Londonite
Londonite is a very rare gem that is closely related to and is the cesium (Cs) dominant analogue of Rhodizite. Rhodizite is potassium (K) dominant. The quantitative chemical analysis is required to differentiate between the two so they are often referred to as Rhodizite-Londonite when specimens have not been chemically tested. Londonite was discovered in the Sahatany Pegmatite Field near Mt. Bity in the Manandona Valley, Vakinankaratra Region, Antananarivo Province, Madagascar and is the only current source of facetable material. Londonite is strongly piezoelectric and pyroelectric and is barely radioactive due to its cesium (Cs) and rubidium (Rb) content. The color of greenish-yellow and yellow gems may fade to colorless with prolonged exposure to sunlight.
| Formula: | (Cs,K,Rb)Al4Be4(B,Be)12O28 |
| Crystallography: | Isometric – Hextetrahedral |
| Crystal Habit: | Typically as well-formed dodecahedra and tetrahedra, to 3.5 cm, modified by [001] and [111] |
| Twinning: | On [111], uncommon. |
| Cleavage: | [111] Indistinct, [111] Indistinct, [111] Indistinct |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Hardness (Mohs): | 8.0 |
| Density: | 3.34 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Barely Detectable, GRapi = 457.11 |
| Other: | Piezoelectric and Pyroelectric |
| Color: | Colorless, White, Sulfur Yellow, pale Yellow, pale Yellowish Green |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.6935 Isotropic |
| Birefringence: | 0.00 (Isotropic) |
| Dispersion: | Weak; 0.018 |
| Pleochroism: | None |
