Important group of non-silicate mineral oxides, including the subgroups spinel series, magnetite series, and chromite series. Members of the spinel series have the general formula XAl₂O₄, where X = Mg in spinel, X = Fe²⁺ in hercynite, X = Zn in gahnite, and X = Mn in galaxite. Complete solid solution exists between all four end-members and also magnesiochromite (MgCr₂O₄); spinels with appreciable Fe²⁺ (Mg:Fe = 1:3) are called pleonaste and those with Fe²⁺ and Cr³⁺ are called mitchellite; sp. gr. 3.5–4.1; hardness 7.5–8.0; spinel is dark green and hercynite is dark bluish-green; all occur as small octohedra. Unlike chromite, which occurs in ultramafic (see ultrabasic) igneous intrusions, spinel may occur in metamorphic schists and gneisses with sillimanite, garnet, and cordierite. In contact metamorphosed impure limestones spinel occurs with chondrodite, olivine, and orthopyroxene, and in emery deposits it occurs with corundum. Spinel may develop in residual aluminous xenoliths enclosed in basic magmatic rocks. Spinel is found in alluvial deposits and gem quality spinel also occurs. Hercynite occurs in metamorphosed laterites; gahnite in granite pegmatites; and galaxite in manganese vein deposits.