- Sinoite-bearing EL6 chondrites ported in part by NASA grant NAGW-5099. EL6 chondrites that contain sinoite include Forrest 033, Hvittis, Jajh deh Kot Lalu, Pillistfer, Ufana, Yilmia, REFERENCES CITED ALHA81021 (and its paired specimen ALH83018), Alexander, C.M.O'D., Swan, P., and Prombo, C.A. (1994) Occurrence and LEW88714, and EET90102 (Lacroix 1905; Andersen et implications of silicon nitride in enstatite chondrites. Meteoritics, 29, al. 1964; Keil and Andersen 1965a, 1965b; Buseck and 79±85. Andersen, C.A., Keil, K., and Mason, B. (1964) Silicon oxynitride: A Holdsworth 1972; Rubin et al. 1997; Schwarz and Mason meteoritic mineral. Science, 146, 256±257. 1983, 1987, 1992; Satterwhite and Mason 1992). The si- Baur, W.H. (1972) Occurrence of nitride nitrogen in silicate minerals. Na- noite grains in these rocks range up to 200 ␮m in length ture, 240, 461±462. and many have subhedral to euhedral morphologies (e.g., Berkley, J.L. and Jones, J.H. (1982) Primary igneous carbon in ureilites: Petrological implications. Proceedings of the Lunar and Planetary Sci- Fig. 1 of Andersen et al. 1964; Fig. 1 of Keil and An- ence Conference, 13th, A353-A364. dersen 1965a). Binns, R.A. (1967) Olivine in enstatite chondrites. American Mineralogist, Some EL6 chondrites are fragmental or impact-melt 52, 1549±1554. 1006 RUBIN: SINOITE IN CHONDRITE IMPACT MELTS

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McCoy, T.J., Keil, K., Bogard, D.D., Garrison, D.H., Casanova, I., Lind- MANUSCRIPT RECEIVED JANUARY 22, 1997 strom, M.M., Brearley, A.J., Kehm, K., Nichols, R.H., Jr., and Hoh- MANUSCRIPT ACCEPTED MAY 13, 1997