Isocubanite is a cubic polymorph of cubanite CuFezS3, discovered in the submarine sulphide deposits of the East Pacific Rise (EPR) and the Red Sea, in association with chalcopyrite, pyrrhotine, pyrite, sphalerite or wurtzite and anhydrite. It was previously obtained artificially and mentioned as iss, 'intermediate solid solution of CuFe2S3 composition'. Crystals are cubic with a = 5.303(3) A and strongest lines in the X-ray powder pattern are 3.059 (10) 111, 1.876 (7) Isocubanite is characteristic of high-temperature (> 200°C) present-day submarine sulphide deposits where hot hydrothermal fluids are quenched by seawater as in EPR deposits or by cooler brines as in Atlantis II Deep, in the Red Sea; it is unstable and therefore unusual in fossil ores. This cubic phase was previously observed and described more or less accurately as cubic cubanite, cubanite II, chalcopyrrhotite and iss; the name isocubanite, proposed in order to clarify the nomenclature, and this new description, were approved by the IMA Commission on New Minerals and Mineral Names.
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