Offretite

offretite

erionite

chabazite

levyne

Images

Formula: KCaMg(Si₁₃Al₅)O₃₆.15H₂O
Tectosilicate (framework silicate), zeolite group
Crystal System: Hexagonal
Specific gravity: 2.13 measured, 2.06 calculated
Hardness: 4
Streak: White
Colour: Colourless, white, yellow, golden
Common impurities: Mg
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. 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 erionite (Dana).

Localities

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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