Potassium Feldspar

Description

Orthoclase is a common tectosilicate mineral, a type of potassium feldspar, with the chemical formula KAlSi3O8. It is one of the most abundant minerals in the Earth's crust, forming a significant component of many igneous, metamorphic, and sedimentary rocks. It is characterized by its monoclinic crystal system, two distinct cleavage planes intersecting at approximately 90 degrees, and a vitreous luster. Its color can vary widely, but it is often white, pink, or reddish. Orthoclase is an end-member of the feldspar group, specifically the alkali feldspar series, and is stable at higher temperatures compared to its polymorph, microcline.

How to Identify

Color

Typically white, pink, salmon-orange, or reddish; less commonly gray, yellow, or colorless. The color is often uniform.

Luster

Vitreous (glassy) to pearly on cleavage surfaces.

Texture

Typically massive in igneous and metamorphic rocks, but can form well-developed euhedral to subhedral crystals, especially in pegmatites.

Crystal Form

Monoclinic system, often forming prismatic or tabular crystals. Carlsbad twinning is common, appearing as a single crystal divided into two halves by a planar boundary.

Cleavage

Two distinct cleavage planes intersecting at approximately 90 degrees (specifically, 90 degrees and 30 minutes, or 90.5 degrees). One cleavage is perfect ({001}), and the other is good ({010}).

Geological Environment

Common in felsic igneous rocks (granite, syenite, rhyolite), high-grade metamorphic rocks (gneiss, schist), and pegmatites. Also found in some hydrothermal veins and as detrital grains in sedimentary rocks.

Key Facts

• Hardness: 6 to 6.5 on the Mohs scale
• Specific Gravity: 2.55 to 2.63 g/cm³
• Crystal System: Monoclinic
• Color: White, pink, salmon-orange, reddish, gray, yellow, colorless
• Luster: Vitreous to pearly
• Transparency: Transparent to translucent
• Fracture: Uneven to conchoidal
• Cleavage: Perfect on {001}, good on {010}, intersecting at 90.5 degrees
• Composition: Potassium aluminum silicate (KAlSi3O8)

Quick Check

• Color: White, pink, salmon-orange, reddish
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Prismatic, tabular, anhedral to euhedral grains, massive. Often twinned (Carlsbad, Baveno, Manebach).
• Cleavage Type: Two distinct cleavages, one perfect ({001}) and one good ({010}), intersecting at approximately 90 degrees (90°30').
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

Formation

Orthoclase forms during the slow cooling of felsic magmas (e.g., granite, granodiorite) and in high-grade metamorphic rocks (e.g., gneiss, schist). It can also form in pegmatites, where large crystals are common, and in some hydrothermal veins. Its formation is favored by relatively high temperatures and pressures.

Usage

Orthoclase is a primary component in the manufacture of ceramics (porcelain, tiles), glass, and glazes, providing alumina and alkalis. It is also used as a flux in metallurgy, as an abrasive, and occasionally as a gemstone (moonstone, amazonite, although amazonite is microcline, a polymorph). In geology, it is a key indicator mineral for rock classification and metamorphic grade.

Age Distribution

Found in rocks of all ages, from Precambrian to Cenozoic, wherever granitic or metamorphic conditions have occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Tectosilicate (Feldspar Group)

Group

Alkali Feldspar Series

Crystal System

Monoclinic

Chemical Formula

KAlSi3O8

Composition

Potassium aluminum silicate

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