Orthoclase is a member of the feldspar group (like plagioclase) and is a framework silicate. Orthoclase, also known as alkali feldspar or K-feldspar, is one end-member of a solid solution between orthoclase and albite.
Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks.
Chemical composition - KAlSi3O8
Hardness - 6
Specific gravity - 2.6
Transparency - Translucent to opaque (rarely transparent)
Colour - Pinkish white, off-white, yellow, or shades of red, orange to brown
Streak - White
Lustre - Vitreous
Cleavage/fracture - Perfect in two directions, seldom twinned / hackly, conchoidal
Crystal habit/mode of occurrence - Prismatic, tabular