Murchisite,
Cr5S6, a new mineral from
the Murchison meteorite
Chi
Ma, John R. Beckett, George R. Rossman
Division
of Geological and Planetary Sciences, California Institute of
Technology,
Pasadena,
California 91125, USA
ABSTRACT
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 P
1c 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.
False-color
SEM image of the type murchisite (Cr5S6).