The Mineral anorthoclase
Anorthoclase is the intermediary member of the Albite-Sanidine series. It is interesting to note that this mineral series contains two end members crystallizing in different crystal systems. Albite crystallizes in the triclinic system, whereas Sanidine crystallizes in the monoclinic system.
When Anorthoclase forms, its atoms form in a specific arrangement. This
arrangement of the atoms forms the crystal structure of the mineral.
Depending on the environment it forms in, it will either form a monoclinic or
triclinic crystal. If it forms a monoclinic crystal, it leans toward
Sanidine in regard to its crystal structure, and if it forms in the
triclinic system, it leans toward Albite. (For this reason, Anorthoclase
crystals can either be triclinic or monoclinic, although the tendency is usually toward triclinic.)
The scientific classification of Sanidine, Anorthoclase,
and Albite can only be determined with x-ray analysis. However, certain
localities produce known examples of these particular Feldspars, in which case the mineral type can be determined without
complex analysis. (Click here for more detailed information on the crystal structure of the Feldspars.)
Colorless, white, cream, yellow, pink, brown, reddish, greenish, gray, black
6 - 6.5
Transparent to opaque
2.5 - 2.6
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Conchoidal to uneven
In well formed crystals, usually prismatic. Crystals frequently form twins. Also as dense aggregates, grainy, and massive.
for more detailed information on the crystal structure of the Feldspars.
Anorthoclase localities are Mt Erebus, Ross Island, Antarctica; Mount Kilimanjaro, Tanzania; Pantelleria Island, Italy; Tjolling, Norway; and Kusukidani, Wakayama Prefecture, Japan. North American localities are Obsidian Cliff in Yellowstone National Park, Wyoming; Franklin, Sussex Co., New Jersey; and Mont Saint-Hilaire, Quebec, Canada.
Common Mineral Associations
Quartz, Muscovite, Biotite
Distingushing Similar Minerals
Albite, Sanidine, Orthoclase, and Microcline - Indistinguishable without x-ray analysis.