Formula: KCaMg(Si13Al5)O36.15H2O Tectosilicate (framework silicate), zeolite group
Specific gravity: 2.10 to 2.13
Colour: Colourless, white, yellow, golden
Common impurities: Mg
Volcanic igneous environments
Offretite occurs in veins and fractures in basaltic rocks, usually intergrown with
erionite or as epitaxial overgrowths on it. It is not found in Si-rich volcanics,
pegmatites or altered volcanic ash
tuff (where erionite is abundant); it may be
intergrown with other zeolites. Offretite is of
hydrothermal origin. It is commonly epitaxial on lévyne or on
At Hunter Valley, Durham county, New South Wales, Australia, offretite occurs on plates of lévyne (Dana).
At Douglas Lake Road, Westwold, Kamloops Mining Division, British Columbia, Canada, offretite occurs as 3 mm long needles on lévyne (Dana).
At the Forcel Rosso Gully, Mt. Foppa, Upper Val Saviore, Brescia Province, Lombardy, Italy, chabazite-offretite epitaxial overgrowths occur (Dana).
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