A petrographic catalogue for the “Onyx Marbles and Alabasters Collection” of The Museum of Mineralogy and Petrography in Turin University

The paper reports information and results concerning 69 rock samples belonging to the “Onyx Marbles and Alabasters Collection”, recently acquired by the Museum of “Mineralogy and Petrography” of the University of Turin. The collection is in loan for use to the “Museo Regionale di Scienze Naturali” in Turin. This set of specimens represents several onyx marble varieties currently available in the Italian ornamental stone market. Most of them are actually calcite-alabasters, sedimentary rocks formed in karst environments. Generally, they show a banded structure having nearly parallel layers that differ, from case to case, in colour, fabric and /or mineral composition. In order to examine the different features of this heterogeneous category, a multi-technique approach has been chosen. Detailed photographic documentation was acquired for each sample of the collection; moreover, a small amount of specimen was collected for chemical analysis and mineralogical investigations. Every sample has been scanned by electron microscopy and, for the most interesting varieties, a thin section was obtained and observed at the optical microscope. MicroRaman spectroscopy was used to determine the mineral phases present in each sample, as well as EDS analysis that added a chemical dataset to the characterization of the mineral species detected. ICP-OES spectroscopic analyses were done for the determination of trace elements of the carbonate fraction of each sample. All the information obtained were organized in a petrographic catalogue reported at the end of the paper.