Anorthite (Feldspar)
Anorthite could be the member that is rarest of Plagioclase Feldspars for the Feldspar number of minerals that includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. The Plagioclase Feldspars form a string between Albite and Anorthite. Albite is the sodium-rich end member while Anorthite may be the calcium-rich end member and Oligoclase is the user that is intermediate. Faceted Anorthite gems are almost always little and although Anorthite is really a widely distributed mineral that is rock-forming treasure quality crystals are quite rare.
Anorthite had been named in 1823 by Gustav Rose through the Greek words αν and ορθός, meaning not angle that is right oblique, in allusion to the oblique triclinic form of the crystals.
Sources of gem quality Anorthite crystals are discovered at Ettringer Bellerberg Mt., Eifel Mts., Rhineland-Palatinate, Germany; the place that is classic of Somma, Naples Province, Campania, Italy; Miyake Island, Izu Archipelago, Tokyo Prefecture, Japan as well as in the US at Iron Hill number 2 District, Sierra County, New Mexico.
| Category: | Feldspar |
| Chemical Formula: | CaAl2SiO8 |
| Calcium Aluminum Silicate | |
| Molecular Weight: | 277.41 gm |
| Composition: | Sodium | 0.41 % | Na | 0.56 % | Na2O |
| Calcium | 13.72 % | Ca | 19.20 % | CaO | |
| Aluminum | 18.97 % | Al | 35.84 % | Al2O3 | |
| Silicon | 20.75 % | Si | 44.40 % | SiO2 | |
| Oxygen | 46.14 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Triclinic – Pinacoidal |
| Crystal Habit: | Crystals commonly short, prismatic, to 2 cm; lamellar, coarse granular, massive. |
| Twinning: | Commonly polysynthetic on the Albite law; also after the Pericline, Carlsbad, Manebach, and Baveno laws. |
| Cleavage: | Perfect on {001}, less so on {010}, Imperfect on {110} |
| Fracture: | Conchoidal to Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 6.0 – 6.5 |
| Density: | 2.74 – 2.76 (g/cm3) |
| Optical Properties: | Biaxial (-) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | White, Grayish, Reddish; Colorless in thin section |
| Transparency: | Translucent, Opaque; rarely Transparent |
| Luster: | Vitreous |
| Refractive Index: | 1.573 – 1.590 Biaxial ( – ) |
| Birefringence: | 0.0110 – 0.0120 |
| Dispersion: | Weak, r < v |
| Pleochroism: | X = colorless, Y = colorless, Z = colorless |
