Study of the structure of struvite stones with scanning electron microscopy and energy-dispersive X-ray microanalysis

Topographic features of eight struvite calculi were investigated with scanning electron microscopy (SEM) equipped with energy-dispersive X-ray microanalysis (EDX). Perpendicularly cracked fragments showed concentric laminations composed of compact and loosely packed strata alternately. Magnesium and phosphorus were detected in the compact strata with their characteristic dispersive X-ray spectra. There existed numerous spherular crystals with smooth or porous surfaces and scattered penta- or hexa-hedral coffin-lid shaped crystals in the loosely packed strata. The former crystals showed the dispersive X-ray spectra of calcium and phosphorus, and were estimated to be calcium phosphate (CaP). The latter ones were presumed to be magnesium ammonium phosphate (MAP) with EDX. The surfaces of the fragments were cracked like eggshells and displayed numerous CaP crystals and scattered MAP crystals in most cases, while in only 1 case some faces of pieces demonstrated wavy MAP phases, sundry areas which were rimmed with aggregated CaP spherulites. The mean molar ratios of Mg/P and Ca/P in each case were 0.88-1.03 and 1.25-1.52, respectively. Though EDX was inadequate to determine the accurate chemical formula of CaP and MAP crystals by detecting their molar ratios of Ca/P and Mg/P with EDX, SEM/EDX is useful to observe these urolith crystals and to surmise them to be CaP or MAP crystals by detecting their atomic elements with EDX.