Formula: Ca2(Si4Al4)O16.8H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.12 to 2.28
Hardness: 4 to 5
Streak: White
Colour: Colourless, white, very pale pink

Sedimentary environments (tuff)
Metamorphic environments
Basaltic cavities

Gismondine is one of the rarer zeolites; it occurs most commonly in olivine basalt, less commonly in nepheline basalt, and tuff, schist or pegmatites.
In zeolite zones in basalt lavas it is associated with chabazite, thomsonite and phillipsite (DHZ 4 p407).

At Alexander dam, Hawaii, USA, gismondine has been reported in cavities associated with stilbite, allophane, olivine and augite (DHZ 4 p407).

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