Stilbite

tectosilicate

basaltic cavities

The stilbite minerals are tectosilicates (framework silicates) and members of the zeolite group, being:
Stilbite-Ca: NaCa4(Si27Al9)O72 .28H2O
Stilbite-Na: Na9(Si27Al9)O 72.28H2O

Specific gravity: (both) 2.1 to 2.2,
Hardness: (both) 3½ to 4
Streak: (both) White
Colour: (both) White, Red, Yellow, Brown, Cream
Solubility: Moderately soluble in hydrochloric acid
Environments:

Basaltic cavities

Stilbite is found in cavities in basalt and related rocks, associated with heulandite, chabazite and other zeolites and calcite.
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that stilbite crystallised at temperatures from 75oC to 200oC at depths between 45m and 1200m.
Stilbite is relatively frequent in crevices in metamorphic rocks, sometimes associated with quartz, epidote, prehnite and adularia (variety of orthoclase).

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