Calcite with Chabazite

Calcite with Chabazite from Cairns Bay, Flinders, Victoria, Australia, measuring 102 × 52 × 36 mm and weighing 163 grams. A long, interesting piece combining a carbonate and a zeolite from the same basalt cavities.

Calcite is calcium carbonate, CaCO3, and it is a regular member of basalt cavity assemblages, often arriving late in the sequence after the zeolites, although the order can only be judged by looking at which mineral sits on which. Calcite and Chabazite are easy to tell apart once you know what to look for: Calcite is soft, hardness 3, fizzes in dilute acid and has perfect rhombohedral cleavage, while Chabazite is harder, does not react to acid and breaks unevenly. Calcite's strong double refraction is another species feature worth knowing.

At Cairns Bay, the zeolite and carbonate minerals fill gas holes in Eocene olivine basalt flows exposed along the shoreline, and the combination of Calcite with Chabazite shows how the late fluids in those cavities shifted between silicate and carbonate chemistry. Pieces like this one from the David Vince Collection make good teaching specimens for anyone learning to separate basalt-cavity minerals.