ABSTRACT

A series of experiments was conducted in which boron minerals were precipitated by water evaporation from solutions containing boron and potassium, sodium or lithium at 25°C, and boron isotope fractionation accompanying such mineral precipitation was investigated. In the boron-potassium ion system, K₂[B₄O₅(OH)₄]·2H₂O, santite (K[B₅O₆(OH)₄]·2H₂O), KBO₂·1.33H₂O, KBO₂·1.25H₂O and sassolite (B(OH)₃) were found deposited as boron minerals. Borax (Na₂[B₄O₅(OH)₄·8H₂O) was found deposited in the boron-sodium ion system, and Li₂B₂O₄·16H₂O, Li₂B₄O₇·5H₂O, Li₂B₁₀O₁₆·10H₂O, LiB₂O₃(OH)·H₂O and sassolite in the boron-lithium ion system. The boron isotopic analysis was conducted for santite, K₂[B₄O₅(OH)₄]·2H₂O, borax and Li₂B₂O₄·16H₂O. The separation factor, S, defined as the ¹¹B/¹⁰B isotopic ratio of the precipitate divided by that of the solution, ranged from 0.991 to 1.012. Computer simulations for modeling boron mineral formations, in which polyborates were decomposed into three coordinated BO₃ unit and four coordinated BO₄ unit for the purpose of calculation of their boron isotopic reduced partition function ratios, were attempted to estimate the equilibrium constant, K_B, of the boron isotope exchange between the boric acid molecule (B(OH)₃) and the monoborate anion (B(OH)₄^-). As a result, the K_B value of 1.015 to 1.029 was obtained. The simulations indicated that the K_B value might be dependent on the kind of boron minerals, which qualitatively agreed with molecular orbital calculations independently carried out.