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Stilbite

Stilbite
Stilbite USA.jpg
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).


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