Abstract:

Aragonite is unstable in fresh water and usually reverts to calcite, but it is actively depositing as a secondary mineral in the vadose zone of some caves in New South Wales. Aragonite deposits were examined to determine whether the material is or is not aragonite. Substrates to the aragonite were examined, as was the nature of the bedrock. The physical, climatic, chemical and mineralogical influences on calcium carbonate deposition in the caves were investigated. The study sites are all located in Palaeozoic rocks within the Lachlan Fold Belt tectonic region of New South Wales. Several factors were found to be associated with the deposition of aragonite instead of calcite speleothems. They included the presence of ferroan dolomite, calcite-inhibitors (in particular ions of magnesium, manganese, phosphate, sulfate and heavy metals), and both air movement and humidity. Chemical inhibitors work by physically blocking the positions on the calcite crystal lattice which would have otherwise allowed calcite to develop into a larger crystal. Often an inhibitor for calcite has no effect on the aragonite crystal lattice, thus aragonite may deposit where calcite deposition is inhibited. Another association with aragonite in some NSW caves appears to be high evaporation rates allowing calcite, aragonite and vaterite to deposit. Vaterite is another unstable polymorph of calcium carbonate, which reverts to aragonite and calcite over time. Vaterite, aragonite and calcite were found together in cave sediments in areas with low humidity.