Pyroxene << PY rok seen >> is any of a group of minerals that play an important part in the formation of many kinds of rocks. Pyroxenes occur widely in the continental and oceanic crusts of the earth, as well as in many of the rocks of the moon’s outer crust.
Most pyroxenes are the direct product of igneous and metamorphic processes of rock formation (see Rock (Igneous rock) (Metamorphic rock)). Geologists study pyroxenes that have crystallized from magma (molten rock material) and, by doing so, they have learned much about the changes undergone by rocks that have formed from magma.
Pyroxenes range in color from greenish black and reddish brown to colorless. All pyroxenes have the same silicate structure. In this structure, a silicon atom at the center with four oxygen atoms attached forms a tetrahedron, a pyramidlike figure with four triangular faces. These groups of atoms band together in a single chain, with two of the four oxygen atoms connecting to adjacent tetrahedra. The chains, in turn, are linked together by positively charged atoms called cations within the unit cell of a pyroxene (see Mineral (Inside minerals)).
Cations of different elements affect the arrangements of the chains within the crystal structures. With a calcium, sodium, or other large cation present, the crystals form a monoclinic pattern (see Crystal). These pyroxenes are called clinopyroxenes. With a small cation, such as iron or magnesium, the crystals form an orthorhombic pattern. These pyroxenes are called orthopyroxenes.
The most common varieties of clinopyroxenes are augites, diopsides, and pigeonites. The most common orthopyroxenes are bronzites and hypersthenes.
