FELDSPAR is the name applied to a group of minerals that are the second
most common of all the earth’s minerals. All feldspars are composed of
aluminum, silicon, and oxygen, combined with varying amounts of one or
more metals, particularly potassium, sodium, calcium, and lithium.
The minerals are hard, have a smooth glassy or pearly luster, and cleave
along two planes nearly at right angles to each other. Feldspars are
fairly light weight. The streak is white, but the color of the mineral
is highly variable, although potassium and sodium-bearing feldspar are
commonly white or pink and most plagioclase feldspar is gray.
Feldspars are essential parts of the crystalline igneous rocks. Their
decomposition products 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 pebbles in glacial drift.