Stilbite

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Stilbite is a tectosilicate mineral of the zeolite group consisting of hydrated calcium aluminium silicate, NaCa₂Al₅Si₁₃O₃₆·14H₂O.

Usually a small proportion of the calcium is replaced by sodium. Crystals are monoclinic, and are commonly twinned, giving rise to complex groups and characteristic sheaf-like aggregates. The color is usually white, sometimes red, and on the perfect cleavage (parallel to the plane of symmetry) the lustre is markedly pearly; hence the name stilbite, from Greek, "to shine". After the separation of heulandite from this species in 1818, the name desmine ("a bundle") was proposed, and this name is now employed in Germany. The hardness is 3.5 - 4 and the specific gravity 2.2. The sievelike crystal structure of the zeolite stilbite enables it to separate hydrocarbons in the process of petroleum refining.

Stilbite is a mineral of secondary origin, and occurs with other zeolites in the amygdaloidal cavities of basaltic volcanic rocks; it is sometimes found in granite and gneiss, and exceptionally in hydrothermal veins. It is abundant in the volcanic rocks of Iceland, Faroe Islands, Isle of Skye, Bay of Fundy, in Nova Scotia and elsewhere. Salmon-pink crystals occur with pale green apophyllite in the Deccan traps near Bombay and Poona, India; white sheaf-like groups encrust the calcite (Iceland-spar) of Berufjord near Djupivogr in Iceland; and crystals of a brick-red color are found at Old Kilpatrick, Scotland.

[edit] References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.