Silicate Glasses

Abstract

Silicate glasses are important cultural, societal and geological materials. Geologic glasses testify for the igneous activity of the Earth and, for instance, represented important source of tools and ornamental objects during the Paleolithic. Nowadays, silicate glasses are used to build technical materials, such as smartphone screens or glass matrix for stabilizing hazardous radioactive wastes. Therefore, silicate glasses are central to the history of the Earth and of the humanity. The compositional landscape of natural and industrial silicate glasses is vast, with various elements that all influence differently the glass properties and structure. The SiO 4 tetrahedral framework, backbone of silicate glasses, is variously influenced by the introduction of network modifier metal cations or network former aluminium cations. Industrial and geologic silicate glasses further contain multivalent elements (e.g., Fe 2+/3+), rare-earth elements, and volatile elements (H, C, S, Cl, F, I) that play different roles on the glass structure and properties. This chapter proposes to review the link between the structure, the properties and the chemical composition of silicate glasses.

Analytical model of the network topology and rigidity of calcium aluminosilicate glasses

“…To establish our topological model of CAS glasses, we take as a reference the structure of glassy silica (SiO 2 ), wherein all the Si and O atoms are four-and two-fold coordinated, respectively-that is, all the O act as BO atoms bonded to two network formers. 14 Starting from this reference structure, we then describe the competitive effects of Ca and Al atoms: (i) each Ca atom consumes two BOs and, in turn, creates two NBO atoms, 46 whereas, in contrast; (ii) each Al atom consumes one NBO and creates one BO. 14 These well-known effects arise from the following mechanisms.…”

“…On one hand, when added to pure SiO 2 , Ca 2+ cations act as network modifiers as they tend to depolymerize the atomic network by breaking some Si-O-Si inter-tetrahedral joints and, in turn, charge-compensating pairs of negatively charged NBOs. 46 On the other hand, starting from a calcium silicate glass, newly added Al atoms act as network formers and tend to repolymerize the network by using available Ca 2+ cations to charge-compensate negatively-charged four-fold coordinated AlO 4 units. [47][48][49][50] all the BOs present in the network, that is, for N BO ≥ 0, whereas mechanism (ii) remains possible until the added Al atoms consume all the NBOs present in the network, that is, for N NBO ≥ 0.…”

“…In this domain, the glass compositions are peraluminous, that is, they exhibit an excess number of Al atoms as compared to the ones that are needed to charge-compensate all the calcium atoms in the glass (i.e., N Al = 2N Ca ). 46 In this regime, all the AlO 4 tetrahedral units cannot be charge-compensated any longer due to the deficit of Ca cations. From this point, two possible mechanisms have been suggested to occur: (i) some Al atoms become overcoordinated (i.e., with a coordination number larger than 4); and (ii) some three-fold coordinated triclusters oxygen atoms (i.e., O atoms that are connected to 3 Si or Al network formers) tend to form.…”

“…These well‐known effects arise from the following mechanisms. On one hand, when added to pure SiO 2 , Ca 2+ cations act as network modifiers as they tend to depolymerize the atomic network by breaking some Si–O–Si inter‐tetrahedral joints and, in turn, charge‐compensating pairs of negatively charged NBOs 46 . On the other hand, starting from a calcium silicate glass, newly added Al atoms act as network formers and tend to repolymerize the network by using available Ca 2+ cations to charge‐compensate negatively‐charged four‐fold coordinated AlO 4 units 47–50 .…”

Effect of Ti4+ on the structure of nepheline (NaAlSiO4) glass

“…For example, the viscous flow of silicate melts is, at least, partially dependent on the bond strength, which is altered by additional melt constituents (such as TiO 2 ) and their respective bond lengths and coordination; see (Le Losq et al, 2019) and (Richet and Mysen, 2019) for a review on the subject.…”