Tilleyite
Tilleyite is a rarely occurring calcium sorosilicate mineral with formula Ca3[Si2O7]·2CaCO3 (sometimes represented as Ca5(CaCO3)2[Si2O7]). It is chemically a calcium silicate with additional carbonate ions. Tilleyite crystallizes in the monoclinic crystal system and forms only poorly developed, irregularly defined, tabular crystals and spherical grains. In its pure form it is colorless and transparent, however due to multiple refractions of light from lattice defects or polycrystalline formation, it can also appear white, with the transparency decreasing accordingly.
| Tilleyite | |
|---|---|
Tilleyite collected from Crestmore quarries, Riverside County, California | |
| General | |
| Category | Sorosilicate |
| Formula (repeating unit) | Ca3[Si2O7]·2CaCO3 |
| IMA symbol | Tly |
| Strunz classification | 9.BE.82 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P21/b |
| Unit cell | a = 15.108(3) Å b = 10.241(1) Å c = 7.579(1) Å β = 105.17(1)° Z = 4 |
| Identification | |
| Colour | Colourless, white |
| Twinning | Simple twinning {100}, often lamellar; α:twin plane ~ 24° |
| Cleavage | {201} perfect; {100} and {010} poor |
| Luster | Vitreous, Dull |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.838 - 2.88 |
| Optical properties | Monoclinic (+) |
| Refractive index | nα = 1.605 - 1.617 nβ = 1.626 - 1.635 nγ = 1.651 - 1.654 |
| Birefringence | δ = 0.035 - 0.046 |
| 2V angle | 85-89° |
| Dispersion | r < v |
| References | |
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