Peridot (Olivine)
Peridot is the green variety of the Olivine Group of minerals. Olivine is not an individual mineral but a group of minerals that includes Fayalite, Forsterite, Liebenbergite, Tephroite and Laihunite. Fayalite (Mg2SiO4) is the iron rich member of the Fayalite – Forsterite mineral series, Forsterite (Fe2SiO4) is the magnesium rich member. Peridot (Mg2Fe2SiO4) is the green variety of Forsterite containing both Iron (Fe) and magnesium (Mg). Peridot has a specific gravity of 3.275 (g/cm3) and the refractive index of 1.635 – 1.772 Biaxial ( + ). Peridot’s color can vary from pale to rich green (for which Peridot is famous) to brown. The considerable variation in shades of green to brown is dependant on the locality of origin and amount of ferrous iron. The ideal green Peridot has an iron content of about 12-15%. More iron results in a muddy color. Very bright green Peridot may also contain traces of chromium.
Peridot is not a recognized mineral name. It’s hard to believe that Peridot is the equivalent of a trade name, not a recognized mineral name since it is one of the most popular and well-known gems in the world. Peridot has been a popular gem throughout history.
Peridot is available from many localities worldwide including these well known sources: Mogok, Sagaing District, Mandalay Division, Burma (Myanmar); St John’s Island (Zagbargad; Zebirget; Topazios), Red Sea, Egypt; Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan; and Peridot Mesa, San Carlos, San Carlos Indian Reservation, Gila Co., Arizona , USA.
| Category: | Category Silicate mineral variety |
| Chemical Formula: | (Mg, Fe)2SiO4 |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | In euhedral or subhedral crystals, typically thick, with striations parallel to elongation, with wedge-shaped terminations, to 17 cm. Commonly granular or compact massive. |
| Twinning: | On [100], [011], [012] |
| Cleavage: | [010] Perfect, [100] Imperfect |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Hardness (Mohs): | 7.0 |
| Density: | 3.275 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Yellowish Green, Olive Green, Greenish Black, Reddish Brown |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.635 – 1.772 Biaxial ( + ) |
| Birefringence: | 0.0330 – 0.0420 |
| Dispersion: | Relatively Weak; r > v or r < v |
| Pleochroism: | None |
