AlkaliFeldspäten

Alkali feldspäten is a group of tectosilicate minerals that together form a solid solution series. This series is primarily composed of potassium feldspat (KAlSi3O8) and sodium feldspat (NaAlSi3O8). The end-member potassium feldspat is known as orthoclase or microcline, while the end-member sodium feldspat is albite. Within this solid solution, various intermediate compositions exist, such as sanidine and anorthoclase. These minerals are characterized by their framework structure of silicon-oxygen and aluminum-oxygen tetrahedra, with alkali metal cations (potassium and sodium) occupying interstitial sites.

Alkali feldspäten are extremely common in igneous rocks, particularly in granites, syenites, and rhyolites, where they are often a primary constituent. They also occur in metamorphic rocks like gneisses and schists, and are found in some sedimentary rocks. Their physical properties include a hardness of 6 to 6.5 on the Mohs scale and a specific gravity ranging from approximately 2.55 to 2.63. They typically exhibit two cleavage directions at or near 90 degrees. Color can vary widely, including white, pink, gray, and colorless, often influenced by impurities or exsolution textures. Many alkali feldspäten display characteristic twinning, such as Carlsbad or albite twins. Their abundance and widespread occurrence make them significant minerals in understanding geological processes and rock formation.