Pyroxene

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Microscopic image Pyroxene

Pyroxene is a group of important rock-forming inorganic silicate minerals commonly found in many igneous rocks and metamorphic rocks. Pyroxenes have a complex chemical composition and structure, with a general formula of XY(Si,Al)₂O₆, where X and Y are both cations, with X typically larger than Y. The X site can be occupied by calcium, sodium, iron, or magnesium, while the Y site can host ions such as manganese, iron, magnesium, chromium, and aluminium. This variability in composition leads to a wide variety of pyroxene minerals, each with unique properties and occurrences.

Classification

Pyroxenes are classified based on their chemistry and crystal structure into two major groups: the orthopyroxenes and the clinopyroxenes. Orthopyroxenes have orthorhombic crystal symmetry, while clinopyroxenes are characterized by monoclinic symmetry.

Orthopyroxenes

Orthopyroxenes, such as enstatite (MgSiO₃) and ferrosilite (FeSiO₃), are typically high-temperature minerals found in igneous rocks like basalt and gabbro, as well as in some high-grade metamorphic rocks.

Clinopyroxenes

Clinopyroxenes include minerals such as augite, diopside (CaMgSi₂O₆), and jadeite (NaAlSi₂O₆). These minerals are more commonly found in a variety of rock types, including both igneous and metamorphic rocks.

Physical and Chemical Properties

Pyroxenes are generally characterized by their dark color, ranging from green to black, and a hardness of 5 to 6 on the Mohs scale of mineral hardness. They have two distinctive cleavage planes at nearly 90 degrees, a feature that helps in their identification.

The chemical composition of pyroxenes allows for extensive solid solution between members, leading to a wide range in their physical and chemical properties. This variability is a result of the substitution of various cations in the crystal lattice, influenced by the conditions under which the pyroxene formed.

Occurrence

Pyroxenes are ubiquitous in the Earth's crust, found in a wide range of igneous and metamorphic rocks. They are significant components of the oceanic crust, particularly in basalt and gabbro, and are also present in many continental rocks. In addition to their presence on Earth, pyroxenes have been identified in meteorites, the Moon, and Mars, indicating their importance in understanding planetary geology.

Economic Importance

While pyroxenes themselves are not typically mined for economic purposes, they are indicators of the presence of other valuable minerals such as platinum, chromium, and nickel. In geology, the study of pyroxene minerals can provide valuable information about the conditions under which rocks form, including temperature, pressure, and the presence of other minerals.

See Also

• Silicate minerals
• Igneous rocks
• Metamorphic rocks
• Mineralogy

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