kalsilite

Kalsilite is a rare potassium aluminum silicate mineral in the feldspathoid group, with the ideal formula KAlSiO4. It forms in silica-undersaturated, alkalic igneous rocks and is typically colorless to white, sometimes pale yellow, with a vitreous to pearly luster. Crystals are usually tabular or platy and may occur as grains or in intergrowths with other feldspathoids.

As a feldspathoid, kalsilite has a framework silicate structure built from linked SiO4 and AlO4 tetrahedra that create cavities large enough to accommodate alkali cations. In kalsilite those cavities are occupied by potassium ions. The mineral has a relatively low density and a Mohs hardness around 5, reflecting its silicate-like composition.

Geologically, kalsilite forms in silica-undersaturated, alkali-rich igneous systems, including nephelinite, phonolite, and related rocks. It is commonly associated with nepheline, leucite, melilite, sodalite, natrolite, and other feldspathoids, as well as with certain mafic minerals.

The name kalsilite derives from potassium (kalium) and silicate, reflecting its chemical composition. The mineral is mainly of interest to mineral collectors and researchers studying alkali igneous rocks and feldspathoid assemblages. See also feldspathoids, leucite, and nepheline for related minerals.