Structural elucidation of silica present in kidney stones coming from Burkina Faso

Hundred kidney stones obtained from the University Hospital of Ouagadougou (Burkina Faso) were finally characterized by a panel of complementary spectroscopic and diffraction tools. The most surprising result is the high occurrence of opaline silica as a component in these kidney stones. Opaline silica is a scarce mineral phase in renal calcification; however, we found that at least 48% of the stones had a detectable proportion of silica. SEM images demonstrate the presence of micrometric objects (of spheroidal shape) in close association with monohydrated calcium oxalate crystals. X-ray fluorescence, XRD and 29Si solid state MAS NMR demonstrate unambiguously the presence of amorphous silica, whose composition is comparable to that of natural opals. As NMR is a local spectroscopic probe, other nuclei can be probed. We demonstrate that traces of aluminium are present in the kidney stones by using 27Al solid state MAS NMR. These experiments may offer the first clues of pathological processes that are responsible for these stones.