Formed via aqueous carbonation of Mg2+ ions, the crystallization of magnesite (MgCO3) is a promising carbon capture and reuse technology, albeit limited by the slow precipitation of MgCO3. Although magnesite is naturally abundant, forming at low temperature conditions, its production is an energy-intensive process due to the temperatures required to prevent the formation of hydrated phases. The principle difficulty arises from the very strong Mg2+···H2O interaction, raising barriers to dehydration. Using atomistic simulations, we have investigated the influence of thirty additive anions (Xn–, n = 1–3), ranging from simple halides to more complex molecules, on the first two steps of MgCO3 aggregation from solution: Mg2+ dehydration and Mg2+∙∙∙CO32– pairing. We have computed the thermodynamic stability of solvent shared ion pairs, Mg2+···H2O···Xn–, and contact ion pairs, Mg2+···Xn–, with Mg2+ to reveal the propensity of solution additives to inhibit Mg2+∙∙∙CO32– formation. We have determined the stabilization of undercoordinated hydrated Mg2+ states with a vacant coordination site to which CO32– can bind, subsequently initiating MgCO3 nucleation or Mg2+ incorporation into the crystal lattice. Extensive molecular dynamics simulations of electrolyte solutions containing Na2CO3 with different sources of Mg2+, MgCl2, MgSO4 and Mg(CH3COO)2, further shows that the degree of dehydration of Mg2+ and the structure of prenucleation MgCO3 clusters changes depending on the type counterion. Through a fundamental understanding of the role of solution additives in the mechanism of Mg2+ dehydration, our computational study can rationalize previously reported experimental observation of the effect of solvation environments on the growth of magnesite. This understanding may contribute to identifying solution composition conditions that could promote the low-temperature CO2 conversion into MgCO3.