Murchisite, Cr5S6, a new mineral from the Murchison meteorite
Chi Ma, John R. Beckett, George R. Rossman
of Geological and Planetary Sciences, California Institute of
Pasadena, California 91125, USA
Murchisite (IMA 2010-003), Cr5S6, is a new chromium sulfide mineral, discovered in the Murchison CM2 meteorite. The type material occurs as one subhedral crystal (1.3 x 4 μm in size) in contact with Ni-bearing iron kamacite , martensitic iron, schreibersite and a Ca-, Al-rich glass, all of which are included in an isolated forsteritic olivine grain in the meteorite's matrix. The mean chemical composition determined by electron microprobe analysis of the type material is (wt%) Cr 53.32, S 42.87, V 1.44, Fe 1.14, P 0.10, Ni 0.10, sum 98.97. The empirical formula calculated on the basis of 6 S atoms is: (Cr4.60V0.13Fe0.09Ni0.01)∑4.83(S6.00P0.01)∑6.01. Murchisite was also identified in another isolated olivine grain from the same meteorite. These crystals are subhedral to round in shape, 300 nm to 1 μm in size, and occur in association with tochilinite and serpentine, within which it is included, chromite and eskolaite. Its electron back scatter diffraction patterns are an excellent match to that of synthetic Cr5S6 with the P1c structure, showing a = 5.982 Å, c = 11.509 Å, V = 356.67 Å3 and Z = 2, based on previously published data from synthetic material. Murchisite is named for the locality. It is a low-temperature phase (≤ 327 °C in the Cr-S system) whose formation is related to the evolution and alteration of Cr-bearing alloy spherules in forsteritic olivine grains that were probably derived from chondrule fragments. Formation of the type murchisite likely predates incorporation of the host olivine onto the Murchison parent body but murchisite included in tochilinite and serpentine may have formed during aqueous alteration on the Murchison parent body.
SEM image of the type murchisite (Cr5S6).