Stilbite | |
---|---|
Stilbite from Kiui Island, Alaska, USA
|
|
General | |
Category | Zeolite |
Formula (repeating unit) |
Stilbite-Ca: NaCa4(Si27Al9)O72·28(H2O) Stilbite-Na: Na9(Si27Al9)O72·28(H2O) |
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) |
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 | 3½ 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 |
Nx = 1.479 to 1.492, Ny = 1.485 to 1.500, Nz = 1.489 to 1.505
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, 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.
At one time heulandite and stilbite were considered to be identical minerals. After they were found to be two separate species, in 1818, the name desmine ("a bundle") was proposed for stilbite, and this name is still employed in Germany. The English name "stilbite" is from the Greek stilbein = to shine, because of the pearly luster of the {010} faces.
Stilbite shows a wide variation in exchangeable cations: silicon and aluminium ions occupy equivalent sites and can substitute for each other. Since silicon and aluminium have a different charge (Si4+ and Al3+) the ions occupying the sodium/calcium site have to adjust to maintain charge balance. There is a continuous series between stellerite, whose formula can be written as Ca4(Si28Al8)O72·28(H2O), and stilbite, and another continuous series between stilbite and barrerite, Na8(Si28Al8)O72·26(H2O).