Feldspar
Silicate · (K,Na,Ca)(Al,Si)₄O₈ · also: Orthoclase, Plagioclase, Labradorite, Moonstone
Feldspar is a group of aluminosilicate minerals that make up the majority of the Earth’s crust, including orthoclase, labradorite and moonstone.
What is feldspar?
Feldspars are a family of aluminosilicate minerals and, taken together, the most abundant minerals in the Earth’s crust. They include potassium feldspar (orthoclase, often pink in granite) and the plagioclase series (white to grey), plus prized varieties like iridescent labradorite and shimmering moonstone. They are the reference mineral for hardness 6.
Properties
- Chemical formula
- (K,Na,Ca)(Al,Si)₄O₈
- Category
- Silicate
- Hardness (Mohs)
- 6
- Crystal system
- Monoclinic / Triclinic
- Lustre
- Vitreous to pearly
- Streak
- White
- Colour
- White, pink, grey, green
- Cleavage / fracture
- Two directions at about 90°
How to identify feldspar
- →Two cleavage directions meeting at about 90° (flat, step-like faces).
- →Hardness 6: scratches glass, not scratched by a knife.
- →Often blocky crystals in granite; pink, white or grey.
- →Labradorite flashes blue-green; moonstone shows a floating sheen.
Where feldspar is found
Feldspar is everywhere granite and related rocks occur. Labradorite comes from Canada, Finland and Madagascar; moonstone from Sri Lanka and India.
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