Circular 116—Solubility and enthalpy of the barium-stontium sulfate solid solution series

By E. Brower and J. Renault, 1971, 21 pp., 2 tables, 9 figs.

A study of certain physiochemical properties which are pertinent to the precipitation and dissolution of barium-strontium sulfate in order to understand the conditions in which hydrothermal sulfates are deposited and recrystallized. The concentration of barium in the Earth's crust is slightly greater than strontium in a ratio of 1.13:1, and the two elements exhibit a similar geochemistry in igneous rocks. However, barite is a common mineral in hydrothermal deposits, whereas celestite and (Ba,Sr)SO₄ solid solutions of intermediate composition are rare. Both celestite and barite are commonly found in sedimentary deposits, but intermediate (Ba,Sr)SO₄ is apparently rare in these rocks. Although the BaSO-SrSO₄ system has been studied by others, some of the factors which may govern the geochemical distribution of barite and celestite are not well understood. The authors have studied certain physiochemical properties which are pertinent to the precipitation and dissolution of (Ba,Sr)SO₄ in order to understand the conditions in which hydrothermal sulfates are deposited and recrystallized.

This report describes experimental studies of solubility and enthalpy of precipitation in the BaSO₄-SrSO₄ system between zero and 100ºC at atmospheric pressure. Tentative conclusions are made regarding the deposition of barite and celestite in hydrothermal veins. Strontium sulfate in equilibrium with aqueous solution leaches selectively from barium-strontium sulfate solid solutions at 25ºC and at 1 atmosphere. Equilibrium concentrations of Sr⁺² and Ba⁺² in aqueous solution are not significantly different from values calculated on the assumption that the chemical activity of each component is proportional to its mole fraction in the solid.

The enthalpy of mixing for barium-strontium sulfate solid solutions was determined by calorimetry to be approximately zero at 25ºC and 61ºC. The thermodynamic properties are, therefore, characteristic of an ideal mixture. Nearly pure barite occurs frequently in hydrothermal deposits, whereas celesite and barium-strontium sulfate solid solutions of intermediate composition are scarce. Separation of the two sulfates could be the result of fractional precipitation in the presence of low sulfate concentrations or by leaching of an antecedent solid solution.