The pH measurements were evaluated and verified by calculations with the aqueous geochemical simulation program PHREEQC v. 3 using the Wateq4f database (wateq4f.dat) and displayed in the Supplementary Material (Supplementary Material: Fig. ). Any missing database entries for the relevant Mg-containing phases were completed with literature K sp values . The thermodynamic modelling of the precipitation reaction (Eq. 1), using our starting conditions as input, with PHREEQC (Supplementary Material: