Imperial College Rock Library


Silicate, Tektosilicate

How to use crystal diagram
Feldspars are alkali and calcic alumino silicates with framework structures. They are the second most common minerals in the crust after quartz. There are two chemical groups of feldspar: (1) alkali feldspars, which form a solid solution series between Na and K-rich endmembers and (2) plagioclase feldspars, which form a solid solution series between Ca and Na-rich endmembers. There are no K and Ca-rich feldspars due to a two phase field.

Physical Properties

Hand Specimen Colour: white, grey, pale yellow, pink, pale green
Transparency: translucent, transparent
Lustre: pearly, vitreous
Streak: white
Hardness: 0
Density: 0
Habit: Euhedral grains often present as tabular laths
Cleavage: Two:

Optical Properties

PPL Colour: colourless
Pleochroism: None
Relief: Low
Birefringence: 0
2V: 0
Optic Type: Biaxial (+) and (-)


Feldspars are common in most igneous rocks except ultrabasic and the most alkaline. They are present in the majority of metamorphic rocks and are common as detrital grains in siliciclastic sediments.

Identification Tips

Feldspars can be distinguished from quartz by the presence of two cleavages. They also are less vitreous than quartz. In thin-section feldspars often exhibit characteristic twinning.

Related Minerals and Mineral Groups:

plagioclase, alkali feldspar

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