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, K 2 [B 4 O 5 (OH) 4 ]·2H 2 O, santite (K[B 5 O 6 (OH) 4 ]·2H 2 O), KBO 2 ·1.33H 2 O, KBO 2 ·1.25H 2 O and sassolite (B(OH) 3 ) were found deposited as boron minerals. Borax (Na 2 [B 4 O 5 (OH) 4 ·8H

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... O) was found deposited in the boron-sodium ion system, (OH)·H 2 O and sassolite in the boron-lithium ion system. The boron isotopic analysis was conducted for santite, K 2 [B 4 O 5 (OH) 4 ]·2H 2 O, borax and Li 2 B 2 O 4 ·16H 2 O. The separation factor, S, defined as the 11 B/ 10 B 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 BO 3 unit and four coordinated BO 4 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.