Formula: KCaMg(Si13Al5)O36.15H2O tectosilicate (framework silicate) zeolite group
Specific gravity: 2.10 to 2.13
Hardness: 4
Streak: White
Colour: Colourless, white, yellow, golden

Volcanic igneous environments
Hydrothermal environments

Offretite occurs in veins and fractures in basaltic rocks, usually intergrown with erionite or as epitaxial overgrowths on it. Offretite is not found in Si-rich volcanics, pegmatites or altered volcanic ash tuff (where erionite is abundant). Offretite is of hydrothermal origin. At Passo Forcal Rosso, Italy, chabazite-offretite epitaxial overgrowths occur. In Hunter Valley, NSW, Australia offretite occurs on plates of lévyne.
Common impurities: Mg

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