PYROXENE (miniature)

Pyroxene minerals are a group of inosilicates characterized by the general formula XY(Si, Al)2O6, where X represents larger cations such as calcium, sodium, iron(II), and magnesium, with occasional occurrences of zinc, manganese, and lithium. The Y site typically contains smaller ions like chromium, aluminum, iron(III), magnesium, manganese, scandium, titanium, vanadium, and sometimes iron(II). Notably, while aluminum can substitute for silicon in various silicates, it rarely does so in pyroxenes. The structure of pyroxenes features chains of SiO3 tetrahedra that alternate direction, creating a repeating pattern of left and right orientations. This arrangement resembles a series of interconnected three-sided pyramids, contributing to the minerals' prismatic to fibrous appearance. The angles formed by the tetrahedral pyramids result in cleavage angles close to 90 degrees, specifically 93 and 87 degrees. Pyroxenes are closely related to the pyroxenoid group, which shares a similar chain structure but exhibits more pronounced kinks. As significant constituents of many igneous and metamorphic rocks, pyroxenes indicate high-temperature crystallization conditions, typically in the absence of water; if water were present, amphiboles would likely form instead. The term "pyroxene" derives from Greek words meaning "fire" and "stranger," reflecting their unexpected occurrence in volcanic lavas, where they can sometimes be found as crystals within volcanic glass, leading to the misconception that they are mere impurities.