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Degradation and biomineralization of building stones: case study of the Diocletian's Palace, Split, Croatia

DEGRADATION AND BIOMINERALIZATION OF BUILDING STONES: CASE STUDY OF THE DIOCLETIAN'S PALACE, SPLIT, CROATIA V. BERMANEC(1), N. TOMAŠIĆ(1) AND D. MUDRONJA (2) (1)University of Zagreb, Faculty of Science, Institute of Mineralogy and Petrography, Horvatovac bb, HR-10000 Zagreb (2)Croatian Conservation Institute, Nike Grškovića 23, HR-1000 Zagreb INTRODUCTION Under atmospheric conditions building stones are eroded by different weathering agents. Precipitations, temperature changes and pollution products are specially responsible for building stone weathering giving rise to the formation of the different weathering products. Living organisms, due to their metabolism, contribute to the building stone erosion but are also involved in the process of biomineralization. The investigation of two samples of the building stones from the Peristyle colonnades, which are part of the Diocletian&#8217 ; ; s Palace in Split, Croatia, is described here. MATERIALS AND METHODS Two samples (P-1, P-2) originating from the surface area of the Peristyle colonnades were investigated by X-ray diffraction and SEM. X-ray diffraction analysis was performed in order to identify mineral species on the sample surface. Philips X&#8217 ; ; Pert powder diffractometer equipped with X-ray mirror was used, providing measuring of the samples without their prior grounding and thus avoiding the mixing of the mineral phases formed by biomineralization with the original material. This is specially important because sometimes weathering product is very thin. The experiments used CuKa radiation excited at 40kV and 40mA, divergence slit of &frac12 ; ; ° and parallel plate collimator. For morphological study SEM was applied. The JEOL JSM-35 electron microscope on 25kVwas used. The samples were sputtered by carbon thread. RESULTS X-ray powder diffraction indicated the presence of gypsum in the case of both samples, and, additionally, the occurrence of weddellite, calcium oxalate dihydrate in the case of P-2. The diffraction patterns of both samples are presented in Figure 1. A selection of micrographs obtained by SEM is given in Figure 2 showing a needle-like appearance of the crystals on the surface of the samples. DISCUSSION This investigation confirms the presence of the secondary phases upon the original building stone material. Presumably, these phases are the result of organic activity. This activity could be described by two processes: the corrosion of the original material and the formation of new minerals through the biomineralization. The dissolution is partly influenced by environmental agents. Organic dissolution is enhanced by production of organic and inorganic acids in metabolic processes. The corrosion of limestone building material due to bioactivity is mostly connected to the production of CO2 or carbonic acid from respiration (1). Fungi, bacteria, algae and lichens are the most frequent organisms related to biomineralization. Here, no identification of organisms involved in the formation of the revealed minerals has been done, but both mineral phases found are most probably formed by bioactivity. The SEM micrographs clearly show needle-like crystals. The needle-like crystals are characteristic for calcium oxalate monohydrate (whewellite)(2), although X-ray analysis indicates the presence of calcium oxalate dihydrate (weddellite). While the weddelite formation is related to metabolism of living organisms, the origin of gypsum could be dubious. Although there are organisms responsible for gypsum formation, its appearance could also be pollution induced. Sulfur dioxide emitted in atmosphere from combustion of fossil fuels, in the form of sulfuric acid readily reacts with limestone to form gypsum. However, it is uncommon for gypsum and weddellite to occur together, since oxalates tend to dissolve in the presence of sulfuric acid (3). REFERENCE (1) Banfield, J. F., Welch, S. A. (2000), EMU Notes in Mineralogy, 2, 5, 173-196 (2) Tait, K., Sayer, J. A., Gharieb, M. M., Gadd, G. M. (1999), Soil Biology and Biochemistry, 31, 1189-1192 (3) Wilkins, S. J., Compton, R. G., Viles, H. A. (2001), Journal of Colloid and Interface Science, 242, 378-385

degradation; biomineralization; building stones; Diocletian's Palace

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