Mineralogical and chemical characteristics of black coatings in Postojna cave system

Abstract

Mineralogical and chemical analyses of black coatings from two sites in Postojna cave system were studied. Scattered samples were taken from the entrance parts of the cave and from Črna Jama. Thin sections, powder X-ray diffraction (XRD) technique and scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM/EDS) were used. Microscopic investigation of thin sections of black coloured material from both locations revealed that the main material is carbonate – calcite, with evenly dispersed prevailingly minute opaque black grains. The XRD analysis on samples from both locations confirmed just a presence of calcite with minor quartz and dolomite, excluding Mn and Fe oxides or apatite-group minerals as reason for black colouring. The SEM/EDS analysis of samples from the entrance parts of Postojna cave system was consistent with XRD analysis, which did not show any Mn oxides. The high content of C measured in the black coatings from the cave entrance parts indicates organic C, which deposited on the cave walls at time of the petrol explosion during WW2. We can attribute black coatings from Črna Jama to one form of organic C as well, but it is certainly different from the one in the entrance parts of Postojna cave system. As in Črna Jama no other evidence indicates for old human inhabitation of the place: torches of first tourists are a more probable origin of charcoal. On both locations black coatings can be at least partly described by microclimate conditions at cave entrances, which caused the deposition of organic material of allogenic origin (for example soot due to the forest fires).