Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 23

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 [(Na_x,Ca_{1 − x})Al_{2 − x}Si_{2 + x}O₈] 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].