R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

[R2-04] Structural variability and stability of FBUs in borate minerals

Nishiyasu Wataru¹, *Atsushi KYONO¹ (1. University of Tsukuba)

Keywords:Borate minerals, Borax, Structural variability, Fundamental Building Units, Quantum chemical calculation

As increasing temperature, tincalconite Na₂B₄O₅(OH)₄·3H₂O appeared at 30^oC as a dehydrated phase and decomposed at 160^oC. Subsequently, gamma-Na₂B₄O₇ crystallized as an anhydrous phase at 600^oC and changed to alpha-Na₂B₄O₇ at 650^oC. The FBU of borax consists of two three-membered rings, each of which is composed of a BO₃ triangle and two BO₄ tetrahedra. Tincalconite has the same FBU as borax. On the other hand, the FBU of gamma-Na₂B₄O₇ consists of a three-membered ring consisting of one BO₃ triangle and two BO₄ tetrahedra and two BO₃ triangles and one BO₄ tetrahedra sharing a BO₄ tetrahedron. Alpha-Na₂B₄O₇ is composed of two types of FBUs: one is the same as gamma-Na₂B₄O₇ and another is a three-membered ring consisting of two BO₃ triangles and one BO₄ tetrahedron. Therefore, the dehydrated and anhydrous phases appearing during the thermal decomposition are characterized by the fact that a three-membered ring consisting of one BO₃ triangle and two BO₄ tetrahedra is common to the borax FBUs. The total energies of the FBUs of representative borate minerals are compared. The most widely occurring borate minerals are borax, colemanite, ulexite, and kernite. The FBU of colemanite is a three-membered ring consisting of one BO₃ triangle and two BO₄ tetrahedra. The FBUs of ulexite and kernite also contain a three-membered ring consisting of one BO₃ triangle and two BO₄ tetrahedra. The FBUs of borax, colemanite, ulexite, and kernite have a common ring, and the total energy of each is relatively lower among borate minerals. Therefore, the structural stability of the FBUs is almost consistent with the frequency of occurrence of borate minerals. The frequency of occurrence of three-membered rings is determined by Burns et al. (1995), which is in good agreement with the results of quantum chemical calculations. The total energies of the FBUs of alpha-Na₂B₄O₇ and gamma-Na₂B₄O₇ were clearly higher than those of borax. This may support the fact that the anhydrous Na₂B₄O₇ phase has five polymorphs and easily changes the FBU geometry without requiring large amounts of energy.