The Malenco serpentinite (Sondrio, northern Italy): petrography, geochemistry and mechanical properties

The Malenco serpentinite was quarried and used as a dimension and building stone since the XI century. Nowadays there are 22 active quarries that produce 70000 m3/yr of “massive” and schistose serpentinite. The schistose variety is mainly splitted in thin slabs for roof covering, whereas the “massive” serpentinite is commercialized in 6 varieties and processed in many ways. All the varieties have been characterised by petrographic analysis, SEM-EDS, quantitative XRPD and whole rock geochemistry. Mechanical properties have been also considered: water absorption, porosity, compressive strength, flexural strength, abrasion resistance. The mineralogical composition is characterized by various generations of antigorite, olivine, diopside, chlorite, magnetite and little amounts of chromite, Fe-Ni alloys, Fe-Ni sulphides, Ti-clinohumite and chrysotile (only in veins). The serpentinite varieties show non-pseudomorphic textures, marked foliation and fine grain size. The mineralogical composition has been quantified by XRPD, running the FULLPAT software, which always evidences an excellent fit and little errors in phase quantification (< 5 wt.%). The antigorite content is extremely variable (50 – 90 wt.%). The geochemical data (majors, minors and trace elements) evidence a moderate variability in Al2O3 (0.78 - 2.83 wt.%) and CaO (0.15 - 3.14 wt.%), whereas Fe2O3 is generally constant. The LOI is variable (4.1 <LOI< 9.9 wt.%), due to the degree of serpentinization. Cr2O3 (up to 0.41 wt.%) and Ni (also > 2000 ppm) contents are rather high. The REE patterns suggest homogeneous condritic trends, sometimes with slight LREE enrichments. The serpentinite shows a good and homogeneous wear resistance (Amsler test, 5.93 - 9.19 mm). The flexural strength is influenced by the grain size and the planar anisotropy, with excellent values between 25.4 MPa and 83.9 MPa. The compression break load (98.9 - 219.4 MPa) is anisotropic among the varieties as a function of the load applied parallel or perpendicular to the foliation of the rock. The water absorption value is very low (0.04 - 0.18 wt.%) for all the commercial varieties, as well as the porosity (0.02 - 0.18 vol.%, determined by Hg-porosimeter). The workability and the mechanical properties of the serpentinite are strongly influenced by textural and mineralogical features. The mechanical properties make the Malenco serpentinite an excellent material especially for paving, shelving, coverings in areas with significant wear.