Stilbite
Stilbite is the name of a series of tectosilicate minerals of the zeolite group. Prior to 1997, stilbite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named:
- Stilbite-Ca
- Stilbite-Na
| Stilbite | |
|---|---|
| General | |
| Category | Tectosilicate, Zeolite |
| Formula (repeating unit) | Stilbite-Ca: NaCa4(Si27Al9)O72·28(H2O) Stilbite-Na: Na9(Si27Al9)O72·28(H2O) |
| IMA symbol | Stb |
| Strunz classification | 9.GE.10 (10 ed) 8/J.23-30 (8 ed) |
| Dana classification | 77.1.4.3 |
| Crystal system | Monoclinic, also triclinic and orthorhombic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/m (monoclinic) Amma (orthorhombic) |
| Identification | |
| Formula mass | Stilbite-Ca: 2,840 g/mol Stilbite-Na: 2,864 g/mol |
| Color | Usually colorless, white or pink |
| Crystal habit | Thin tabular, aggregates sheaf-like or in bow-ties, also fibrous and globular. |
| Twinning | Very common on {001} |
| Cleavage | Perfect on {010} |
| Fracture | Conchoidal or uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 3+1⁄2 to 4 |
| Luster | Vitreous, pearly on {010} |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.12 to 2.22 |
| Optical properties | Biaxial (−) |
| Refractive index | Nx = 1.479 to 1.492, Ny = 1.485 to 1.500, Nz = 1.489 to 1.505 Nx = 1.484 to 1.500, Ny = 1.492 to 1.507, Nz = 1.494 to 1.513 |
| Fusibility | Easily fused by blowpipe (~1050 °C) to produce a white "enamel" |
| Solubility | Decomposes in HCl |
| References | |
Stilbite-Ca, by far the more common of the two, is a hydrous calcium sodium and aluminium silicate, NaCa4(Si27Al9)O72·28(H2O). In the case of stilbite-Na, sodium dominates over calcium. The species are visually indistinguishable, and the series name stilbite is still used whenever testing has not been performed.
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