Feldspar is so common in the crust that its weathered remains make most mud
Feldspar minerals crystallise in almost every kind of igneous rock, but they form at high temperatures and fall apart easily at the surface. Water breaks them down into clays, which is why about 40 percent of the minerals in sedimentary rocks are clay. The same mineral also ends up in glass bottles.
Feldspars are aluminium silicate minerals that also contain sodium, calcium, potassium or occasionally barium. The two main families are the plagioclase feldspars, mixing sodium and calcium, and the alkali feldspars, mixing potassium and sodium. Structurally they are tectosilicates: each aluminium or silicon ion sits inside four oxygens, and every oxygen is shared with a neighbouring unit, building a continuous three-dimensional framework.
The name comes from the German Feldspat, joining field with a word for rock that splits into flakes, introduced in the eighteenth century perhaps because the mineral turned up so often in fieldstones. English spar, meaning a clear mineral with good cleavage, shaped the modern spelling, and the variant felspar has faded. Geologists use feldspar to classify igneous rocks: the balance of alkali feldspar, plagioclase and quartz underpins the standard QAPF scheme. As magma cools, calcium-rich plagioclase crystallises first and later crystals grow steadily richer in sodium. Rock made almost entirely of calcic plagioclase is called anorthosite.
Temperature decides a lot. Potassium and sodium feldspars do not mix well at low temperatures, so intermediate alkali compositions form only in hot settings; sanidine is the hot-temperature form, while microcline belongs to cooler conditions. Plagioclase shows a graded series from sodium-rich to calcium-rich varieties such as labradorite. Unusual members include buddingtonite, which contains ammonium, and barium feldspars. At the surface, hydrolysis turns feldspar into clays such as kaolinite and illite, and it dissolves faster in acidic or alkaline water than in neutral water.
People put it to work widely. In the United States about two thirds is used in glassmaking, where it supplies alkalis for melting and alumina for strength, and it serves as a flux in ceramics and glazes. Some household cleaners use it as a gentle abrasive, and geologists and archaeologists date samples with it. The US Geological Survey put world production in 2020 at 26 million tonnes. In 2012 the Curiosity rover found a feldspar-rich rock on Mars.
Source: Feldspar
