The Structural Forms of Quartz and Other Silicas

Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature [1] and pressure [4], as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica poly­morph at atmospheric pressure above 1,470°C. It is built on an fcc lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable poly­morphs have been synthesized. These include vitreous silica, clathrasils, and zeo­lites [2]. Except for stishovite, all these structures are based on frameworks of

Crystallographic Bravais

form lattice

2,000

1,723 (melting point)

Подпись: fcc

Подпись: 1,500 Подпись: 1,470

High cristobalite

High tridymite Hexagonal

Подпись: 500 Подпись: 867 573 Подпись: High quartz Hexagonal

T(°C) 1,000-

Low quartz Hexagonal

0

Подпись: Pressure (kbar) Fig. 3 Phase diagram for the SiO2 system [4]

Fig. 2 Principal silica polymorphs at atmospheric pressure [1]

SiO4 tetrahedra. These silica structures have been determined mainly by X-ray and neutron diffraction methods and, more recently, by Si and Al magic angle spinning solid-state NMR studies.

The various framework silica structures arise from the different ways that the (SiO4)4- tetrahedra are linked into 1-, 2-, and 3-dimensional arrangements. Although the basic tetrahedra are present in most silica structures, the connectivity varies widely.

Both ionic and covalent natures of the Si-O bond contribute to the preference for (SiO4)4- tetrahedron formation in both crystalline and glassy silicas. In addition, each O anion is coordinated by two Si cations, corresponding to corner sharing of the oxide tetrahedra, preventing the close-packing of anion layers and resulting in relatively open structures [5].