Spinel Group

The spinel group is a family of oxide minerals sharing the general formula AB2O4 and a common cubic crystal structure, in which metal cations occupy two kinds of site within a framework of oxygen. Wide chemical substitution between the members makes the group a textbook example of solid solution.

Members

The group includes gem spinel (MgAl2O4), the iron oxide magnetite (Fe3O4), the chromium ore chromite (FeCr2O4), zinc-rich gahnite, iron-rich hercynite, and the Franklin zinc mineral franklinite, among others.

Properties and occurrence

Spinel-group minerals are typically hard (often 7.5–8 for the aluminous members), form octahedral crystals, and several are magnetic. They occur in high-temperature metamorphic rocks, mafic and ultramafic igneous rocks, and as durable detrital grains in placers.