Chi Ma, John R. Beckett, George R. Rossman
of Geological and Planetary Sciences, California Institute of
Pasadena, California 91125, USA
Browneite (IMA 2012-008), MnS, is a new member of the sphalerite group, discovered in Zakłodzie, an ungrouped enstatite-rich achondrite. The type material occurs as one single crystal (~ 16 μm in size) in contact with and surrounded by plagioclase; enstatite and troilite are nearby. Low-Ni iron, martensitic iron, tridymite, quartz, cristobalite, sinoite, schreibersite, buseckite, keilite and graphite, are also present in the type sample. Browneite is yellowish brown and transparent. The mean chemical composition of browneite, as determined by electron microprobe analysis of the type material, is (wt%) S 36.46, Mn 62.31, Fe 0.62, Ca 0.10, sum 99.49, leading to an empirical formula calculated on the basis of 2 atoms of (Mn0.993Fe0.010Ca0.002)S0.995. Electron back-scatter diffraction patterns of browneite are a good match to that of synthetic b-MnS with the F3m structure, showing a = 5.601 Å, V = 175.71 Å3 and Z = 4. Browneite is a low temperature (<200°C) phase metastable relative to alabandite that is likely derived from vapor or fluid deposition following the impact melting and subsequent crystallization of an enstatite-rich rock.
entire triangular area. Image credit: Chi Ma