Thomsonite-Ca: NaCa2(Al5Si5)O20.6H2O
Thomsonite-Sr: NaSr2(Al5Si5)O20.6H2O
Tectosilicate (framework silicate) zeolite group
Specific gravity: 2.25 to 2.44
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, pink, red, green, orange, yellow, blue

Sedimentary environments
Metamorphic environments
Basaltic cavities

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 sandstone (Dana).
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 chabazitethomsonitecalcite (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:
thomsonitealbiteprehniteepidote (DHZ 4 p376).

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