Here, Ni was concentrated in the outer rims while the cores of the crystals were enriched in Co, respectively to each other. In addition, the calculated Ni# from bulk ICP-MS data agreed well with the EDS data (ESI: Table ). Thus, the fractionation of Ni and Co occurred in all crystals, following the same temporal and spatial patterns of the elemental distribution. Interestingly, in the NixCo1-x-mixture with x = 0.8, where M-struvite was also present at the end of the reaction with M-phosphate octahydrate in a binary phase mixture (ESI: Figure ), the calculated Ni# agreed with the Ni# in the solution (ESI: Figure , Figure , Table ).