Spinellit

Spinellit, or spinels, are a family of oxide minerals that share the spinel structure, with the general chemical formula AB2O4. In this arrangement, A2+ cations occupy tetrahedral sites and B3+ cations occupy octahedral sites within a close-packed oxide lattice. Natural spinels display a range of cation distributions, from normal to inverse spinel, and many minerals in the group can be described by AB2O4 with varying elements.

The spinel structure is cubic (isometric) and typically forms crystals that are commonly seen as octahedra or related shapes. The most familiar spinels include magnesium aluminate (MgAl2O4, magnesio-spinel), chromite (FeCr2O4), and hercynite (FeAl2O4). Magnetite (Fe3O4) also adopts a spinel-related structure and is described as an inverse spinel. The distribution of elements among the A and B sites can influence color, density, and other physical properties.

Spinellit occur in a variety of geological settings. They form in high-temperature environments such as ultramafic and mafic igneous rocks, as well as in metamorphic rocks where they can indicate pressure, temperature, and chemical conditions. They are common in chromitites and other mantle-derived rocks and are prized as indicator minerals in geology.

Physical properties of spinellit include a hardness of about 7.5–8 on the Mohs scale and a specific gravity around 3.5–4.0. They are typically vitreous and have a cubic refractive character, with little to no birefringence. Color in spinellit ranges from colorless to red, pink, blue, green, or black, driven by trace impurities such as chromium, iron, cobalt, and titanium. Red spinel is especially well known as a gemstone, and some spinellit crystals show asterism or chatoyancy due to inclusions.

Synthetic spinel is produced for gemstones and industrial uses, including lasers and durable, affordable alternatives to natural stones. Spinellit continues to be studied for its wide color range, diverse chemistry, and geological significance.