Borates: a survey of main trends concerning crystal-chemistry, polymorphism and dehydration process of alkaline and pseudo-alkaline borates

摘要

The crystal chemistry of five families of hydrated alkaline or pseudo-alkaline (M) borates, MBO_2.vH_2O, M_3B_5O_9.wH_2O, M_2B_4O_7. xH_2O, MB_3O_5. yH_2O and MB_5O_8 zH_2O are presented. The main trends of their dehydration process were deduced from experiments carried out on some of them. Generally, upon heating, no lower hydrated borate appeared; the complete dehydration was obtained from 85 to 520 deg C, and led to an amorphous anhydrous compound with the same B/M ratio. Upon further heating, a crystallization and, very often, several transitions occurred. Crystal chemistry and polymorphism were examined for several families of anhydrous borates: M_3BO_3, M_3RB_2O_6 (R = trivalent element), MBO_2, M_2B_4O_7, M_3B_7O_(12), MB_3O_5, M_5B_(19)O_(31), M_2B_8O_(13), Cs_3B_(13)O_(21), MB_5O_8 and CsB_9O_(14). Numerous Fundamental Building Blocks (FBBs) were found in all of these borates. Some of them were encountered in both hydrated and anhydrous borates; in the former, the FBBs were often isolated or in chains; in the latter, they generally formed three-dimensional networks. They contained from one to sixty-three boron atoms. However, complex FBBs often are built from shorter borate groups with one (BO_3, triangle open or BO_4, T), three or five boron atoms, these last two consisting of one and two B_3O_3 rings, respectively. A few new FBBs containing more linked B_3O_3 rings were also encountered. The largest FBB was found in Cs_3B_7O_(12) that has a structure built from sheets; its shorthand notation is 63: _(infinity)~2[(7: 3 triangle open + 4 T) + 10(5: 3 triangle open + 2 T) + ((5: 2 triangle open + 3 T) + (1: T) or (5: 3 triangle open + 2 T) + (1: triangle open))].