Feldspars are a type of rock-forming minerals that contain silica and alumina. The group of minerals includes aluminum silicate of potassium, soda, or lime, and makes up approximately 41% of the Earth's continental crust by weight. The minerals are found in abundance in metamorphic and igneous rock, as well as some forms of sedimentary rock. Feldspars can be divided into three composition types: plagioclase feldspars, alkali feldspars, and barium feldspars.
All feldspars have a hardness of about six on the Mohs scale, which falls between a steel knife (5.5) and quartz (7). Most feldspar minerals have a whitish appearance with a glassy luster, although a few contain shades of orange. Feldspar minerals account for the largest percentage of minerals in rocks, and a glassy mineral in a rock slightly softer than quartz is likely to be part of the feldspar family.
Types of Feldspar
There are three types of feldspars: plagioclase, alkaki, and barium. Plagioclase feldspars break along fine parallel lines and are often white, grey or aluminum in color. The minerals include albite, oligoclase, andesine, labradorite, anorthite, and bytownite. Alkali feldspars are divided into two groups: the first group is composed of minerals with potassium combined with silicon, sodium, and aluminum, such as orthoclase, sanidine, monoclinic, microcline, and anorthoclase, while the second group consists of minerals in which barium replaces potassium. These include celsian and hyalophane.
Uses of Feldspar
Feldspar is extracted from rocks and crushed into fine powder or granules that are used in the manufacturing of ceramic tile, plate glass, pottery, paint, plastics, and fiberglass insulation. Each year about 600,000 tons of feldspar is mined in the United States from quarries in South Dakota, Oklahoma, Virginia, California, North Carolina, and Idaho. Vast deposits of felspar exist throughout the world, but the greatest challenge remains extracting and transporting the minerals to the point of consumption. Furthermore, the global demand for the minerals is low.
Some varieties of feldspar have unique characteristics that make them attractive. Sunstone, labradorite, and moonstone are sought for their optical properties. Moonstone is made up of several alternating layers of feldspar that scatter light in different directions to produce a glowing adularescence. Sunstone is made up of microscopic plate-shaped and reflective particles that produce a glittery aventurescence when struck by light. Labradorite is intergrown with microscopic layers of plagioclase minerals such as albite to produce a polished gem that scatters light in different wavelengths, resulting in iridescent reflection. Feldspars with these characteristics are used in the manufacturing of ornaments.