Hercynite

Hercynite is the iron-bearing end-member of the spinel group, an iron aluminium oxide (FeAl2O4). It is the dark counterpart of gem spinel: typically black to very dark green or brown, opaque, and rarely of facet quality. Its name comes from the Hercynian Forest of central Europe.

Properties

Like other spinels it is isometric and hard (7.5–8), often forming octahedral crystals or granular masses, and it can be weakly magnetic where it grades toward magnetite. It frequently forms in solid solution with other spinel-group oxides.

Occurrence

Hercynite forms in high-temperature, aluminium-rich metamorphic rocks (emery deposits, granulites) and in some mafic igneous rocks. It is a member of the wider spinel group of oxides.