plagioclasebearing

Plagioclasebearing is a petrographic descriptor used to indicate that plagioclase feldspar is a major constituent of a rock or magma. Plagioclase is a tectosilicate solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with intermediate members such as oligoclase, andesine, labradorite, and bytownite. In igneous rocks, plagioclase commonly coexists with other feldspars and mafic minerals; its abundance is a key factor in classifying rocks on the QAPF diagram. Rocks described as plagioclasebearing include basalts, andesites, rhyolites, and granites, among others, where plagioclase constitutes a visible or modal portion of the mineral assemblage.

The presence and composition of plagioclase provide information about magmatic differentiation and crystallization history. For example, a more calcic plagioclase (higher calcium) suggests earlier crystallization from higher-temperature magmas, whereas more sodic plagioclase indicates differentiation. Plagioclase zoning, exsolution textures, and the ratio of calcium to sodium within plagioclase crystals can reveal cooling rates and magmatic processes. In metamorphic rocks, plagioclase may be preserved as a dominant feldspar phase, especially in calc- to low-grade assemblages.

The term plagioclasebearing does not imply a unique mineral assemblage beyond the presence of plagioclase; it is a descriptive qualifier used across rock types to emphasize plagioclase as a principal mineral component. It complements more specific classifications that note the dominant feldspar, silica saturation, and overall rock type.