Orthoclase
Mineral KAlSi3O8
Also called Potassium feldspar and K-feldspar
About it
Blocky prisms, often Carlsbad or Baveno twins, with two cleavages at right angles; a main mineral of granite. Clear yellow gem orthoclase comes from Madagascar, and adularia moonstone from Sri Lanka.
- Type
- Mineral
- Group
- Silicate (tectosilicate), feldspar group
- Formula
- KAlSi3O8
- Hardness (Mohs)
- 6
- Specific gravity
- 2.55–2.63
- Streak
- white
- Lustre
- vitreous (glassy), pearly
- Crystal system
- Monoclinic
- Colours
- colourless, white, pink, cream, yellow, grey
- Transparency
- transparent, translucent
- Cleavage and fracture
- perfect and good in two directions at 90°
Typical values: real specimens vary. For more, see mindat.org.