olivineorthopyroxene

Olivineorthopyroxene refers to a geological context where both olivine and orthopyroxene minerals are present within the same rock. These minerals are both silicate minerals, but they belong to different crystal structures. Olivine is a nesosilicate, characterized by isolated SiO4 tetrahedra, and typically has a forsterite-fayalite composition. Orthopyroxene is an inosilicate, featuring double chains of SiO4 tetrahedra, and its common members are enstatite-ferrosilite. The co-occurrence of these minerals in a rock is geologically significant.

The presence of both olivine and orthopyroxene often indicates a specific range of formation conditions, particularly regarding temperature and pressure. In many igneous rocks, their coexistence suggests that the magma from which they crystallized cooled through a temperature range where both minerals were stable. For instance, in basaltic magmas, the crystallization sequence can involve olivine forming first, followed by orthopyroxene as the temperature decreases. This pairing is common in mafic and ultramafic rocks, such as peridotites, gabbros, and some basalts.

The relative proportions and textures of olivine and orthopyroxene can provide further clues about the petrogenesis of the rock. In some cases, orthopyroxene may form as a reaction product rimming olivine crystals if the magma composition or oxygen fugacity changes during cooling. Studying the mineral chemistry of both olivine and orthopyroxene in a sample can help geologists reconstruct the thermal and chemical history of their formation environment, offering insights into mantle processes or igneous differentiation.