Geochemistry and bonding of thiospinel minerals

The thiospinel group of minerals present a variety of interesting problems in mineral chemistry. Carrollite (CuCo₂S₄), linnaeite (Co₃S₄), siegenite [(Co, Ni)₃S₄], polydymite (Ni₃S₄) and violante (FeNi₂S₄) occur in ore deposits, daubréelite [(Fe, Mn, Zn)Cr₂S₄] is present in meteorites and greigite (Fe₃S₄) is found in lacustrine sediments. This paper illustrates how the crystal chemistry, geochemistry and certain physical properties may be interpreted by molecular orbital and band theories of the chemical bond. In the spinel structure, transition metal ions occur in tetrahedral and octahedral coordinations, and 3d electrons of cations bonded to sulphur atoms are distributed amongst non-bonding and anti-bonding molecular orbitals. The composition ranges, geochemistry, and variations of cell parameters, microhardness, reflectivities and relative stabilities of thiospinels are directly related to the numbers of electrons in antibonding molecular orbitals. Linnaeite, which is the most stable thiospinel mineral in the system Cu-Co-Ni-Fe-S, has the smallest number of antibonding electrons. It has the smallest cell edge, highest reflectivity and largest microhardness. With increasing electron occupancy of the antibonding orbitals, each of these physical properties, together with the thermal stabilities, decrease along the linnaeite-siegenite-polydymite, linnaeite-carrollite and violarite-polydymite series. In each of these minerals, the unusually small cell edge may be correlated with the occurrence of transition metal ions in low-spin states. The metallic conductivity and Pauli-paramagnetism of many of these minerals is related to electron delocalization in antibonding molecular orbitals. Iron cations occur in high-spin states in greigite, giving rise to increased numbers of electrons in antibonding orbitals. As a result greigite has a larger cell edge and lower thermal stability than other thiospinel minerals. The semiconducting and ferrimagnetic properties of greigite indicate that the 3d electrons are more localized on the cation than sulphospinels in the Cu-Co-Ni-Fe-S system. Absence of solid-solution between violarite and greigite is attributed to differing spin-states of octahedral Fe(II) ions, which are low-spin in violarite and high-spin in greigite. The high thermal stability of daubréelite is due to the large octahedral site preference energy of Cr³⁺ in the structure. The preference of transition metal ions for octahedral coordination is reflected by the ease in which thiospinel phases transform to a cation defect NiAs structure at elevated pressures and temperatures. As a result, polymorphic transitions involving thiospinel phases are postulated in the mantle and in shocked meteorites.