plagioklas

Plagioclase is a major group of feldspar minerals forming a continuous solid-solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The plagioclase series includes common varieties such as albite, oligoclase, andesine, labradorite, and bytownite, with composition varying primarily in the proportion of sodium and calcium. In general terms, plagioclase minerals range from sodium-rich to calcium-rich along the albite–anorthite axis.

Crystallography and properties: Plagioclase feldspars are tectosilicates that typically exhibit two cleavage directions near 90 degrees. They are usually colorless to white or pale gray, often showing zonation from core to margin due to changes in Na–Ca ratio during growth. A distinctive feature of some varieties is fine parallel striations on cleavage surfaces, caused by exsolution of the Na-rich and Ca-rich phases. Certain specimens display labradorescence, an iridescent play of color, particularly in the lava-feldspar member labradorite.

Occurrence and significance: Plagioclase is a dominant constituent of many igneous rocks, including basalt, andesite, diorite, granodiorite, and rhyolite. It also occurs in metamorphic rocks such as gneiss and schist derived from granitoid sources. In petrography, plagioclase serves as a key diagnostic mineral for rock classification and provides evidence of magmatic differentiation and cooling history. It contrasts with alkali feldspar, which has different end-member compositions and textures.

Endmembers and common varieties: The series spans from albite (sodium-rich) to anorthite (calcium-rich). Intermediate members include oligoclase, andesine, labradorite, and bytownite, each defined by increasing calcium content. Approximate anorthite content ranges are used in classifications: albite 0–10% An, oligoclase ~10–30% An, andesine ~30–50% An, labradorite ~50–70% An, bytownite ~70–90% An, with anorthite 90–100% An.