ABSTRACT

A series of experiments was carried out in which boron minerals were precipitated from pH- and chemical composition-controlled solutions at 25°C and boron isotope fractionation accompanying the mineral deposition was measured. Borax, sassolite and sborgite were synthesized from aqueous boric acid-sodium hydroxide solutions. Borax was obtained from solutions with higher pH and sassolite from solutions with lower pH, irrespective of the mole ratio of B and Na in the solution. The fractionation factor, S, defined as S = (¹¹B/¹⁰B)_mineral/(¹¹B/¹⁰B)_solution increases with increasing the pH of the solution for both borax and sassolite. A cross-over point at which S = 1 was found at about pH = 9.5 for borax, while S was always larger than unity for sassolite. The present results of boron isotope fractionation are consistent with theoretical prediction of equilibrium isotope effects, although the experimental equilibrium constant of the boron isotope exchange reaction between B(OH)₃ and B(OH)₄^– at 25°C is larger than that theoretically predicted (1.03–1.05 compared to 1.0194).