Bertrandite
Bertrandite is a fairly common mineral but rarely faceted since crystals are rarely large enough or clean enough for faceting. It is an important ore of beryllium second only to Beryl. Bertrandite and Beryl are often found associated together. Bertrandite is an alteration product of Beryl and it is not unusual to find occurrences of Bertrandite completely replacing Beryl crystals forming a pseudomorph. A pseudomorph is when one mineral replaces another on an atom by atom basis, replacing the chemistry and structure with the new mineral but preserving the external shape of the original crystal. Bertrandite gems are usually very small, colorless and included.
Bertrandite is found in small amounts at numerous localities but gem quality crystals are rare. A few locations include Golconda mine, near Governador Valadares, Minas Gerais, Brazil; Barbin quarries at Petit-Port, Loire-Atlantique, France; Akchatau, Kara-Oba, and Konrad, Kazakhstan; and in the USA on Mt. Antero, Chaffee County, Colorado; at the Strickland quarry, Portland, Middlesex County, Connecticut.
|
Category: |
Sorosilicate |
|
Chemical Formula: |
Be4(Si2O7)(OH)2 |
| Beryllium Silicate Hydroxide | |
| Molecular Weight: | 238.23 gm |
| Composition: | Beryllium | 15.13 % | Be | 42.00 % | BeO |
| Silicon | 23.58 % | Si | 50.44 % | SiO2 | |
| Hydrogen | 0.85 % | H | 7.56 % | H2O | |
| Oxygen | 60.44 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Orthorhombic – Pyramidal |
| Crystal Habit: | Crystals thin tabular, commonly prismatic to needle-like, to 5 cm; in radial aggregates. |
| Twinning: | Common on [011] or [021]; twins heart-shaped or V-shaped with axes crossing at angles of about 60° and 120°. |
| Cleavage: | Perfect on {001}; Distinct on {100}, {010}, and {110} |
| Fracture: | n/a |
| Tenacity: | Brittle |
| Moh’s Hardness: | 6.0 – 7.0 |
| Density: | 2.59 – 2.60 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Other: | Pyroelectric |
| Color: | Colorless, pale Yellow |
| Transparency: | Translucent to Transparent |
| Luster: | Vitreous, Pearly on cleavage {001} |
| Refractive Index: | 1.583 – 1.614 Biaxial ( – ) |
| Birefringence: | 0.0230 |
| Dispersion: | Weak; r < v |
| Pleochroism: |
None |
