Orthoclase

Orthoclase is a primary rock-forming mineral that crystallizes from potassium-rich silicate melts during the cooling of granitic and syenitic magmas. In a typical granite, orthoclase begins crystallizing at temperatures around 700 to 900 degrees Celsius as potassium, aluminum, silicon, and oxygen ions organize into the monoclinic feldspar framework structure. The mineral often forms large phenocrysts in porphyritic granites, sometimes reaching 10 centimeters or more, when slow cooling in plutonic environments allows extended crystal growth. Orthoclase typically makes up 40 to 60 percent of granite by volume, making it one of Earth's most abundant minerals.

In pegmatites, orthoclase can form enormous crystals due to the volatile-rich, slowly cooling conditions. The mineral also occurs in high-temperature metamorphic rocks (granulite facies) and in some volcanic rocks where rapid cooling preserves the monoclinic structure. At lower temperatures, potassium feldspar tends to adopt the triclinic structure of microcline instead. The boundary between orthoclase (monoclinic) and microcline (triclinic) stability lies at approximately 500 degrees Celsius, though the transition is kinetically slow and many orthoclase specimens persist metastably at surface conditions.

The moonstone variety of orthoclase forms when thin alternating layers of orthoclase and albite (sodium feldspar) develop through a process called exsolution. As the feldspar cools below approximately 600 degrees Celsius, the initially homogeneous potassium-sodium feldspar becomes unstable and separates into potassium-rich and sodium-rich layers. When these exsolution lamellae are thin enough (on the order of visible light wavelengths), they produce the billowy blue-white sheen called adularescence that makes moonstone so distinctive.

Orthoclase is identified by its two good cleavage directions meeting at approximately 90 degrees (the name means straight fracture in Greek), vitreous to pearly luster, and typical cream, pink, or white color. Hardness is exactly 6 on the Mohs scale, as orthoclase is the defining mineral for that hardness value. Carlsbad and Baveno twins are common and diagnostic. Distinguish from plagioclase feldspar by the absence of fine parallel twinning striations on cleavage surfaces (plagioclase shows these, orthoclase does not). Distinguish from quartz by orthoclase's two prominent cleavage planes (quartz has no cleavage and shows conchoidal fracture). In granite specimens, orthoclase is typically the pink or cream-colored mineral alongside grey quartz and dark biotite mica. The moonstone variety shows a distinctive blue-white adularescent sheen when moved under light. Distinguish from microcline by crystal system (orthoclase is monoclinic, microcline is triclinic), though this often requires X-ray diffraction for confirmation.