This paper investigates the formation of amorphous calcium carbonate under high magnesium concentration (Mg/Ca = 6) via several novel methods. While feeding calcium chloride solution into the mixed solution of magnesium chloride and sodium carbonate, a large amount of calcium ions (>0.13 mol/L) stably coexisted with the amorphous calcium carbonate in the solution. The calcium concentration detected by the calcium ion selective electrode was proportional to the calcium concentration added. Surprisingly, the ratio of magnesium to calcium in the amorphous calcium carbonate was just 0.07, and the concentration of calcium continued to increase in the solution after the formation of the amorphous calcium carbonate. We propose that stable prenucleation cluster formed in the solution, which stably coexisted with the amorphous calcium carbonate because of the incorporation of hydrated magnesium ions. The high dehydration free energy barrier of magnesium ions led to its slow dehydration. Also, the hydroxide ion helped to bypass the high dehydration free energy barrier. This research indicates the possible significant role of prenucleation cluster pathway in the calcium carbonate crystallization. (C) 2018 Elsevier B.V. All rights reserved.