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The present study investigates the gradual replacement of Ca²⁺ with Mg²⁺ ions in brushite (CaHPO₄·2H₂O). To date, this approach has not been systematically explored and may prove beneficial for the production of Ca_1−xMg_xHPO₄·nH₂O materials with tailored properties which are suitable for environmental and medical applications. For their production, solutions of sodium dihydrogen orthophosphate dehydrate, NaH₂PO₄·2H₂O, calcium nitrate tetrahydrate, Ca(NO₃)₂·4H₂O, magnesium nitrate hexahydrate, Mg(NO₃)₂·6H₂O and ammonium hydroxide solution, NH₄OH, were used. At low Mg/Ca molar ratios (up to 0.25) in the starting solution, partial replacement of Ca with Mg takes place (Mg doping) but no struvite is produced as discrete phase. When the Mg/Ca molar ratio increases gradually to 1.5, in addition to Mg-doped brushite, struvite, NH₄MgPO₄·6H₂O, precipitates. The microstructure of the materials produced for different degrees of Ca replacement with Mg has been analyzed in depth with the use of powdered XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), thermogravimetric (TG) analysis and SEM (scanning electron microscopy). The results of this study prove that the Mg/Ca ratio in the starting solution can be monitored in such a way that materials with tailored composition are obtained.