Thomsonite with Analcime

Thomsonite with Analcime from Cairns Bay, Flinders, Victoria, Australia, measuring 68 × 68 × 50 mm and weighing 230 grams. A compact, chunky specimen pairing two of the characteristic zeolites of the Flinders basalt.

Thomsonite has the formula NaCa2Al5Si5O20·6H2O, and its name honours Thomas Thomson, the Glasgow chemist who analysed many early mineral samples. It is orthorhombic and most often forms radiating groups of blades. Analcime is a sodium zeolite whose trapezohedral crystals, with 24 faces, are among the most recognisable shapes in the zeolite family, and the two are often found sharing the same cavities in basalts around the world.

The Flinders area on the Mornington Peninsula is one of Victoria's longest-known zeolite localities, and its coastal exposures of Older Volcanics basalt have given collectors and museum mineralogists a steady supply of well-documented material. This Thomsonite with Analcime is one of several Cairns Bay zeolites from the David Vince Collection, and a good example of what the bay produces.