Petalite
Petalites are beautiful colorless gems which are usually free of inclusions. The Chemical formula of Petalites is LiAlSi4O10 and its molecular weight is 305.15 gm. They are bright white and somewhat rare. Petalite gems could be used in jewellery, but with care since they are a bit brittle. Most gem quality Petalite is currently coming from Brazil.
The name Petalite is from the Greek pedalos meaning leaf in regard to its perfect basal cleavage. Petalite has a specific gravity of 2.412 – 2.422 (g/cm3) and refractive index 1.504 – 1.523 Biaxial ( + ). The name for the crystals that Petalite is cut from is Castorite, named after Castor. Castor and Pollux were heavenly twins and brothers of Helen of Troy from Greek mythology. Pollux is the basis for the naming of Pollucite.
The main sources of gem quality crystals are Paprok, Nuristan Province, Afghanistan; Bahia, Northeast Region, Brazil; Minas Gerais, Southeast Region, Brazil; and Mogok, Sagaing District, Mandalay Division, Burma (Myanmar).
| Category: | Tectosilicate |
| Chemical Formula: | LiAlSi4O10 |
| Molecular Weight: | 305.15 gm |
| Crystallography: | Monoclinic – Prismatic |
| Crystal Habit: | Crystals tabular on [010], or elongated, to 2 m; some faces may be striated. Commonly foliated cleavable, massive. |
| Twinning: | Common on [001], lamellar |
| Composition: | Lithium | 2.09 % | Li | 4.50 % | Li2O |
| Aluminum | 8.75 % | Al | 16.54 % | Al2O3 | |
| Silicon | 36.72 % | Si | 78.56 % | SiO2 | |
| Oxygen | 52.43 % | O | |||
| 100.00 % | 99.61 % | = TOTAL OXIDE |
| Cleavage: | [001] Perfect, [201] Imperfect |
| Fracture: | Imperfectly Conchoidal |
| Tenacity: | Brittle |
| Hardness (Mohs): | 6.0 – 6.5 |
| Density: | 2.412 – 2.422 (g/cm3) |
| Luminescence: | Thermoluminescent |
| Radioactivity: | Not Radioactive |
| Color: | Colorless, White, Gray, Yellow, Pink |
| Transparency: | Translucent to Transparent |
| Luster: | Vitreous, Pearly on cleavages |
| Refractive Index: | 1.504 – 1.523 Biaxial ( + ) |
| Birefringence: | 0.0120 – 0.0140 |
| Dispersion: | Weak; r > v |
| Pleochroism: | None |
