CRYSTALLOGRAPHY AND HIGH–TEMPERATURE CRYSTAL CHEMISTRY OF ANHYDROUS BOROSILICATES OF ALKALI AND ALKALI–EARTH METALS
M. G. Krzhizhanovskaya1, R. S. Bubnova1,2, S. K. Filatov1
1Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia
2Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
Keywords: borosilicates, silicoborates, crystal chemistry, polymorphism, thermal expansion
Abstract
The crystal structures and thermal behavior of anhydrous borate-silicates of alkali and alkali-earth metals are briefly overviewed. In silicoborates (B/Si > 1), the boron atoms occur in triangle and tetrahedral coordination, as in borates; in two of three known silicoborates, the unique layered and chained polyanions are found. The known borosilicates (B/Si ≤ 1) have a framework structure formed from tetrahedrally coordinated atoms of boron and silicon; some borosilicates belong to the known structural types of aluminosilicates, while the others are only structurally similar to them and silica modifications, whereas their tetrahedral frameworks are topologically identical to that of aluminosilicates and SiO2. The exception are borosilicates Li4B4Si8O24 and lisitsynite KBSi2O6, having the original frameworks. In line with the similarity of chemical composition and commonality of crystal structure, thermal behavior of borosilicates appears to be similar to that of aluminosilicates and silicates, however, borosilicates have considerably lower temperatures of formation, polymorphic transformations, and melting.
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