Alkalifelsparien

Alkalifelsparien, known in English as alkali feldspars, are a major group of feldspar minerals in which potassium and/or sodium dominate the chemical composition. They are tectosilicates with the general formula (K, Na)AlSi3O8 and form a continuous solid solution between the endmembers potassium feldspar (KAlSi3O8) and sodium feldspar (NaAlSi3O8). The group includes potassium-rich varieties such as orthoclase, sanidine, and microcline, as well as sodium-rich varieties such as albite, oligoclase, and andesine.

In the alkali feldspar series, the endmembers combine to create a wide range of compositions. Orthoclase, sanidine, and microcline are primary K-feldspars, whereas albite, oligoclase, and andesine are Na-rich feldspars. Microcline is the low-temperature, ordered form of K-feldspar, while orthoclase and sanidine are higher-temperature polymorphs. The minerals often exhibit exsolution textures, producing perthite intergrowths where small patches of one composition are embedded within another.

Alkali feldspars crystallize mainly in felsic igneous rocks such as granites, granodiorites, and rhyolites, but they also occur in metamorphic rocks and pegmatites. They form during the fractional crystallization of silica-rich magmas and can be altered by subsequent weathering to clay minerals. In many rocks, alkali feldspar is a principal source of potassium and a key component in ceramic and glassmaking industries.

Geologically, alkali feldspars are also used in radiometric dating, notably potassium-argon dating, which leverages the potassium content of these minerals to determine the age of rock samples.