Feldspar (Orthoclase)

KAlSi3O8

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Orthoclase, a potassium feldspar with the formula KAlSi3O8, ranks among the most abundant minerals in the Earth's crust and is an essential component of granite and many other igneous rocks. It crystallizes in the monoclinic system as prismatic or tabular crystals, showing a vitreous luster, a white streak, and perfect {001} cleavage. Colors include white, pink, cream, and gray, with a Mohs hardness of 6 to 6.5 and a specific gravity between 2.55 and 2.63. Moonstone, valued for its floating adularescent sheen, is a gem variety of orthoclase. Its uneven to conchoidal fracture and two good cleavage directions are typical of the feldspar group.

Orthoclase is a potassium feldspar and one of the most common minerals in the Earth's crust. It is an essential component of granite and many other igneous rocks. Moonstone is a gem variety of orthoclase.

Crystal System Monoclinic
Mineral Class 09 Silicates
Mohs Hardness 6.0-6.5
Specific Gravity 2.55-2.63
Color White, pink, cream, gray
Luster Vitreous
Streak White
Cleavage Perfect {001}, good {010}
Fracture Uneven to conchoidal
Transparency Translucent
Crystal Habit Prismatic, tabular

Mohs Hardness

Mohs Hardness: 6.0 – 6.5
1 Soft10 Hard
Feldspar (Orthoclase) has a Mohs hardness of 6–6.5 on the 1–10 mineral hardness scale.
Mohs Hardness Reference Mineral
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond

Crystal System

Monoclinic
Feldspar (Orthoclase) crystallizes in the Monoclinic crystal system.
Property Value
Crystal System Monoclinic
Axial Relationships a ≠ b ≠ c, α = γ = 90°, β ≠ 90°

Frequently Asked Questions

What is the hardness of Feldspar (Orthoclase)?
Feldspar (Orthoclase) has a Mohs hardness of 6.0-6.5.
What crystal system does Feldspar (Orthoclase) belong to?
Feldspar (Orthoclase) crystallizes in the Monoclinic crystal system.
What color is Feldspar (Orthoclase)?
Feldspar (Orthoclase) is typically White, pink, cream, gray.
What type of luster does Feldspar (Orthoclase) have?
Feldspar (Orthoclase) exhibits a Vitreous luster.
What is the chemical formula of Feldspar (Orthoclase)?
The chemical formula of Feldspar (Orthoclase) is KAlSi3O8.