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MINERALOGY and MINERAL CHEMISTRY of NOBLE METAL GRAINS in R CHONDRITES. H. Schulze, Museum Für Naturkunde, Institut Für Mineralogie, Invalidenstr

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MINERALOGY and MINERAL CHEMISTRY of NOBLE METAL GRAINS in R CHONDRITES. H. Schulze, Museum Für Naturkunde, Institut Für Mineralogie, Invalidenstr Lunar and Planetary Science XXX 1720.pdf MINERALOGY AND MINERAL CHEMISTRY OF NOBLE METAL GRAINS IN R CHONDRITES. H. Schulze, Museum für Naturkunde, Institut für Mineralogie, Invalidenstr. 43, D-10115 Berlin, Germany, hart- [email protected]. Introduction: Rumuruti-chondrites are a group of [1]. Mostly, NM grains can be encountered as mono- chondrites intermediate between ordinary and carbo- mineralic grains, but multiphase assemblages of two naceous chondrites [1]. The characteristic feature of or three phases were found as well. Host phase of most these chondrites is their highly oxidized mineral as- NM grains are sulfides, either intact like in Rumuruti semblage. Equilibrated olivine usually has a fayalite or weathered to Fe-hydroxides in the desert finds. content around ~39 mole-% Fa. There is almost no Also silicates are frequent host phases (mostly olivine free NiFe-metal in these chondrites [1]. As a conse- and plagioclase). Several grains were found in or ad- quence, noble metals form separate phases [1,2,3] jacent to chromite. which otherwise, due to their siderophilic behavior, Mineralogy and mineral chemistry: Table 1 lists certainly would be dissolved in the FeNi metal. De- the NM minerals detected in R chondrites. Summa- spite the occurrence of noble metal minerals the rizing the results for all R chondrites, tetraferroplati- chemistry does not show any enrichment in these ele- num, sperrylite, irarsite, and osmium are the most ments which are CI-like except for very volatile and abundant NM minerals. Gold seems to be a common chalcophile elements which are even lower [4]. Noble phase, too, but was not so frequently observed due to metal grains were described so far from carbonaceous the lower chemical abundance. Nevertheless, there are chondrites, i.e., the so-called fremdlinge or opaque some significant differences between the five R chon- assemblages in the CAIs of CV chondrites [5,6] and drites indicating a formation under different condi- from CK chondrites [7]. tions. The occurrence, mineralogy and mineral chemistry of these phases is described in detail. Five R chon- Table: Mineralogy of noble metal grains in R chon- drites were investigated: Acfer 217 (R3.8-5), Dar al drites and their abundance Gani 013 (R3.5-6), Hammadah al Hamra 119 (R4), Phases Formula A DaG HaH Hu R S Rumuruti (R3.8-6), and Hughes 030 (R3-6). 217 013 119 030 Method: Polished thin sections were carefully and completely scanned with a scanning electron micro- Tetraferro- PtFe (1) 36 - 3 4 44 scope in the backscatter mode at high contrast and a platinum magnification of 200-400x. Under these conditions, Sperrylite PtAs2 9 - 7 2 19 37 phases with high atom numbers become striking. It is Niggliite PtSn - - 5 - 1 6 estimated that the majority of noble metal grains equal Rusten- Pt3Sn - - 3 - 1 4 or larger than 1 µm could be detected in the thin sec- burgite tions. Analyses were performed with an EDX as well Unnamed Pt2PdSn2 - - 1 - - 1 as with a microprobe. Moncheite PtTe2 1 - - - - 1 Osmium Os 1 14 10 - 6 31 Size and occurrence: The grain size of NM grains Ruthenos- RuOsIr 1 - 1 - - 2 in R chondrites is generally in the range of 0.5-2 µm. miridium The largest grain is ~8x4 µm, but also several grains Erlich- OsS 2 - - - - 2 only 200-300 nm in size could be detected. The modal 2 manite abundance of NM grains is almost the same in all Laurite RuS 4 - - - - 4 chondrites with approximately 1-2 ppm by volume. 2 2 Irarsite IrAsS 14 7 6 4 10 41 On average 10-15 grains per cm were found. Their Gold Au 5 1 1 - 4 11 shape is mostly rounded. Several grains show idio- morphic shapes (especially irarsite and sperrylite), other are irregular (e.g., tetraferroplatinum, and os- Grains [n] 38 58 34 9 45 184 Invest. area 4.1 ~10 2.7 1.1 2.9 20.8 mium metal). 2 The NM grains are spread through the whole [cm ] Grains per 9 ~6 13 8 16 9 chondrite and were found in almost all petrologic 2 components except of chondrules. This is similar to cm the occurrence of NM grains in CK chondrites, but different to CV chondrites where they are restricted to Pt-phases. Sperrylite is the most abundant Pt- CAIs. The NM grains are, however, most abundant mineral in A 217 and Rumuruti. It was frequently within oval sulfide-plagioclase-chromite-assemblages observed in HaH 119 and Hughes 030, too. Often it is Lunar and Planetary Science XXX 1720.pdf NOBLE METALS IN R CHONDRITES: H. Schulze almost pure PtAs2. In A 217 and Rumuruti micro- these grains are not much more abundant than NM probe analyses revealed several