Crystal chemistry and durability of the spinel structure type in natural systems

Minerals of the spinel group, ideally AB₂O₄ oxides of diverse chemistry, are widespread in nature and occur in a number of geological environments including igneous intrusive and volcanic rock suites, regional and contact metamorphic rocks, various sedimentary environments, lunar rocks, and meteorites. In the structure of normal spinel, the A-site is tetrahedrally coordinated and generally occupied by divalent cations (e.g. Mg, Mn, Fe, Ni, Zn). The B-site is octahedrally coordinated and occupied by trivalent cations (e.g. Al, Cr, Fe) or in part by tetravalent Ti. Inverse spinel is characterized by the occupation of one of the B-sites by the divalent cation with one trivalent cation taking its place on the A-site. Vacancies are also possible in the structure, as shown by solid solution from AB₂O₄ toward A(B_1.67□_0.33)O₄ stoichiometry in certain compositions. Spinels having compositions close to the ideal end-member spinel, MgAl₂O₄, have proven to be highly durable in natural systems and often occur in placer deposits following the breakdown of the host rock by weathering. At higher temperatures, spinel may be replaced by hoegbomite, an OH-bearing mineral of similar composition, A_2-2x-0.5yTi_xB₄O_8-y(OH)_y, where A and B are cations derived from the replaced spinel. The durability in natural systems is corroborated by laboratory studies demonstrating that spinel is highly resistant to dissolution in acidic fluids.