Epistilbite

tectosilicate

basaltic cavities

Formula: Ca3[Si18Al6O48].16H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.22 to 2.28
Hardness: 4 to 4½
Streak: White
Colour: Colourless, white, orange, red

Environments:

Metamorphic environments
Basaltic cavities

Epistilbite occurs in cavities in silica-rich basalt and olivine basalt, associated with laumontite, heulandite and mordenite (DHZ 4 p385). It also occurs in gneiss. No diagenetic epistilbite has been reported.
In Iceland epistilbite occurs in geothermal wells in basalt at 80 to 160oC (Dana).

Common impurities: Fe,Mg,Na,K

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