X-ray diffraction analysis of urinary calculi: need for heat treatment

Although X-ray diffraction (XRD) is the most reliable method for analysis of urinary stones, it has its specific limitations. It fails to detect amorphous phases, cannot distinguish between chemically different phases having identical lattice geometry (e.g., brushite CaHPO₄·2H₂O and gypsum CaSO₄·2H₂O) and may miss some phases (e.g., apatite and calcium urates) due to peak overlaps. XRD of urinary stones was performed using a DRON 2.0 diffractometer with CuKα radiation and repeated after calcining the sample, preferably with weighing. XRD of the calcined samples enabled detection of amorphous magnesium phosphates, poor crystallized apatite mixed with struvite, weddellite and/or organic matter, hidden organic calcium salts mixed with uric acid; unambiguously discriminated between brushite and gypsum, struvite and its potassium analogue; confirmed presence of quartz in one stone. Statistical study of 341 samples from Rostov region has shown that three-phase mixtures are most frequent (32.3%). Redoing XRD phase analysis after heat treatment, preferably at 500 and/or 900°C, considerably enhances capabilities of the method due to (i) avoiding peak overlaps; (ii) crystallization of amorphous phases; (iii) concentrating minority inorganic components in organic stones; (iv) different decomposition products from indistinguishable phases; (v) semi-quantitative information from the weight loss data.