## 3 Analytical Methods  
For a complete characterization of the stone materials and degradation products (i.e., black crusts), several complementary techniques were employed to investigate the textural, morphological and compositional features, as well as the interaction with rock substrate. Optical Microscopy (OM) observations were carried out on polished and stratigraphic thin sections using a Zeiss Axioskop 40 microscope (Carl Zeiss Microscopy GmbH, Jena, Germany, 2007). OM made it possible to: a) determine the textural features; and b) detect weathering rate on superficial layers by evaluating the morphology and growth of black crusts.  
Scanning Electron Microscopy (SEM) coupled with energy dispersive X-ray spectrometry (EDS) analyses were performed on polished cross-sections previously covered by carbon coating, to obtain information about micromorphology and chemical composition (in term of major elements) of the black crusts. Analyses were performed with a SEM (Cambridge Instruments, version 360 Stereoscan, UK., Cambridge), equipped with a microanalyzer in energy dispersive spectrometry (EDS) (EDAX model) with an ultrathin (UHT) window in order to ensure the detection of light elements. The operating conditions were set at an accelerating voltage of 20 kV, beam current of 0.2 mA, acquisition time of 100 s, and dead time of 30-35%.  
Chemical analyses were carried out according to standard mode and normalized by weight (%). The detection limit for the EDS system is approximately 0.1% weight. The accuracy of the analysis was periodically tested on standard USGS samples. Chemical analyses of the crust surfaces were performed in raster mode.  
X-ray powder diffraction (XRPD), performed to investigate the crusts' mineralogical composition, was recorded on an X-ray diffractometer (version D8 Advance, Bruker, UK) using X-ray Cu K\(\alpha\) radiation. The operative conditions were 40 kV voltage, 30 mA current, 0.02\({}^{\circ}\) 20 step size, and 3.0 sec step time with a 20 range of 10-80\({}^{\circ}\).  
The analysis of trace elements was conducted by using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This method enables the detection and quantification of several elements with spot resolutions of approximately 40-50 \(\upmu\)m, leading to the determination of compositional variations on a micrometric scale [41; 42; 43]. Measurements were obtained using the LA-ICP-MS instrument (model Elan DRCe, Perkin Elmer/SCIEX, MA, USA), connected to a New Wave UP213 solid-state Nd-YAG laser probe (213 nm). The ablation was performed with spots of 40-50 \(\upmu\)m with a constant laser repetition rate of 10 Hz and fluence of \(\sim\)20 J/cm\({}^{2}\). Calibration was performed using the NIST 612-50 ppm glass reference material as the external standard [44]. Internal standardization to correct instrumental instability and drift was achieved using CaO concentrations from SEM-EDS analyses [45]. The accuracy was evaluated on BCR 2G glass reference material and on an in-house pressed-powder cylinder of the standard Argillaceous Limestone SRM1d of NIST [46]. The resulting element concentrations were compared with reference values from the literature [47]. The accuracy, as the relative difference among the reference values, was always better than 12%, and most elements plotted in the range of \(\pm\)8%. Investigations were performed on cross-sections 100 \(\upmu\)m thick. Each sample was subject to several spot analyses, depending on the thickness of the black crust, to assess the compositional variability within the crust and the differences between the degraded portion and the underlying unaltered substrate.