Anorthoclase minerals is member of the sodium- and potassium-rich feldspar group takes its name from the Greek word anorthos, which means “not straight”—a reference to its oblique cleavage. Anorthoclase is colorless, white, cream, pink, pale yellow, gray, or green. Its crystals are prismatic or tabular and are often multiply twinned. Anorthoclase crystals can show two sets of fine lines at right angles to each other like microcline, but the lines are much finer. Specimens can also be massive or granular. Anorthoclase forms in sodium-rich igneous zones. It commonly occurs with ilmenite, apatite, and augite. Much anorthoclase exhibits a gold, bluish, or greenish schiller effect, making it one of several feldspars known as moonstone when cut en cabochon. A type of the igneous rock syenite called larvikite has large schillerized crystals of anorthoclase and is highly prized as an ornamental stone. Anorthoclase is widespread, but fine examples come from Cripple Creek, Colorado, USA; Larvik, Norway; and Fife, Scotland.
Name: From the Greek for oblique and fracture, descriptive of the cleavage.
Mineral Group: Feldspar (alkali) group; intermediate between low sanidine and high albite.
Contents
Chemical Properties of Anorthoclase
| Chemical Classification | Silicates Minerals |
| Chemical Composition | (Na,K)AlSi3O8 |
Physical Properties of Anorthoclase
| Color | White, colourless, greyish pink |
| Streak | White |
| Luster | Vitreous to pearly on cleavage planes |
| Cleavage | Perfect |
| Diaphaneity | Transparent |
| Mohs Hardness | 6 – 6½ on Mohs scale |
| Specific Gravity | 2.57 – 2.60 |
| Crystal System | Triclinic |
| Tenacity | Brittle |
| Fracture | Uneven |
Optical Properties of Anorthoclase
| Type | Anistropic |
| Color / Pleochroism | Colorless |
| Twinning | Polysynthetic twinning produces a grid pattern on [100] |
| Optic Sign | Biaxial (-) |
| Birefringence | δ = 0.008 |
| Relief | Low |
Occurrence
In high-temperature sodic volcanic and hypabyssal rocks.
Association
Typically in a fine-grained groundmass or weathered out as loose crystals
Distribution
Rather abundant worldwide. Some localities for well-characterized material include:
- on Pantelleria and Ustica Islands, Italy.
- At Larvik, Norway.
- From Berkum, North Rhine-Westphalia, Germany.
- On Grande Caldeira Island, Azores.
- At Ropp, Nigeria.
- On Mt. Kenya, Kenya.
- From Kilimanjaro, Tanzania. At Chilposan, near Minchon, North Korea.
- From Ogaya, Toyama Prefecture, and Madarajima, Saga Prefecture, Japan.
- At Kakanui, New Zealand.
- From Mt. Anakie and Mt. Franklin,
- Daylesford, Victoria, Australia.
- Large crystals from Mt. Erebus, Ross Island, Antarctica.
- At Boron, Kern Co., California, USA.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019). Anorthoclase: Mineral information, data and localities.. [online] Available at: https://www.mindat.org
