plagioklaas

Plagioklaas, known in English as plagioclase, is a broad group of feldspar minerals forming a continuous solid solution between the sodium-rich endmember albite (NaAlSi3O8) and the calcium-rich endmember anorthite (CaAl2Si2O8). The plagioclase series includes albite, oligoclase, andesine, labradorite, and bytownite as progressively calcium-rich members. Together, these minerals are among the most common rock-forming minerals in the Earth's crust.

Plagioclase minerals are tectosilicates with a framework of aluminum and silicon tetrahedra coordinated by oxygen. They typically exhibit two cleavage directions at about 90 degrees and often show fine surface striations produced by polysynthetic twinning. Some varieties can display a variety of optical effects, including the iridescence known as labradorescence in labradorite. Hardness is about 6 to 6.5 on the Mohs scale, and specific gravity ranges roughly from 2.6 to 2.8.

Geological occurrence and significance: plagioclase is a major constituent of many igneous rocks, including granites, diorites, and basalts, and is also found in metamorphic rocks such as gneiss and schist. It forms by crystallization from siliceous magmas and through fractional crystallization produces the characteristic mineral assemblages of related rocks. Its compositional variability helps define rock types and magmatic histories.

Uses and identification: plagioclase is widely used as a raw material in the ceramic and glass industries and as an ornamental stone. In hand sample and under the microscope, the presence of two near-90-degree cleavages, white to gray color, and, in some varieties, labradorescence, aid in identification and distinction from alkali feldspars.