Colourful speleothems in the Wieliczka Salt Mine

Secondary halite deposits in the Wieliczka Salt Mine are represented by various forms of speleothems. Some of them, e.g. stalactites, cauliflowers or crusts, reveal various hues, yellow to red to brown. These hues are related to iron compounds, but the forms, occurrence and composition of these compounds has not previously been studied. To explain the origin of their colours a detailed study of samples collected in the mine, including brines and water soluble extracts and produced experimentally in the laboratory, has been undertaken using SEM, XRD and AAS. The colourful speleothems are compared, using SEM-EDS, with the forms and composition of rusting metal artefacts, where several iron oxide minerals have been identified. We suggest that coloured halite is generally pure halite of which the surface was covered by a ferric minerals suspension in the brine. Colours of the speleothems stem from the various colours of iron minerals whereas their intensity is related probably to the type of fluid that coated the crystals, a continuous film or finely dispersed micron-size mineral aggregates.