Figure 4 The abundant beds of black flint present in a 80‐m high limestone cliff of the English Channel at the Pointe du Chicard in Yport (Normandy). Same beds of the Upper Cretaceous used in the past for making flint glass in England on the other side of the Channel. Height visible on the picture: 10 m.
Source: Photo P. Richet.
In passing, one can also note that silica has been biogenically produced relatively late in evolution compared with calcite and aragonite, the main CaCO3 polymorphs, but then met with immense success especially with diatoms. A major reason was the advantages of an amorphous compared with a crystalline substance in terms of optical or mechanical properties for the materials protecting the living organisms (Chapter 8.1); amorphous calcium carbonates do exist, but they serve instead as intermediate reaction steps, which are short lived and thus end up crystallizing [21], which is not surprising as molten CaCO3 is not itself a good glass‐forming liquid. Interestingly, formation of biogenic silica would have first been a way to evacuate toxic Si at too high concentrations from cells. By a twist of evolutionary history, it would have become a protecting device so efficient for organisms [22] that it has since then played a major role in the global ecosystem, causing, for instance, the Si concentrations to be so low in seawater.
1.4.2 A Quantum‐Chemical Factory: The Production of Silica Sand
Although glassmaking would have been possible without sand, it is unlikely that flint would have led to the invention of glass as it requires thorough grinding to become a reactive raw material. Regardless of grinding costs, it is also doubtful that flint would have been a silica resource widespread and convenient enough for an expanding glass industry. The fundamental importance of silica sand thus remains undisputed. Geologically, sand is produced via the weathering of granite and related SiO2‐rich igneous rocks. The most abundant rock of the Earth's crust, granite is made up of quartz and alkali [(Na,K)AlSi3O8] and plagioclase [(Nax,Ca1 − x)Al2 − xSi2 + xO8] feldspars. Whereas feldspars progressively transform into clay under the action of meteoric waters, quartz resists and accumulates as sand either on the spot or downstream.
The very presence of quartz at the Earth's surface appears to be a clear geochemical anomaly, however, which thus deserves some explanation. With typical 75 wt % SiO2, the melts from which granite crystallizes represent the end products of magma differentiation (Chapter 7.2). Owing to their very high viscosities, they rarely rise up to the Earth's surface to erupt as obsidian flows but crystallize slowly instead at some depth to yield large-grained rocks. These melts are the last produced after partial crystallization of primary magmas, which form themselves deep in the Earth's mantle by partial melting of SiO2‐poor, MgO‐rich rocks (~45 wt % for both oxides, along with ~7 % FeO, 2 % Al2O3, 1 % CaO, and a few ‰ at most alkali oxides). Because oxygen bonds more strongly with silicon than with the other elements (Table A.1), one might think that SiO2‐rich minerals should be the most refractory. As a result, the SiO2 content of primary magmas should be lower than that of their source rock and decrease further through partial crystallization on their way up to the Earth's surface. Such a trend is opposite to the SiO2 increase observed. It is in contrast consistent with the fact that cristobalite, the high‐temperature polymorph of SiO2 at room pressure, is less refractory than lime (CaO), periclase (MgO), and even forsterite (Mg2SiO4) whose melting temperatures are about 600, 800, and 175° higher than the 2000 K of cristobalite, respectively.
The paradox lies in the fact that bond strengths are usually considered within the framework of ionic forces, which are by definition nondirectional. Now, directionality is an inherent feature of Si–O bonding in view of its markedly covalent character. Because electron delocalization through polymerization and creation of Si–O–Si linkages is not large enough to constrain geometrically the arrangements of the SiO4 tetrahedra, the same energy variations are, for instance, caused in H6Si2O7 clusters by a small 0.02 Å change of the Si–O bond length and by a large 20° modification of the O–Si–O inter‐tetrahedral angles (Figure 5). Bending of these linkages is thus so easy that configurational rearrangements take place without involving much energy [24]. The fact is most simply illustrated by the transitions of α‐quartz and α‐cristobalite to their dynamically disordered β‐forms near 573 and 250 °C, respectively. Hence, fusion of these minerals does not require the breaking of bonds involved in ionic crystals. The SiO2 enrichment and resulting quartz crystallization induced by magma differentiation are thus mainly driven by the sp3 hybridization of silicon orbitals, which causes largely polymerized crystals to melt at temperatures much lower than would be expected from the Si–O and Al–O bond strengths [24]. In other words, the existence of silica sand originates in a quantum‐chemical effect, without which glassmaking would not have existed.
Figure 5 The strong contrast between the potential energy changes induced by variations of Si–O distances and S–O–Si angles indicated by the calculated surfaces of constant energy of H6Si2O7 clusters.
Source: After [23].
2 Some Basic Concepts of Glass Science
2.1 From Metastability to Relaxation
The silica issue illustrates how answers to apparently simple problems can require in‐depth analyses for which theoretical concepts presented in various chapters of the Encyclopedia should prove useful. To help readers whose knowledge of the glassy state is minimal, however, the rest of this introduction will be devoted to a brief presentation of some basic concepts pertaining to glass and nonequilibrium systems, which will thus not need to be commented upon in specific chapters.
In preamble, it would be useful to define precisely what a glass is before discussing any of its properties. In accordance with its intrinsically disordered nature, however, glass might be pleasantly defined as a material that is difficult to define in an unambiguous or fully consistent manner. In Chapter 10.11, a glass is nonetheless defined as a macroscopically homogeneous amorphous solid whose properties (physical, chemical, or structural) vary with its preparation conditions. Usual definitions differ depending on whether the emphasis is put on the disordered atomic structure of the material or on the existence of a glass transition separating a solid material at lower temperature from a supercooled liquid at higher temperatures. Because glass structures depend on the type of system considered, they are described in widely different ways for oxides, metals, or organic polymers so that they do not lend themselves to a brief, general presentation.
Although a glass transition cannot always be observed, its phenomenology and its implications on glass properties are in contrast common not only to all glass‐forming liquids, but also to partially disordered systems such as plastic crystals. In view of their dual practical and theoretical importance, the main features of the glass transition will thus be summarized here in a qualitative way. Without making any reference to recent advances in the field, the purpose is simply to describe the phenomenology of vitrification and its effects on physical properties, to introduce some
