Orthoclase Feldspar (or K-Feldspar)

Hardness: 6-6.5 on Mohs scale. Color: Typically pink, salmon-orange, white, or gray. Luster: Vitreous to pearly. Crystal Structure: Monoclinic; commonly forms stout prismatic or tabular crystals, often twinned (Carlsbad twinning is common). Cleavage: Two distinct cleavages at nearly right angles (90 degrees). Specific Gravity: 2.56-2.58.

Orthoclase is a common tectosilicate mineral that forms in various igneous rocks (such as granite, granodiorite, syenite) as they cool and crystallize from magma. It can also form during regional metamorphism in certain metamorphic rocks like gneisses, and in some sedimentary rocks as detrital grains. Its formation temperature is typically high, and it's a primary constituent of Earth's continental crust. Geological age: Present throughout Earth's geological history, as it's a fundamental rock-forming mineral.

Industry: Used in the manufacture of glass and ceramics, as a flux in glazes, and in some abrasives. Construction: A major component of many building stones like granite. Jewelry: Transparent varieties or those with optical effects (like moonstone, which is a variety of orthoclase or an intergrowth of orthoclase and albite) are used as gemstones. Collecting: Well-formed crystals are sought after by mineral collectors.

Orthoclase is one of the most abundant minerals in the Earth's crust, forming one of the three end-members of the feldspar group (along with albite and anorthite). The name 'orthoclase' comes from the Greek words 'orthos' (straight) and 'klasis' (fracture), referring to its two cleavage planes being at right angles. It plays a crucial role in forming the bulk of many intrusive and volcanic igneous rocks.

In the field, orthoclase can be identified by its distinctive pink to salmon-orange color, its good cleavage at 90 degrees, and its hardness (it will scratch glass but can be scratched by quartz). It's commonly found in granites, pegmatites, and syenites, often appearing as large, blocky crystals. Locations: Found worldwide in various igneous and metamorphic terrains, with notable occurrences in places like the Appalachian Mountains (USA), various European countries (e.g., Norway, Germany), and Madagascar. Tips for collectors: Look for samples exhibiting good crystal habit or prominent cleavage planes. Its relatively high hardness makes it quite durable.