Anorthite
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Anorthite is one of the plagioclase feldspars, an important group of minerals abundant in the Earth's crust. Anorthite's chemical formula is CaAl2Si2O8.
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Mineralogy
Anorthite is the calcium-rich endmember of the plagioclase system, the other endmember being albite (sodium-rich). The range of intermediate compositions have varying ratios of calcium and sodium ions.
Occurrence
Anorthite occurs in igneous and metamorphic rocks. Its type localities are Monte Somma and Valle di Fassa, Italy,
Details
| Crystallographic system | Triclinic |
| Colours | Normally colourless or white, colourless in thin section. |
| Lustre | Vitreous |
| Hardness | 6-6.5 |
| Refractive index | 1.51 |
| Cleavage | 3: 1 perfect, 1 good, 1 poor |
| Twinning | Multiple lamellar and Carlsbad twins |
| Pleochroism | Yellow to dark green to transparent to opaque |
