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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, SodiumCalcium4(Silicon27Aluminium9)Oxygen72·28(Hydrogen2O). 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.
Epistilbite is a distinct zeolite species unrelated to stilbite.
Non-endmember forms of stilbite may be triclinic or even truly orthorhombic, indeed the framework can have symmetry ranging from orthorhombic to triclinic in a single crystal.
Stilbite is biaxial (-) with Refractive Index:
The unit cell can be considered as a monoclinic cell with β close to 130° and one formula unit per unit cell (Z = 1), or as a larger pseudo-orthorhombic cell with β close to 90° and Z = 2.
The framework of stilbite is pseudo-orthorhombic with the open channels typical of zeolites. It has 10-member rings and 8-member rings forming channels parallel to a and pseudo-orthorhombic c respectively.
Iceland is generally considered to be the type locality for stilbite-Ca. It is presumed to be the Helgusta Iceland Spar Mine, along Reydarfjordur. Excellent white bow ties of stilbite are found here on calcite and quartz, associated with heulandite and laumontite in cavities.
The type locality for stilbite-Na is Cape Pula, Pula, Cagliari Province, Sardinia, Italy. Small, lustrous, white or pink, pointed blades of stilbite-Na, and formless masses, up to 5 cm in diameter, have been found there, covering a thin crust of reddish heulandite in large fractures and cavities in the highly weathered volcanic andesite or rhyolite.
The Tertiary period Deccan basalts of western India are the most prolific sources of stilbite in the world. Stilbite is the most abundant zeolite in the tholeiitic basalt plateaux near Nasik and Pune and decreases in abundance toward the coast at Mumbai.
Habit
are typically thin tabular, flattened parallel to the dominant cleavage and elongated along the a axis. Aggregates may be sheaf-like or in bow-ties, also fibrous and globular. Crystal twinning, cruciform and penetration, is extremely common on {001}.
Physical and optical properties
The color is usually colorless or white, also yellow, brown, pink, salmon, orange, red, green, blue or black. The luster is generally vitreous, and on the perfect cleavage parallel to the plane of symmetry it is markedly pearly. The streak is white and crystals are transparent to translucent. The Mohs hardness is to 4 and the specific gravity 2.12 to 2.22. Cleavage is perfect on {010}, poor on {001}. The mineral is brittle, with a conchoidal or uneven fracture. It is not radioactive.
Unit cell and structure
Where sources give cell parameters for stilbite-Na, they are the same as those for stilbite-Ca.
Cell Parameters for the monoclinic cell:
Cell parameters for the pseudo-orthorhombic cell:
Uses
The open channels in the stilbite structure act like a molecular sieve, enabling it to separate hydrocarbons in the process of petroleum refining.
Environment
Stilbite is a low-temperature secondary hydrothermal mineral. It occurs in the amygdaloidal cavities of volcanic rocks, in , gneiss and hydrothermal veins. It also forms in Hot spring, and as a cementing agent in some sandstones and conglomerates. Stilbite has not been found in sedimentary tuff deposits or deep-sea deposits. Associated minerals are other zeolites, prehnite, calcite and quartz.
Localities
Stilbite is abundant in the volcanic rocks of Iceland, Faroe Islands, Isle of Skye, Bay of Fundy, Nova Scotia (where it is the provincial mineral), northern New Jersey and North Carolina. Salmon-pink crystals occur with pale green apophyllite in the Deccan Traps near Mumbai (Bombay) and Pune District, India; white sheaf-like groups encrust the calcite (calcite) of Berufjord near Djupivogr in Iceland; brown sheafs are found near Paterson, New Jersey in the United States; and crystals of a brick-red color are found at Old Kilpatrick, Scotland.
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