Orthoclase Mineral Specimen
HISTORY, NAME, LOCALITIES: Orthoclase has been mined since Roman times for use in glassmaking. Mineralogists identified orthoclase as a species in 1823, and in 1849 assigned its name, which stems from the Greek words orthos, meaning “upright,” and klasis, or “breaking,” in reference to its prominent, right-angle cleavage. Orthoclase is collected in Brazil, Mexico, Russia, England, and the United States (California, Colorado, Maine, New Mexico).
MINERALOGY, PROPERTIES, OCCURRENCE: Orthoclase [potassium aluminum silicate, KAlSi3O8], pronounced ORE-tho-claze, is an abundant feldspar mineral that crystallizes in the monoclinic system as short-to-long prisms with square or rectangular cross sections. Crystals are usually off-white or pale hues of brown, gray, or yellow, transparent to opaque, and frequently twinned. Orthoclase has a Mohs hardness of 6.0, good right-angle cleavage in two directions, and a specific gravity of 2.5-2.6. As an alkali feldspar and common rock-forming mineral, orthoclase is an essential component of all granitic rocks and many volcanic and metamorphic rocks.
METAPHYSICAL PROPERTIES, LORE, USES: According to modern metaphysical belief, the moonstone variety of orthoclase brings good fortune while enhancing passion, sensitivity, intuition, and general physical abilities. Transparent, yellowish orthoclase crystals are faceted into gems for both collector and jewelry purposes. Moonstone is a milky, adularescent gem variety of orthoclase. Orthoclase is mined extensively and used in the manufacture glass, ceramics, and glazes and as a filler in paints, paper, and plastics.
COLLECTORS’ INFORMATION: Orthoclase is collected primarily for its gemstone varieties and its excellent examples of crystallographic twinning.