Tectosilicate (framework silicate) zeolite group
Specific gravity: 2.25 to 2.44
Hardness: 5 to 5½
Colour: Colourless, white, pink, red, green, orange, yellow, blue
Thomsonite is found in cavities in low silica basalt, often associated with
chabazite and other zeolites.
Less commonly it
may be found in
contact metamorphic environments and pegmatites. It is not found in
altered volcanic ash deposits. It occurs as an authigenic (formed in place) cement in some
Crystals of gonnardite often have rims of thomsonite.
In the vicinity of Meshkinshahr, Ardabil Province, Iran, thomsonite occurs mainly in basalt (R&M 92.6.541-542).
At the Aranga quarry, Northland, New Zealand, distinctively coloured blue-green thomsonite-Ca, as well as the colourless variety, occur in cavities, encrusting thin joint planes and as thin veins in the host lava flows. The host rock is basalt and basaltic andesite. The thomsonite generally occurs on chabazite and sometimes on later calcite. The colour is likely caused by vanadium, along with gallium, substituting for aluminium. Another vanadium-bearing mineral, cavansite, is also present in the secondary mineral assemblage. The probable paragenesis is chabazite → thomsonite → calcite (AJM 19.2.7-13).
In Northern Ireland, UK, thomsonite is common in the zeolite localities, associated with chabazite, natrolite and analcime. It can be an alteration product of anorthite variety labradorite, or of nepheline, and also forms pseudomorphs after nepheline (DHZ 4 p376).
In the Isle of Mull, Scotland, UK, thomsonite is formed in basaltic cavities after albite, epidote and prehnite. On metamorphism by later intrusives recrystallisation occurs in the reverse order:
thomsonite → albite → prehnite → epidote (DHZ 4 p376).
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