Feldspar

Description

Feldspar is not a single mineral but a group of tectosilicate minerals that make up approximately 60% of the Earth's crust. They are characterized by a framework structure of interconnected silica and alumina tetrahedra. The group is broadly divided into two main series: the alkali feldspars (potassium feldspar and sodium feldspar) and the plagioclase feldspars (sodium-calcium feldspar solid solution series). Their widespread occurrence and diverse compositions make them fundamental to understanding igneous, metamorphic, and sedimentary petrology.

How to Identify

Color

Highly variable, ranging from white, gray, pink, red, green, to brown. Alkali feldspars often exhibit pink to reddish hues (orthoclase, microcline), while plagioclase can be white, gray, or bluish-gray (labradorite).

Luster

Vitreous to pearly on cleavage surfaces.

Texture

Typically blocky or tabular crystals, often with striations on plagioclase cleavage faces.

Crystal Form

Monoclinic (orthoclase, sanidine) or triclinic (microcline, plagioclase series). Crystals are often prismatic, tabular, or anhedral masses.

Cleavage

Two distinct cleavage planes intersecting at or near 90 degrees (orthoclase, microcline) or slightly less than 90 degrees (plagioclase).

Geological Environment

Found in virtually all types of igneous rocks (granites, basalts, syenites, gabbros), metamorphic rocks (gneisses, schists), and some sedimentary rocks (arkoses, greywackes).

Key Facts

• Hardness: 6 - 6.5 (Mohs scale)
• Specific Gravity: 2.55 - 2.76
• Crystal System: Monoclinic (orthoclase, sanidine) or Triclinic (microcline, plagioclase series)
• Color: Highly variable: white, gray, pink, red, green, brown
• Luster: Vitreous to pearly
• Transparency: Transparent to translucent
• Fracture: Uneven to conchoidal
• Cleavage: Two distinct cleavages, typically at or near 90 degrees
• Composition: Aluminosilicates of potassium, sodium, and calcium (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8)

Quick Check

• Color: White, gray, pink, red, green, brown (variable)
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Prismatic, tabular, blocky, anhedral masses; often twinned (e.g., Carlsbad, Albite, Pericline twins)
• Cleavage Type: Perfect in two directions, typically at 90 degrees (orthoclase) or slightly less (plagioclase)
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to pearly on cleavage surfaces

Formation

Feldspars form under a wide range of geological conditions. They are primary constituents of igneous rocks, crystallizing from magma and lava. In metamorphic rocks, they form through recrystallization under elevated temperature and pressure. They are also significant components of many sedimentary rocks, particularly arkoses, where they are derived from the weathering of feldspar-rich source rocks.

Usage

Feldspars are economically important. They are used extensively in the ceramics industry (e.g., pottery, porcelain, tiles) as a fluxing agent to lower melting temperatures and improve strength. In the glass industry, they provide alumina, which enhances durability and reduces devitrification. They are also used as fillers and extenders in paints, plastics, and rubber, and as mild abrasives. Certain varieties, like labradorite and amazonite, are used as gemstones and ornamental stones.

Age Distribution

Ubiquitous throughout Earth's geological history, found in rocks of all ages from Precambrian to Cenozoic.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Tectosilicates

Group

Feldspar Group

Crystal System

Monoclinic or Triclinic

Chemical Formula

KAlSi3O8 (Orthoclase/Microcline) - NaAlSi3O8 (Albite) - CaAl2Si2O8 (Anorthite)

Composition

Solid solution series between K-feldspar (orthoclase, microcline), Na-feldspar (albite), and Ca-feldspar (anorthite). The alkali feldspars are KAlSi3O8 and NaAlSi3O8. The plagioclase feldspars are a continuous solid solution between NaAlSi3O8 (albite) and CaAl2Si2O8 (anorthite).

Explore Feldspar