Isocubanite is a cubic polymorph of cubanite CuFe2S3, discovered in the submarine sulphide deposits of the East Pacific Rise (EPR) and the Red Sea, in association with chalcopyrite, pyrrhotine, pyrite, sphaterite 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) Å and strongest lines in the X-ray powder pattern are 3.059 (10) 111, 1.876 (7) 220, 1.602 (5) 311. Euhedral grains range from a few microns up to 400 µm, with a Vickers microhardness of 175(25) kg/mm2 for P = 100 g. Isocubanite is usually intimately intergrown with iron- and zinc-rich chalcopyrite and is opaque with a metallic lustre and a bronze colour. Pinkish brown and isotropic in polished sections; reflectance values (SiC standard) are: 420 nm 22.73%, 460 nm 26.87, 500 nm 31.34, 540 nm 34.79, 580 nm 37.35, 620 nm 39.11,660 nm 40.32, 700 nm 41.33, 740 nm 41.91, 780 nm 42.50. Electron microprobe analyses gave (wt. %): Fe 41.64–42.49, Cu 20.79–21.52, Zn 0.77–1.09, S 35.49–35.82, corresponding to the formula (Cu,Zn)Fe2S3.
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.