Offretite

tectosilicate

erionite

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
Environments:

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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