Orthopyroxene
Introduction
Orthopyroxene is a significant mineral group within the pyroxene family, characterized by its orthorhombic crystal system. These minerals are commonly found in igneous and metamorphic environments, playing a crucial role in the petrology of these rocks. Orthopyroxenes are primarily composed of a solid solution series between enstatite (MgSiO₃) and ferrosilite (FeSiO₃), with variations in their chemical composition influencing their physical properties and stability.
Crystal Structure and Composition
Orthopyroxenes crystallize in the orthorhombic system, which is defined by three mutually perpendicular axes of unequal length. This structure contrasts with the monoclinic system of clinopyroxene, another subgroup of the pyroxene family. The general formula for orthopyroxenes is (Mg,Fe)₂Si₂O₆, where magnesium and iron can substitute for each other in the crystal lattice, forming a continuous solid solution series.
The orthorhombic structure of orthopyroxenes is characterized by chains of silica tetrahedra linked by octahedrally coordinated cations. This arrangement results in a distinct cleavage pattern, typically at angles of approximately 90 degrees, which is a diagnostic feature of pyroxenes.
Physical Properties
Orthopyroxenes are typically found in shades ranging from green to brown, with their color intensity often correlating with their iron content. The mineral exhibits two prominent cleavage directions at nearly right angles, a vitreous to dull luster, and a hardness of 5 to 6 on the Mohs scale. The specific gravity of orthopyroxenes varies between 3.2 and 3.9, depending on their composition.
The optical properties of orthopyroxenes are also significant in mineral identification. They are biaxial minerals, exhibiting two optic axes, and display pleochroism, where the mineral appears to change color when viewed from different angles under polarized light. This property is particularly useful in thin section analysis using a petrographic microscope.
Occurrence and Formation
Orthopyroxenes are commonly found in mafic and ultramafic igneous rocks, such as gabbro, norite, and peridotite. They also occur in high-grade metamorphic rocks, including granulite facies rocks, where they form under conditions of high temperature and moderate pressure.
In igneous environments, orthopyroxenes crystallize from magma as part of the early stages of fractional crystallization. Their stability is influenced by the geochemical environment, particularly the availability of magnesium and iron. In metamorphic settings, orthopyroxenes form through the recrystallization of pre-existing minerals under elevated temperature conditions, often in association with garnet and plagioclase.
Geochemical Significance
The presence of orthopyroxene in rocks provides valuable insights into the petrogenesis of the host rock. The Mg/Fe ratio in orthopyroxenes can be used to infer the oxygen fugacity of the magma from which they crystallized. This information is crucial for understanding the redox conditions during rock formation and can be used to reconstruct the thermal history of the rock.
Orthopyroxenes also play a role in geothermobarometry, a technique used to estimate the temperature and pressure conditions during rock formation. The exchange of elements between orthopyroxene and coexisting minerals, such as clinopyroxene and garnet, can be used to calculate these conditions with considerable accuracy.
Industrial and Economic Importance
While orthopyroxenes are not typically mined for their own sake, they are an important component of many industrial minerals and rocks. Their presence in dunite and harzburgite is significant for the mining of chromite and platinum group elements. Additionally, orthopyroxenes contribute to the refractory properties of certain rocks, making them valuable in the production of heat-resistant materials.