Phosphates precipitating from artificial urine and fine structure of phosphate renal calculi

Phosphates precipitating from artificial urine in the pH range 6-8 were identified using X-ray diffraction, chemical analysis and scanning electron microscopy. The influence of magnesium and citrate on phases precipitating from urine was established. From urine containing a normal quantity of magnesium (around 70 ppm), brushite accompanied by hydroxyapatite (HAP) precipitated at pH less than or equal to 7.0 and struvite with HAP at pH > 7.0. HAP was formed exclusively from magnesium deficient urine at pH 7.0. Newberyite, octacalcium phosphate and whitlockite were not identified. The chemical and phase composition and inner fine structure of 14 phosphate calculi were studied. Three types of stones were distinguished based on their magnesium content: (I) stones rich in magnesium composed of struvite, hydroxyapatite and abundant organic matter, (ii) stones with low magnesium content constituted by calcium deficient hydroxyapatite, up to 5% of struvite, considerable amount of organic matter and occasionally brushite, and (iii) calculi without magnesium consisting of brushite, hydroxyapatite and little organic matter. Conditions prevaling during stone-formation assessed for each type of stone were confirmed by corresponding urinary biochemical data and corroborate the in vitro studies of phosphates precipitation.