Composition formulas of silicate glasses

Abstract

AbstractWe here extend the cluster‐plus‐glue‐atom model, originally developed for metallic glasses, to the interpretation of silicate glass compositions. By referring to β‐SiO2 crystal structure and to the widely recognized random network model, our model identifies a 16‐basic‐unit (or 32‐cation) composition formula {M1+2O}n–{M3+2O3}16−(m+n)–{M4+2O4}m, where monovalent unit {M1+2O} and quadrivalent unit {M4+2O4} cannot coexist and the trivalent units {M3+2O3} are constructed from pure trivalent cations such as {(Al,B)2O3} and from combinations of monovalent and divalent cations with the base quadrivalent Si such as {(Na,K)2/3Si4/3O3} and {(Mg,Ca)SiO3}. After analyzing some glasses of historical importance, it is pointed out that most of soda–lime–silica and aluminosilicate glasses satisfy closely {M3+2O3}16, as exemplified by ancient glasses as well as by modern ones, such as Container glass, 1980, Jena Standard Glass, and Corning Gorilla glasses of the first generation. On the other hand, borosilicate glasses feature extra {Si2O4} units ranging from 4 to 9.5 in their 16‐unit formulas, as exemplified by Corning E‐glass and Schott thermometer glass with {Si2O4}4 and Corning Pyrex with {Si2O4}9.5 containing the highest silica proportion. The number of monovalent cations in 32‐cation formulas follows linear dependences on the average cation valence e/c and on the 16‐unit parameters 2 m − 4n, showing that, except alkali‐free glasses, the compositions of soda–silicate glasses converge to 2N2 + N3 ≈ 8, where N2 and N3 are, respectively, the numbers of divalent and trivalent cations in the 32‐cation formula. The revelation of the composition rule shows the capacity of the cluster‐plus‐glue‐atom model in understanding the compositions of complex glassy systems.