Orthoclase
| Colour | White, Yellow, Clear |
|---|---|
| Mohs hardness | 6 (defining mineral) |
| Lustre | Vitreous, pearly on cleavage surfaces |
| Streak | White |
| Crystal system | Monoclinic |
| Transparency | Transparent to translucent |
| Cleavage | Has perfect cleavage on {001} and good cleavage on {010}. Cleavages intersect at 90°. It can be difficult to see cleavage in thin section due to orthoclase's low relief |
| Fracture | Uneven |
| Chemical formula | KAlSi 3 O 8 |
| Specific gravity | 2.55–2.63 |
What is Orthoclase?
Orthoclase, or orthoclase feldspar (endmember formula KAlSi3O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles to each other. It is a type of alkali feldspar, also known as potassium feldspar or K-spar. The gem known as moonstone (see below) is largely composed of orthoclase.
How to identify Orthoclase
- Lustre: Vitreous, pearly on cleavage surfaces.
- Hardness: Mohs 6 (defining mineral) — about as hard as a steel knife.
- Streak: White.
- Habit: Monoclinic crystal system.
Orthoclase in different forms
Stones often confused with Orthoclase
Frequently asked questions
How hard is Orthoclase?
Orthoclase is Mohs 6 (defining mineral) on the hardness scale.
What colour is Orthoclase?
Orthoclase is typically white, yellow, clear (Colorless, greenish, greyish yellow, white, pink).