Orthoclase

Lore & Background

Orthoclase is a common constituent of most granites and other felsic igneous rocks, often forming huge crystals and masses in pegmatite. The pure potassium endmember forms a solid solution with albite; slow cooling causes exsolution of sodium-rich albite lamellae, enriching the remaining orthoclase with potassium, producing an intergrowth called perthite. Higher-temperature polymorphs include sanidine, common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, notably in trachytes of the Drachenfels, Germany. The lower-temperature polymorph is microcline.

Reader's Guide

Orthoclase is significant both as a rock-forming mineral and as an industrial and gemological material. Together with other potassium feldspars, it is a common raw material for manufacturing some glasses, ceramics such as porcelain, and scouring powder. In gemology, intergrowths of orthoclase and albite with an attractive pale luster are called moonstone, characterized by a creamy or silvery white adularescence; it is the state gem of Florida. The mineral is also one of the ten defining minerals on the Mohs scale of hardness, rated at 6. Notably, NASA's Curiosity rover discovered high levels of orthoclase in Martian sandstones, suggesting that some Martian rocks may have experienced complex geological processing such as repeated melting.

Did You Know?

• The name orthoclase comes from Ancient Greek for 'straight fracture' because its two cleavage planes are at right angles.
• Orthoclase is one of the ten defining minerals on the Mohs scale of mineral hardness, with a hardness of 6.
• NASA's Curiosity rover discovered high levels of orthoclase in Martian sandstones, suggesting complex geological processing.