DISCOVERY OF A NEW CHROMIUM SULFIDE MINERAL, Cr5S6, IN MURCHISON

Introduction: During a nano-mineralogy investigation of the Murchison CM2 meteorite, a new chromium sulfide mineral, Cr5S6, was identified as inclusions in two isolated olivine grains. High-resolution SEM, electron-backscatter diffraction (EBSD) and electron microprobe analyses have been used to characterize its composition and structure. The phase is known from laboratory studies of the Cr-S system [1] but has not been previously reported in nature. The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2010003). Occurrence, Chemistry, and Crystallography: One subhedral micrograin [(Cr4.59Fe0.23V0.13Ni0.02) P 4.97(S5.98P0.05) P 6.03] (the type material) occurs associated with a 10 lm alloy inclusion hosted by an irregular olivine (Fo99Fa1, 400 lm in size) grain; it is bounded in section by schreibersite (Fe2.45Ni0.55P) on one side, which largely separates the Cr5S6 from the kamacite (Fe0.929Ni0.060P0.008Cr0.003), and, on the other, a Si-rich glass. Submicron-sized grains of Cr5S6 were also observed in another isolated olivine (Fo99Fa1) grain in Murchison, occurring in fine-grained mixtures of serpentine and tochilinite with chromite and eskolaite nearby. EBSD patterns of this new phase were indexed using the structure and unit cell data for synthetic Cr5S6 from [1]. The structure is trigonal, P31c (a = 5.982 Å, c = 11.509 Å, V = 356.67 Å , Z = 2) and consists of close packed S layers in hexagonal stacking with Cr in octahedral voids and ordered vacancies in every second interlayer [1, 2]. Origin and Significance: This mineral (Cr5S6) is a new meteoritic chromium sulfide, joining the Cr-dominant meteoritic sulfide minerals brezinaite (Cr3S4) and daubreelite (FeCr2S4). Cr5S6 is a low temperature phase, limited in the Cr-S system, by a eutectoid at 327 C. Given the absence of additional coexisting sulfides, it seems unlikely that this new sulfide mineral formed through eutectoid or peritectoid decomposition of a high temperature Cr-S solid solution (e.g., formed through sulfidation of an original Cr-, P-bearing Fe-Ni, which also exsolved schreibersite). It is more likely a low-temperature secondary phase formed during parent body aqueous alteration with Cr derived from the alloy. The interposition of schreibersite between Cr5S6 and kamacite suggests that growth may have occurred in a chemical potential gradient. References: [1] Jellinek F. 1957. Acta Crystallogr. 10:620–628. [2] Van Laar B. 1967. Phys. Rev. 156:654–662. 5155 USING GRAIN DENSITY AND MAGNETIC SUSCEPTIBILITY TO QUANTIFY WEATHERING IN CHONDRITE FINDS R. J. Macke, D. T. Britt and G. J. Consolmagno. University of Central Florida. E-mail: [email protected]. Specola Vaticana.