Dolomite in speleothems of Snežna Jama cave

Povzetek

In Snežna Jama cave, Slovenia, extensive speleothems composed of dolomite, aragonite and hydromagnesite have been found, occurring as 5 cm thick globular crusts coating the host rock. Arborescent aragonite constitutes the skeleton of the crust, whereas dolomite is cementing, coating and replacing the aragonite. The dolomite displays two distinctive fabrics: coarse rounded to spheroidal crystals, frequently showing fibrous-radial and concentric patterns, and microcrystalline aggregates. Dissolution of the dolostone host rock has provided Mg, which is the main control on the precipitation of aragonite, dolomite and hydromagnesite. Dolomite precipitation could be promoted by increased Mg/Ca ratios due to the prior precipitation of calcite and aragonite and by forced degassing due to ventilation caused by the existence of shafts cutting the main cave passage and a former entrance to the cave. However, in many caves such conditions do not lead to the formation of dolomite and so we discuss other mechanisms which might promote dolomite precipitation, like the possible contribution of microbes, or the transformation of precursor phases such as amorphous Ca-Mg carbonates, or hydromagnesite.