Orthopyroxene
Orthopyroxene is a group of pyroxene minerals with an orthorhombic crystal structure, typically low in calcium.
Orthopyroxene is one of the two main branches of the pyroxene family. Its crystals have orthorhombic symmetry, which simply means the three axes of its building block meet at right angles but have different lengths. Orthopyroxenes contain little calcium and range from magnesium-rich enstatite to more iron-rich compositions.
Orthopyroxene is a key mineral in many meteorites. Diogenites, which come from deep inside the asteroid Vesta, are made almost entirely of it. It is also abundant in ordinary chondrites, where its composition, along with that of olivine, helps define the H, L and LL groups. ALH 84001, the famous Martian meteorite, is an orthopyroxenite, a rock dominated by this mineral.
Under the polarising microscope orthopyroxene shows straight extinction, going dark when its long cleavage lines are parallel to the crosshairs. That property helps a petrologist distinguish it from clinopyroxene.
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