The thermodynamic simulation of the systems Na+, Mg2+/Cl-, CO32-//H2O and Na+, Mg2+/SO42-, CO32-//H2O using the Pitzer method at 25 degrees C revealed that both stable and metastable crystallizations take place in these systems. MgCO3 center dot 3H(2)O is always a metastable phase. Thermodynamic modeling of Mg2+ removal as a part of desalination of hyper-saline sea-salt waste brines, which can be described by these systems, pointed to the coprecipitation of various salts. Ensuing from the obtained data and considering the crystal chemistry and kinetic crystallization of the co-crystallizing salts we designed the precipitation of MgCO3 center dot 3H(2)O from sea-salt brines avoiding its further transformation to metastable 4MgCO(3)center dot Mg(OH)(2)center dot 4H(2)O and stable MgCO3. Our experimental studies confirmed the designed method and optimized the conditions for precipitation of high-purity MgCO3 center dot 3H(2)O with good filtration characteristics. (C) 2014 Elsevier B.V. All rights reserved.