In the Provence region (south-eastern France), a large part of the building heritage was erected using a bioclastic limestone called “Pierre du Midi”. Under this common name, the heterogeneity of the material and, specifically, its stability upon climatic exposure depend on the actual extraction location. In some cases, building stones are affected by spalling decay which is commonly related to the presence of clay minerals although without quantitative support. The present study aims at characterizing and quantifying the clay mineralogy of eight “Pierre du Midi” samples showing various spalling degrees.

The combination of transmission electron microscopy (TEM) coupled to energy-dispersive X-ray spectrometer (EDX) and of X-ray diffraction (XRD) full-profile modeling of 00ℓ reflections patterns is used for quantitative phase analysis. Identification of clay minerals present in the < 4 μm fractions is performed on the basis of qualitative analysis of experimental XRD patterns recorded in both air-dried (AD) and glycolated (EG) states. The crystal-chemistry of clay minerals is then refined from TEM-EDX analyses which are used as an essential constraint for the modeling of both AD and EG XRD patterns.

The results highlight that the complex clay mineralogy of limestones can be unraveled and quantified when using the present coupled approach. Finally, the contrasting sensitivity of samples to spalling decay can be explained by the overall proportion of expandable layers, essentially present in Fe-rich mixed-layers, in the clay fraction. A content of expandable layers higher than 0.80 wt. % results in a high sensitivity to spalling decay, whereas rocks with less than 0.20 wt. % of expandable layers remain essentially unaffected.

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