The mineralogical and chemical methods in investigations of decay of the Devonian black ‘marble’ from Debnik (Southern Poland)

Abstract

Abstract Optical microscopy, scanning electron microscopy with energy dispersive spectrometry, X-ray diffraction, infrared (IR) spectroscopy, Rock-Eval pyrolysis and gas chromatography combined with mass spectroscopy were used to examine deterioration of the black limestone from Dębnik near Cracow. Owing to its unique colour and good polishing properties the rock is called the ‘Debnik marble’. The samples were taken from various monuments and natural outcrops exposed to weathering. The material is a compact limestone whose black colour is caused by an admixture of bitumens or pyrite. Its horizontal layers are separated by discontinuities filled with clay minerals. Surface exfoliation is one of the damage signs and results in the formation of irregular or lensoidal fractures. The discontinuities provide an easy access for acid rain that in reaction with calcite produces gypsum. Crystallization of gypsum leads to alveolar weathering, cracking and chipping of the otherwise compact material. The presence of alveoles or surface exfoliation depends on the orientation of stone blocks. When they are cut along the discontinuities, destruction results in exfoliating and cracking. Perpendicular cutting gives rise to the formation of alveoles. The changes affect the original black colour of the stone surface that alters to grey or even white.