alkaliveldspaat

Alkaliveldspaat, or alkali feldspar, is a group of potassium- and sodium-rich feldspar minerals. Its general formula is (K,Na)AlSi3O8, and the main members are sanidine, orthoclase, and microcline. Sanidine is the high-temperature endmember, orthoclase represents an intermediate form, and microcline is a low-temperature, highly ordered variety. In microcline, exsolution of Na-rich lamellae can create a perthitic texture, giving intergrowths that look like a mosaic under the microscope.

Alkali feldspars crystallize in felsic igneous rocks and are common in granites, granodiorites, rhyolites, and pegmatites. They also occur in some high-grade metamorphic rocks resulting from the reworking of granitic compositions. The potassium-rich nature of these minerals makes them important reservoirs of K in the crust and they often record magmatic cooling histories and metamorphic conditions.

In hand specimen, alkali feldspars are typically colorless to pink, white, or gray, with a vitreous luster and two cleavage directions at about 90 degrees. Microcline can show a distinctive grid-like (tartan) twinning pattern under polarized light, while orthoclase and sanidine may display different twinning features. Weathering can alter alkali feldspar to clay minerals such as illite or sericite, releasing potassium into secondary minerals or fluids.

Geologically, alkali feldspar is a key indicator of granitoid and felsic compositions and is widely used in K-Ar and Ar-Ar radiometric dating due to its high potassium content.