Spinels are one of a few relatively common isotropic minerals. Minerals of the spinel series are generally combinations of four or so end members: spinel (MgAl2O4), hercynite (FeAl2O4), gahnite (ZnAl2O4), galaxite (MnAl2O4). The mineral name spinel refers to a spcific composition, MgAl2O4.

Spinels can be found in some aluminous, Si-poor, metapelitic rocks, associated with other aluminous minerals. They occur in metacarbonate rocks, often associated with other Mg silicates. They are also found in some mafic and ultramafic igneous rocks, typically with olivine or pyroxenes.

Important properties:
 ·Appearance and habit - Spinels are isotropic, have high relief, and show no cleavage. They may form octahedra but, more commonly, are anhedral.
 ·Color - Color is highly variable. Common spinels may be colorless, green, red, brown, blue, or black.

Similar minerals:
 ·There are few common isotropic minerals. Spinel is the only one that typically has strong coloration and so should be easily identified.

Spinel with Pyroxenes in a Peridotite Xenolith from Kilbourne Hole, New Mexico

These photos show a high-relief green spinel grain in the upper right (PP). Most of the rest of the view is clinopyroxene (augite). A large grain of orthopyroxene, with slightly greater relief than the clinopyroxene, is on the left edge of the photos. Note that the spinel (isotropic) appears black in the XP view.

The field of view is 4.5 mm.