Graphical Description and Classification of Crystal Structure of Borate Minerals

Borate minerals have a great structural diversity of bonding modes of FBUs(fundamental building units)by Bφ3 triangular and Bφ4 tetrahedral coordination groups(φ=O,OH). This paper introduces the description methods for bonding modes of FBUs in the borate minerals: one is to describe the B-B graphs, and the other is to use descriptors. This paper also introduces the thermal decomposition process of borax Na2[B4O5(OH)4]・8H2O and structural development of the FBU's bonding modes in the borax during the thermal decomposition. With increasing temperature, borax dehydrates(dehydroxylates)and transforms to anhydrous Na2B4O7 through tincalconite Na2[B4O5(OH)4]・3H2O. Anhydrous Na2B4O7 first occurs as γ-Na2B4O7 phase, then changes to α-Na2B4O7 phase. In the thermal decomposition process from borax to α-Na2B4O7 phase, bonding modes of the occurring phase are exactly the same as some of the bonding modes in the previous phase.