Orthoclase
Otero County
New Mexico
This crystal measures 4cm x 2cm
Orthoclase KAlSi3O8
Tectosilicate; potassium aluminum silicate, sometimes with considerable amounts of sodium
Color and Luster: White, pink, brown, gray, green, colorless, yellowish; vitreous, streak white
Hardness: 6-6.5
Cleavage: Good, two directions at 90°
Fracture: Uneven
Crystals: Monoclinic or triclinic; predominantly as single crystals of rectangular or square cross section and as tabular crystals; twin crystals very common; also cleavage masses, granular, disseminated grains
Note: Orthoclase, sanadine, adularia, and microcline are the potash feldspars
Andularia is often called moonstone, with a transparent or opalescent appearance
Sanadine is glassy
Microcline is white to pale yellow, rarely red, and when bright green it is called amazonite
Photo by Mary Sutherland
Descriptions taken from
The Audubon Society Field Guide to North American Rocks and Minerals