Mineraalassemblages

Mineraalassemblages are the characteristic sets of minerals that co-occur in a rock at a given time and under specific physical conditions. They serve as the mineralogical fingerprints of how a rock formed and how it was subsequently altered. An assemblage is more than a simple list of minerals; it reflects the paragenetic sequence—the order in which minerals crystallized, grew, or re-equilibrated during geological processes.

Assemblages occur in various geological settings. Primary assemblages form during the crystallization of igneous rocks or during metamorphism, while secondary or alteration assemblages develop during hydrothermal activity, metamorphic overprint, or diagenesis in sedimentary rocks. Diagenetic and weathering processes can create distinct, often simpler, mineral combinations such as calcite, dolomite, clays, and quartz cement in sediments, or evaporite minerals like halite and gypsum in evaporitic environments.

Common examples include metamorphic facies assemblages: greenschist facies with chlorite, epidote, albite, and quartz; amphibolite facies with hornblende, plagioclase, quartz, and biotite; granulite facies with orthopyroxene, garnet, plagioclase, and quartz. In sedimentary rocks, diagenetic assemblages may involve calcite, dolomite, quartz, and clays, while evaporites may contain halite and gypsum. Ore-forming and hydrothermal systems often exhibit assemblages such as pyrite, sphalerite, and galena with gangue minerals like quartz or barite.

Determining mineral assemblages relies on petrographic microscopy, X-ray diffraction, and electron probe techniques. They are used to infer pressure–temperature histories, depositional environments, and ore potential, making assemblages a fundamental concept in petrography and economic geology.