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Gemstones 250x250

The Mineral anorthoclase

Anorthoclase Crystals

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.)

Chemical Formula

(Na,K)AlSi3O8

Color

Colorless, white, cream, yellow, pink, brown, reddish, greenish, gray

Crystal System

Triclinic

Properties

Streak
White
Hardness
6 - 6.5
Transparency
Transparent to opaque
Specific Gravity
2.5 - 2.6
Luster
Vitreous
Cleavage
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Fracture
Conchoidal to uneven
Tenacity
Brittle

Crystal Habits

In well formed crystals, usually prismatic. Crystals frequently form twins. Also as dense aggregates, grainy, and massive.

Click here for more detailed information on the crystal structure of the Feldspars.

Additional Information

Composition
Sodium potassium silicate, with a varying ratio of sodium to potassium.
In Group
Silicates; Tectosilicates; Feldspar Group
Striking Features
Crystal habits, cleavage, and hardness
Environment
High-temperature, sodium-rich igneous environments.
Rock Type
Igneous

Other Names

Anorthose

Noteworthy Localities

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.

anorthoclase Photos



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