plagioclasedominated

Plagioclase-dominated refers to a mineralogical condition in which plagioclase feldspar is the most abundant mineral phase in a rock. The term is used in igneous petrology to describe rocks whose modal assemblage is controlled by plagioclase over other common minerals such as pyroxene, olivine, quartz, or alkali feldspar. In practice, plagioclase may account for a majority of the mineral content, and rocks can be described as plagioclase-dominated even when other minerals are present.

Plagioclase is a continuous solid solution series between the sodium-rich endmember albite and the calcium-rich endmember anorthite. In plagioclase-dominated rocks, the feldspar component is primarily this plagioclase group, and it commonly forms the framework crystals that define the texture. The relative abundance of plagioclase versus other minerals influences the rock’s color, density, and crystallization history.

Common occurrences include mafic to intermediate igneous rocks such as basalt, gabbro, diorite, and related rocks, where plagioclase is typically the dominant feldspar. Anorthosite is a classic example in which plagioclase constitutes virtually all the feldspar. In some felsic systems, plagioclase can also be prominent, especially in rocks with high plagioclase content and relatively little alkali feldspar or quartz.

The designation helps petrologists infer crystallization conditions, including magma differentiation, pressure, and water content, because the composition and stability of plagioclase track the magmatic evolution of the rock.