Application of X-ray diffraction to the study of limestone patinas

The aim of this study in monumental limestone alteration is to characterize the superficial stone zone called patina where transformation processes due to air and water occur. The patina studied presents four thin coloured layers. It is difficult to separate precisely each layer to prepare powder samples; therefore X-ray diffraction study has been carried out with bulk samples, This allows us to study the stone in thickness with shifting it in translation. Patina mineralogical composition can be known every 100 mu m with a precision of 100 mu m on the beam position on the sample. It is difficult to have: quantitative results with bulk natural stone sample because of X-ray beam size: the mineralogical composition is inhomogeneous in the 0.15 to 0.6 mm(1) studied and there exist crystallite orientation phenomena. This study has permitted us to differentiate patina as it contains one more mineralogical phase : gypsum.