FeldsparFeldspar is the name applied to a group of minerals that is the second most common of all the minerals. All feldspars are composed of aluminum, silicon, and oxygen combined with varying amounts of one or more metals, particularly potassium, sodium, and calcium.

Feldspars have a hardness of 6, have a smooth, glassy or pearly luster, and show good cleavages along two planes at nearly right angles to each other. Specific gravity is about 2.6. The streak is white, but the color of the mineral is highly variable. Potassium feldspars (or K-feldspars) contain potassium, and color is commonly pink to reddish, but otherwise white, gray, yellowish, or pale green (amazonite variety).

Plagioclase feldspars contain varying amounts of calcium and/or sodium, and color is commonly white or gray. A diagnostic feature is fine lines or striations seen with a hand lens on cleavage faces.

Feldspar is used as flux for glass and ceramic manufacture, abrasives, and fillers in paint and plastics.

Feldspars are essential minerals in most crystalline igneous rocks. Their decomposition products are clay minerals that are present in most soils. In Illinois, relatively small feldspar crystals can be found associated with quartz and other minerals in granite and gneiss boulders, and larger crystals occur in some pebbles in glacial drift.