Orthoclase

Orthoclase properties at a glance

Geology & background

Orthoclase is a potassium feldspar that crystallizes from cooling magma in granites, syenites, and pegmatites, and also occurs in some metamorphic rocks formed at moderate temperatures.

Orthoclase has been used for centuries in ceramics, glass, and glaze production, and as a familiar rock-forming feldspar it became a reference point in mineral hardness and petrology.

How to identify Orthoclase

• Look for blocky cream, pink, or colorless crystals with two cleavage directions at right angles.
• Orthoclase has a hardness of 6 and a vitreous luster, often with a pearly sheen on cleavage faces.
• It is hard to separate from microcline by eye; twinning and optical tests confirm the species.

Where Orthoclase is found

Orthoclase occurs in granitic and metamorphic rocks worldwide, with notable specimen and gem localities in Madagascar, Germany, Italy, the United States, Sri Lanka, and Myanmar.

How much is Orthoclase worth?

Value depends on crystal form, color, transparency, twinning, locality, and aesthetic quality of the specimen.

Common uses

• Ceramics and porcelain
• Glass manufacturing
• Glazes and enamels
• Gem orthoclase
• Collector specimens

Interesting facts

• Orthoclase is the index mineral for hardness 6 on the Mohs scale.
• Its name means 'straight fracture,' referring to its two cleavage planes meeting at right angles.
• Transparent yellow orthoclase from Madagascar is faceted as a collector gemstone.
• The moonstone effect can appear in feldspar where orthoclase and albite are finely intergrown.

Care & safety

Orthoclase is fairly hard but brittle and can split cleanly along its perfect cleavage.

Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.

Related rocks & minerals

Quartz Quartz Family Albite Minerals Muscovite Minerals Biotite Minerals Hornblende Minerals Jadeite Silicate Minerals