Kiyoshi Kaneshima

Solubilities of calcite, aragonite and protodolomite in supratidal brines of Minamidaito-Jima Island, Okinawa, Japan

Abstract Chemical compositions of brines in Minamidaito-jima, where protodolomite is widely distributed, have been studied. Chlorinity of the brines ranges from 19.5 to 215 g kg −1 , and the Mg 2+ / Ca 2+ molar ratio ranges from 4.5 to 88, whereas Ca 2+ /Cl − , SO 4 2 /Cl − ,and alkalinity/Cl − ratios in the brines are smaller than those in ‘normal’ seawater. Such variations are caused by the precipitation of CaCO 3 and CaSO 4 through the evaporation of seawater. Apparent solubilities of calcite, aragonite and protodolomite (Ca 1.06 Mg 0.94 (CO 3 ) 2 ) in these supratidal brines have been studied. It was found that protodolomite is unstable in seawater and brines with chlorinity up to 30 g kg −1 . Above 30 g kg −1 , protodolomite becomes more stable with increasing chlorinity, though it is still less stable than aragonite and calcite under these conditions. The pH values of the brines saturated with aragonite and protodolomite are similar at > 150% Cl. The thermodynamic solubility product of protodolomite K sp pd,o , was estimated to be 10 −14.91+0.03 from the saturometric study at 20°C.