Orthoclase

Orthoclase is one of the most abundant minerals in Earth's crust and a principal member of the potassium feldspar (K-feldspar) group. Its name, meaning "straight fracture," refers to its two cleavage planes that meet at right angles.

It is the standard mineral for hardness 6 on the Mohs scale. Orthoclase typically appears white, cream, pink, or pale yellow, and it is a defining constituent of granite and many other felsic rocks. Transparent yellow gem-quality crystals from Madagascar are faceted as collector gemstones, and the moonstone gem variety is associated with orthoclase and its intergrowths.

Chemically it is potassium aluminosilicate, forming a series with albite (sodium feldspar) at high temperatures.

Orthoclase crystallizes from silica- and potassium-rich magmas and is a primary component of granite, syenite, and granodiorite, as well as their volcanic equivalents. Large, well-formed crystals occur in pegmatites, where slow growth from late-stage fluids allows them to reach great size.

It also forms during metamorphism of suitable rocks and persists in sediments such as arkose because of its relative resistance to weathering. Notable gem and specimen localities include Madagascar (yellow gem crystals), the Itrongay deposit, plus occurrences across Europe, the United States, and Brazil. Over geologic time orthoclase can alter to clay minerals like kaolinite through chemical weathering.

Look for a hard (Mohs 6-6.5) silicate with two cleavage directions meeting at about 90 degrees, blocky crystals, and a vitreous to slightly pearly luster. Color is usually white, cream, pink, or pale yellow; streak is white.

Orthoclase is hard enough to scratch glass and cannot be scratched by a steel knife. It is distinguished from quartz by its cleavage (quartz has none and fractures conchoidally) and from plagioclase feldspars, which often show fine parallel striations (twinning lines) on cleavage faces that orthoclase lacks.

Microcline, a chemically identical K-feldspar, can look identical and may require optical or microscopic methods to separate, though green amazonite color suggests microcline.

Orthoclase is industrially important as a feldspar raw material. It is used in the manufacture of ceramics and porcelain, where it acts as a flux to lower firing temperatures, and in glassmaking, glazes, enamels, and as a mild abrasive in scouring powders.

Gem-quality transparent yellow orthoclase, especially from Madagascar, is faceted for collectors, and orthoclase-related moonstone is a popular jewelry gem prized for its adularescence.

As a rock-forming mineral it is geologically significant for dating and for understanding igneous and metamorphic processes. Metaphysically, orthoclase and moonstone are linked in folklore to intuition and emotional balance, though these are traditional beliefs.