pyrokseenista

Pyrokseenista is a term used in Finnish geology to denote pyroxenite, a class of ultramafic igneous rocks in which the mineral pyroxene constitutes the dominant phase. In English-language geology, the corresponding term is pyroxenite, and pyrokseenista is the Finnish equivalent used in field and laboratory descriptions.

Composition and varieties: Pyroxenites are dominated by pyroxene minerals, which may be orthopyroxene (enstatite) or clinopyroxene (diopside–augite). Olivine is commonly present in smaller amounts, and accessory minerals such as magnetite, chromian spinel, and amphibole can occur. Subvarieties include orthopyroxenite (mostly orthopyroxene), clinopyroxenite (mostly clinopyroxene), and websterite (a pyroxene-rich rock that includes olivine and both pyroxene varieties). Textures range from massive, granular rocks to layered or cumulate textures formed in magma chambers.

Formation and occurrence: Pyroxenites typically form as cumulate rocks in mafic to ultramafic magmatic systems, where pyroxene crystallizes early from basaltic magmas and accumulates as layers. They also occur as metamorphic or metasomatic rocks in mantle-derived sequences and within ophiolites and layered intrusions. They may be associated with peridotites and dunites in mantle sections.

Significance: Pyroxenites provide information on mantle chemistry, crystallization processes, and tectonic history. They are not generally major ore hosts, but accessory minerals such as chromian spinel and magnetite can be of mineralogical interest for researchers and collectors.

See also: Pyroxene, Pyroxenite, Ophiolite.