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Potassium feldspar (K-feldspar) is a group of tectosilicate minerals that are end-members of the feldspar mineral group, characterized by the presence of potassium (K) as the dominant alkali metal. The most common polymorphs are orthoclase, microcline, and sanidine, which differ in their crystal structure and formation temperatures. K-feldspar is one of the most abundant minerals in the Earth's crust, forming a significant component of many igneous, metamorphic, and sedimentary rocks. It typically exhibits a vitreous luster, a hardness of 6 on the Mohs scale, and two distinct cleavage directions at or near 90 degrees. Its color can vary widely, including white, pink, salmon-orange, red, green (amazonite), and gray.
How to Identify
- Color
- Typically white, pink, salmon-orange, red, or gray. Green varieties (amazonite) also exist. Color is often a key indicator, especially the characteristic salmon-pink of orthoclase.
- Luster
- Vitreous (glassy) to pearly on cleavage surfaces.
- Texture
- Generally smooth on cleavage surfaces, rougher on fracture surfaces.
- Crystal Form
- Often forms euhedral to subhedral crystals, commonly prismatic or tabular. Twinning (Carlsbad, Baveno, Manebach) is common and can be diagnostic. Microcline often shows characteristic 'gridiron' twinning under a petrographic microscope.
- Cleavage
- Two distinct cleavage planes intersecting at or near 90 degrees (typically 90 degrees for orthoclase and sanidine, slightly less than 90 degrees for microcline). This is a critical diagnostic feature.
- Geological Environment
- Common in felsic igneous rocks (granite, granodiorite, syenite, rhyolite, trachyte), pegmatites, high-grade metamorphic rocks (gneiss, schist), and as a detrital component in arkosic sandstones.
Key Facts
- Hardness: 6-6.5 (Mohs scale)
- Specific Gravity: 2.55-2.63 g/cm³
- Crystal System: Monoclinic (Orthoclase, Sanidine), Triclinic (Microcline)
- Color: White, pink, salmon-orange, red, green (amazonite), gray
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Two distinct cleavages, one perfect {001} and one good {010}, intersecting at or near 90 degrees.
- Composition: KAlSi3O8 (Potassium Aluminum Silicate)
Quick Check
- Color: White, pink, salmon-orange, red, green, gray
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, tabular, anhedral masses; often twinned (Carlsbad, Baveno, Manebach). Microcline can show 'gridiron' twinning.
- Cleavage Type: Perfect on {001}, good on {010}, intersecting at 90° (orthoclase, sanidine) or slightly less than 90° (microcline).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Potassium feldspar forms primarily during the crystallization of felsic igneous rocks (e.g., granite, rhyolite) at various depths and cooling rates. It also forms during regional and contact metamorphism of argillaceous and siliceous rocks, and as a detrital mineral in sedimentary rocks. Hydrothermal alteration can also produce K-feldspar.
Usage
Potassium feldspar is a crucial industrial mineral. It is used extensively in ceramics (porcelain, tiles, sanitary ware) as a fluxing agent to lower melting temperatures and improve glaze quality. It is also used in glass manufacturing as a source of alumina and alkalis, in fillers and extenders for paints, plastics, and rubber, and as a mild abrasive. Gem varieties like amazonite are used in jewelry.
Age Distribution
Found in rocks of all geological ages, from Precambrian to Cenozoic.
Where to Find
Pikes Peak, Colorado, USA
Famous for large, well-formed pink orthoclase crystals, often associated with smoky quartz and amazonite.
Broken Hill, New South Wales, Australia
Known for significant deposits of microcline, particularly the green variety amazonite.
Madagascar
A major source of gem-quality amazonite and other K-feldspar varieties.
Norway
Numerous pegmatite localities yield large crystals of orthoclase and microcline.
Brazil
Various localities produce K-feldspar, including gem-quality material.
Finding Tips
Look for Igneous Rocks
Target granitic and rhyolitic outcrops, as K-feldspar is a primary constituent of these felsic igneous rocks.
Explore Pegmatites
Pegmatite dikes are excellent places to find large, well-formed K-feldspar crystals, often associated with quartz, mica, and other rare minerals.
Check Metamorphic Terrains
High-grade metamorphic rocks like gneisses and schists can contain significant amounts of K-feldspar.
Observe Cleavage
The two distinct cleavage planes at or near 90 degrees are a key identifier. Look for flat, reflective surfaces.
Note Color
The characteristic salmon-pink to reddish-brown color of many K-feldspars is often a good initial indicator.
Similar Rocks
Plagioclase Feldspar
Albite, Oligoclase, Andesine, Labradorite, Bytownite, Anorthite
Also known as: Na-Ca Feldspar
Quartz
SiO2
Also known as: Silica
Nepheline
(Na,K)AlSiO4
Also known as: Elaeolite
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Feldspar Group
- Crystal System
- Monoclinic (Orthoclase, Sanidine), Triclinic (Microcline)
- Chemical Formula
- KAlSi3O8
- Composition
- Potassium Aluminum Silicate
Explore Potassium Feldspar
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