Crystallization of a magnesium hydroxychloride (basic magnesium chloride) mineral may play an important role in buffering pH in concentrated marine evaporite brines. Laboratory synthesis of Mg2(OH)3 C1-4H2O as an alteration product of Mg(OH)2 in 2M MgCl2 solutions at 25°C suggests that this salt may precipitate in hypersaline environments. Brine compositions from a succession of salt pans near Portoroz, Yugoslavia, support the hypothesis. After converting concentrations to ion activities with a modified version of the computer program WATEQ, it is seen that the ion activity product of Mg2 (OH)3 Cl reaches a maximum midway through the Portoroz succession and then remains essentially constant in the more concentrated brines. This suggests that IAPmax = Ksp. Since crystallization of only minute amounts of this salt would generate the observed pH decline and since the salt would be extremely soluble in subsequent pore fluids, it is not surprising that the phase remains unobserved in marine evaporite assemblages.