Gismondine

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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
Environments:

Pegmatites
Sedimentary environments (tuff)
Metamorphic environments
Basaltic cavities

Gismondine is one of the rarer zeolites occurring in silica-undersaturated environments, intimately associated with phillipsite; it is likely that the temperature of crystallizasion is between 25°C and about 90°C (IZA). 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).

Localities

At Brisbane, Queensland, Australia, gismondine occurs in miarolitic cavities in granite, associated with prehnite, laumontite, and sulphides (IZA).

At several localities in Austria gismondine occurs in cavities in nepheline basalt associated with gonnardite and phillipsite (IZA).

At Ontario, Canada, gismondine occurs in metapyroxenite with other zeolites (IZA).

At Mont Saint Hilaire, Quebec, Canada, gismondine occurs in breccia cavities in nepheline syenite, associated with analcime (IZA).

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

In Bohemia, Czech Republic, gismondine occurs in cavities in basalt and leucite tephrite (IZA).

In the Eifel District, Germany, gismondine occurs in vesicles in nepheline basalt and in xenoliths in the basalt. Associated minerals include phillipsite and gonnardite (IZA).

In eastern Iceland gismondine occurs in vesicles in olivine basalt (IZA).

At County Antrim, Northern Ireland, gismondine occurs in cavities in olivine basalt flows (IZA).

At the first recorded locality, Capo di Bova in the Rome area of Italy, gismondine occurs in cavities in leucitic basalt at (IZA).

At Round Top, Honolulu, Hawaii, USA, gismondine occurs in cavities in basalt and in melilite nepheline in the scoria cone (IZA).

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