Fig. 3: Abiotic AMSU and its transformation into crystalline MSU.

a Time evolution of conductivity and transmittance during titration experiments resulting in the initial precipitation of AMSU (marked by the drop in transmittance, vertical blue dashed line). Shaded areas mark std dev. b XRD patterns of precipitates collected at the onset of precipitation (point 1 in (a)) and at the end of the titration test (point 2 in (a)). Note the marked increase in intensity and reduction in peak broadness in the latter case; c FESEM image of the nanogranular structure of early precipitates (AMSU); d TEM image of an aggregate of AMSU, as revealed by the diffuse haloes in the SAED pattern (e). f TEM image of the transformation of AMSU into MSU. g The EDS spectrum confirms that these phases are sodium urates. h FESEM image of an aggregate of poorly crystalline MSU showing a nanogranular surface structure; i FESEM and j TEM images of an aggregate of well-formed acicular (blade-like) MSU crystals. The latter (i–j) correspond to crystals formed in a silica gel, whereas the former (c–h) correspond to precipitates obtained in titration tests.