percent Ir either in grains and their modal abundance is estimated to be solid solution or due to intimate intergrowths with on average less than 10 ppm by volume. Detected sul- irarsite. In HaH 119 sperrylite always contains ap- fides comprise molybdenite in DaG 013 and Rumuruti proximately 5-10 atom-% Sn and Sb substituting As. (lath-like, up to 10 µm) and heazlewoodite (DaG 013). Tetraferroplatinum is the major Pt-carrier (and only Discussion: Noble metals in R chondrites occur in Pt-phase) in DaG 013 and perhaps in Hughes 030. It separate phases with manifold mineralogy. There are always contains Ni (6-10 atom-%) and up to ~10 % Ir. some common minerals, but also some remarkable In Rumuruti an interesting difference has been ob- differences. Most noteworthy are the S-rich mineral served with sperrylite being more abundant in equili- assemblage in A 217, the dominance of Sn-containing brated clasts and the sulfide-plagioclase-chromite- Pt-phases in HaH 119, and the „primitive“ assemblage assemblages whereas tetraferroplatinum is more in DaG 013 with tetraferroplatinum as the only Pt- abundant in the matrix. HaH 119 is different from the phase and the lack of sperrylite. other chondrites containing abundant Sn-phases like Certainly, the minerals were formed under differ- niggliite, rustenburgite (both with up to 8 % Sb), and ent conditions as there is no evidence of differences in Sn-containing sperrylite. Such tin phases were also the chemical composition. A metamorphosis model found in Rumuruti. In A 217 a telluride, moncheite, can explain some but not all of the facts. Clearly, good was observed which was also described from CK thermodynamic data are needed. Petrographic evi- chondrites [7]. dence indicates that tetraferroplatinum forms as the Ir-phases. Irarsite is the major Ir-phase in all R first Pt-phase. The major evidence for this assumption chondrites. It is often intergrown with sperrylite comes from the high number of PtFe-grains in the and/or forms a solid solution with this mineral as in- least metamorphosed chondrite DaG 013 with its type dicated by petrologic evidence and up to ~8 atom-% Pt 3.5-clasts and in unequilibrated Rumuruti matrix. in the analyses, respectively. Tetraferroplatinum is PtFe was probably formed after the matrix. This is another important Ir-carrier. The Ir-content in this indicated by an irregular shape of a large PtFe-grain mineral is variable from almost 0 to 10 % Ir. In DaG in Rumuruti residing in clastic silicate matrix. The 013 almost half the Ir resides in tetraferroplatinum. outer edges are precipitated with gold which obviously Detectable amounts of Ir were also found in sperrylite is a later product. If this assumption is right then also (up to 10 %), osmium (up to 11 %), and ruthenos- Os which is sometimes intergrown with PtFe formed miridium (~15 %). under these conditions. Sn-compounds as found in Os-phases. Osmium metal with ~10-20 % Fe, HaH 119 probably formed at higher pT-conditions as ~10 % Re and Ir is the major Os-carrier in DaG 013, it is out of the investigated R chondrites the only R4- HaH 119, and Rumuruti. In A 217 Os metal occurs as chondrite [8]. This meteorite perhaps displays a nar- well, but here erlichmanite (OsS2) is the major phase row range of formation conditions as it is also the only indicating a formation under higher S-fugacity. unbrecciated R chondrite investigated so far. The ar- Ru-phases. In HaH 119 and A 217 very small senides irarsite and sperrylite are often intergrown and grains of ruthenosmiridium were found. In A 217 ru- their formation probably occurred under similar con- thenium occurs quite abundant as laurite (RuS2) ditions. They could be related to the equilibrated clasts forming a solid solution with erlichmanite. as indicated by the enrichment of sperrylite in such a Other noble metals. Gold occurs as native metal at clast in Rumuruti. However, idiomorphic grains en- least in four R chondrites. It contains several percent closed in silicates can have a different history as con- Fe, Cu, and Ag. Bischoff et al. described a gold sulfide densates. Furthermore, it is not clear how A 217 fits from A 217 [2]. The only Pd-phase detected so far is a into a metamorphosis scheme or whether this meteor- hard to define Pt-Pd-Sn compound found in A 217. Re ite formed under higher S-fugacity. was analyzed with up to ~10 % in Os metal. Micro- References: [1] Schulze H. et al. (1994) Mete- probe analyses revealed traces of Rh (0.1-0.2 %) in oritics, 29, 275–286. [2] Bischoff A. et al. (1994) tetraferroplatinum. Meteoritics, 29, 264–274. [3] Schulze H. (1998) Me- Additional minerals. In the course of this exami- teoritics & Planet. Sci., 33, A139. [4] Kallemeyn G. nation further noteworthy minerals were detected. W. (1996) Geochim. Cosmochim. Acta, 60, 2243– Awaruite and/or nickel grains (up to 90 % Ni) were 2256.–1345.
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