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Formulae:
Thomsonite-Ca: NaCa2(Al5Si5)O20.6H2O
Thomsonite-Sr: NaSr2(Al5Si5)O20.6H2O
Tectosilicate (framework silicate) zeolite group
Crystal System: Orthorhombic
Specific gravity: Thomsonite-(Ca) 2.23 to 2.29 measured, 2.366 calculated
Thomsonite-(Sr) 2.47 measured, 2.62 calculated
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, pink, red, green, orange, yellow, blue
Environments:
Pegmatites
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.
Localities
At the Marron Volcanics of the Olalla Area South-Central British Columbia, Canada, the variety of habits and forms observed
in thomsonite includes blocky, equant crystals, complex prisms, simple to chisel-like blades and even acicular crystals.
The colour is pinkish due to included hematite, to white, to clear and colourless.
Crystals range in size from 0.5 to 8 mm. The more common, bladed habit is found in the Upper Yellow Lake outcrops. At the
White Lake Road site, thomsonite is present as linings in
calcite-dolomite filled nodules to 10 cm
in diameter. Some of the nodules also contain analcime and/or
natrolite. At this site are also white radiating groups of thomsonite
coated with orange analcime, natrolite
and calcite. The Green Mountain Road site produces chisel-shaped blades and blocky
pseudohexagonal prisms to 4 mm, some associated with analcime
(R&M 96.6.524).
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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