A new potassium-iron-nickel sulphide from a nodule in kimberlite

A new K-rich sulphide phase is described from a clinopyroxene-ilmenite intergrowth from the Frank Smith kimberlite diatremes. The mineral occurs as ovoid blebs, which also contain pyrrhotite and pentlandite, in close spatial association with the ilmenite lamellae, and as individual grains in cracks in the clinopyroxene host. Chemical analyses from twelve grains show the following ranges of composition (wt.%): K, 8.12–13.01; Cu, 1.24–2.99; Fe, 38.47–41.76; Ni, 11.65–15.04; S, 32.09–34.34. Electron diffraction data indicate a primitive cubic unit cell,a = 10.29 ± 0.03Å, with no systematically absent reflections. Three models for the origin of this sulphide, liquid immiscibility, multiple exsolution, and metasomatism, are considered. The discovery of potassic sulphide in a kimberlite nodule has important implications, not only as a potential source of K in the upper mantle, but also for those models which propose removal of K and S from the lower mantle into the core.