Natrolite with Analcime

Natrolite with Analcime from Cairns Bay, Flinders, Victoria, Australia, measuring 90 × 75 × 52 mm and weighing 446 grams. At nearly half a kilogram, this is the heaviest of the Flinders zeolites in this sale and a substantial cabinet piece.

Natrolite takes its name from natron, the old word for soda, and it is indeed a sodium zeolite, Na2Al2Si3O10·2H2O. It is famous for its slender prismatic to needle-like crystals, which often radiate in sprays or fill cavities with fine tufts. Analcime is the other common sodium zeolite at Flinders, and its name comes from the Greek for "weak", because it develops only a weak electric charge when rubbed or heated. Both are typical products of sodium-rich fluids moving through the basalt after it cooled.

The Flinders zeolites sit in Eocene basalt flows exposed along the coast of the Mornington Peninsula, and one idea put forward for their formation is that zeolites already present near the top of an older flow were buried and reheated when a younger flow covered them. That makes the local cavity mineralogy a little more varied than in a single simple flow, something a substantial piece like this from the David Vince Collection shows well. As with all fine zeolite sprays, handle it by the matrix rather than the crystals.