Tilleyite
Tilleyite is a rare calcium silicate carbonate mineral that is rarely (if ever) transparent but may occassionally be found as small white translucent crystals that can make attractive gems for collectors of the unusual. It is always white by itself but may appear pale bluish-white when associated with blue Calcite at the type locality of Crestmore, Riverside County, California. The best location for gem quality crystals is from Redcap Creek, Queensland, Australia.
Locations for Tilleyite include the type locality of Crestmore, Riverside County, California; North Peak, Iron Mountains, Sierra County, New Mexico; and near Helena, Lewis and Clark County, Montana, USA. In Ireland, at Barnavave, near Carlingford, County Louth. From near Kilchoan, Ardnamurchan, Argyllshire, and Camas Mòr, Isle of Muck, also on the Isles of Skye and Rhum, Scotland. From the Güneyce-Ikizdere area, Trabzon Province, Turkey. In Russia, from near Anakit Creek, at the mouth of the Lower Tunguska River, central Siberia, and several other less-well-defined localities. At Kushiro, Hiroshima Prefecture, and the Akagané mine, Iwate Prefecture, Japan; and from Redcap Creek, Queensland, Australia.
| Chemical Formula: | Ca5Si2O7(CO3)2 |
| Calcium Silicate Carbonate | |
| Molecular Weight: | 488.58 gm |
| Composition: | Calcium | 41.02 % | Ca | 57.39 % | CaO |
| Silicon | 11.50 % | Si | 24.60 % | SiO2 | |
| Carbon | 4.92 % | C | 18.02 % | CO2 | |
| Oxygen | 42.57 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Monoclinic – Prismatic |
| Crystal Habit: | As subhedral crystals, rounded grains, and irregular plates. |
| Twinning: | On {100}, simple; rarely, lamellar |
| Cleavage: | Perfect on {201}; poor on {100} and {010}; very poor on {001} |
| Fracture: | No data |
| Tenacity: | Brittle |
| Moh’s Hardness: | No data |
| Density: | 2.82 – 2.84 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | White, Colorless in thin section |
| Transparency: | Translucent to Transparent |
| Luster: | Vitreous, Dull |
| Refractive Index: | 1.612 – 1.654 Biaxial (+) |
| Birefringence: | 0.0370 – 0.0400 |
| Dispersion: | Perceptible; r < v |
| Pleochroism: | None |
