feldspatti

Feldspatti, or feldspars, are a group of rock-forming minerals in the silicate family known as tectosilicates. They are among the most abundant minerals in the Earth's crust, forming roughly 60 percent of crustal rocks. Feldspars are aluminosilicates in which aluminum, silicon, and oxygen form a framework that accommodates potassium, sodium, and/or calcium ions, giving rise to two main subgroups: alkali feldspars and plagioclase feldspars.

Alkali feldspars include potassium-rich varieties such as orthoclase, microcline, and sanidine. Plagioclase feldspars form a solid solution series from sodium-rich albite to calcium-rich anorthite, with intermediate members like oligoclase, andesine, labradorite, and bytownite. The two groups differ in chemical composition and occur in a wide range of igneous, metamorphic, and sedimentary rocks.

Feldspars crystallize from magma and are particularly common in felsic rocks such as granites and rhyolites, as well as in some mafic rocks where plagioclase is present. In weathering, feldspars break down to clay minerals, releasing alkali metals into soils and influencing sediment composition and soil fertility.

Physically, feldspars typically have a hardness around 6 on the Mohs scale, a vitreous luster, and two good cleavages at or near 90 degrees. They occur in a broad color range, often white, pink, gray, or brown, and can be transparent to translucent.

Industrially, feldspars are important as fluxing agents in glassmaking and ceramics, lowering melting temperatures and aiding glaze formation. They are also used as fillers in paints, plastics, and rubber, and certain microcline varieties are valued as gemstones.