Hypersthene is a relatively common mineral and is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. It forms a solid solution series with the minerals enstatite and ferrosilite. A solid solution series occurs when two or more elements can substitute for each other in a crystal structure without much alteration of the structure. In this case, the elements are iron and magnesium and enstatite is the magnesium end member of the series.
Hypersthene is the intermediate member with around 50% iron and ferrosilite is the iron rich end member of the series. Enstatite is fairly common but ferrosilite is extremely rare. The iron deeply colours the minerals and therefore any deeply coloured specimens of this series are usually called hypersthene.
In fact the two most common members of the series are often considered together as enstatite-hypersthene in many mineral guides and texts. Hypersthene is an orthopyroxene or a pyroxene with an orthorhombic symmetry. At high temperatures, hypersthene's structure changes to a structure with a monoclinic symmetry, a clinopyroxene or more specifically, clinohypersthene. Clinohypersthene is a polymorph of hypersthene, meaning that it has the same chemistry but a different structure.