PALYGORSKITE FROM BOLCA, ITALY - A CHARACTERIZATION BY HIGH-RESOLUTION SYNCHROTRON-RADIATION POWDER DIFFRACTION AND COMPUTER MODELING

A new find of the mineral palygorskite from Bolca, Vicenza, Italy is d escribed. The mineral occurs in association with calcite and diaspore along the shear fractures and in the veins of secondary minerals in th e Eocene marls of the Bolca quarry, a locality known for its well-pres erved fossil specimens. Selected regions of the X-ray powder diffracti on spectra (XRPD) were collected using X-ray synchrotron radiation and high-resolution parallel-beam geometry. The data are interpreted by c omparison with the computer-simulated powder diffraction patterns rela tive to the proposed strcture models for palygorskite. Cell parameters and relative phase weight evaluation were obtained by model-constrain ed Rietveld refinement. The sample was also investigated by electron p robe micro-analysis (EPMA), TGA and DTG analysis, and SEM imaging. The average material is a mixture containing about 60% orthorhombic palyg orskite, 30 % monoclinic palygorskite, and less than 10 % calcite and diaspore impurities. The association of the two different varieties of palygorskite is in agreement with recent re-evaluation studies of the mineral from different localities, showing the ubiquitous presence of the two forms.