Thomsonite with Natrolite, Analcime

Thomsonite with Natrolite and Analcime from Cairns Bay, Flinders, Victoria, Australia, measuring 80 × 63 × 42 mm and weighing 193 grams. Three zeolites on one piece, which makes this a handy specimen for showing the range of the Flinders cavities.

Thomsonite is a sodium-calcium zeolite named after the Scottish chemist Thomas Thomson, and it commonly forms bladed to prismatic crystals in radiating sprays or compact spherical aggregates. Its sprays can look very much like Natrolite, and having both on one specimen is a good opportunity to compare them; Thomsonite blades tend to be flatter, while Natrolite forms slimmer, squarer needles. Analcime, the third mineral here, crystallises in quite different trapezohedral forms, so it is easy to pick out.

These minerals all formed in gas cavities within the Eocene basalt flows of the Flinders coast, where sodium and calcium carried by groundwater determined which zeolite grew where. Cairns Bay has a well-recorded zeolite list on Mindat, and this three-species specimen, formerly in the David Vince Collection, captures much of that variety in one hand.