Despite its good technical properties, the “Macigno” sandstone undergoes a typical process of decay when used outdoor, consisting prevalently in a flaking of the stone surface with detachment of rock fragments. In order to understand the causes of this decay, the relationships between mineralogy and physical rock properties were investigated. Twenty-six rock samples were collected from quarries situated near the villages of Fivizzano and Pontremoli (the Lunigiana area of northwestern Tuscany, Italy) and examined for their chemical (XRF), mineralogical (XRPD) and petrographic (optical microscopy) characteristics, as well as physical properties such as real and bulk densities, water absorption capacity and linear dilatation coefficient of water-saturated samples. The phyllosilicate minerals of the sandstone were studied on oriented aggregates of the <4 μm fractions by X-ray powder diffraction, and their chemical composition determined by EDS microprobe analyses. The cationic exchange capacity (C.E.C.) was measured on fifteen selected samples.
The studied sandstones are fine- to coarse-grained, arkoses and lithic arkoses whose colour ranges from grey to yellowish. The normative carbonate fraction consists essentially of calcite; its amount is generally low (<1–10%), with the exception of three samples collected in the Turlago-Labiano quarry, which contained calcite concentrations of up to 14, 15 and 38%, respectively. The normative non-carbonate fraction is made up of quartz (19–34%), K-feldspars (7–13%), plagioclase (14–33%), mica-like minerals (4–10%) and chlorite s.l. (5–26%). Mica-like minerals include muscovite (illite), minor amounts of biotite and sporadic illite/smectite mixed-layers (I/S). Chlorite s.l. includes chlorite s.s. and chlorite/smectite mixed-layer phases (Chl/S). The content of the sandstone smectitic component, predominantly associated to Chl/S, ranges from 1 to 7 (wt.%). C.E.C. varies from 2.4 to 11.1 meq/100g.
Water absorption capacity (wt.% of dry samples) as well as hydric dilatation of the stone upon water saturation increase with increasing smectite content, indicating that this swelling component may play an important role in the “Macigno” sandstones decay.