Tetrahedra Silicates

Describe the structures of a silicate in which the silicate tetrahedra share (a) one O atom (b) two O atoms. 

What is the empirical formula of a potassium silicate in which the silicate tetrahedra share (a) two O atoms and form a chain or (b) three O atoms and form a sheet In each case, there are single negative charges on the unshared O atoms. 

A bar of talc feels like a bar of soap which is why it is often called soapstone. Its exceptional softness (it is the softest of the Mohs minerals) is a direct result of its unusual crystal structure. This consists of sheets of silicate tetrahedra without metal ions between the sheets. Thus the sheets are bonded only by London polarization forces. The latter are particularly weak because silicate tetrahedra have relatively small polarizabilities. 

The fundamental structural unit of all crystalline silicates is a tetrahedron that has an anion at each of its four corners and a Si" cation in the center. The silicate tetrahedra can be arranged in a variety of ways giving rise to a great variety of crystalline silicate minerals. Most of these arrangements involve some degree of sharing of the corner 0 anions between adjacent silicate tetrahedra. 

Biogeochemists use the terms dissolved silica (DSi) or dissolved silicate to collectively refer to all of the dissolved silicon. Silicic acid exhibits tetrahedral geometry with the silicon atom at the center and a hydroxyl group occupying each of the four corners. This structure is similar that of the mineral silicate tetrahedra (Figure 14.3c). Chemical weathering of the silicate minerals is the major source of DSi to the ocean, giving rise to the term dissolved silicate, which is usually abbreviated to just silicate. ... 

Some of the diversity that characterizes the properties and compositions of the silicate minerals stems from the ability of the aluminum ion (Al ) to substitute for silicon in the tetrahedral unit. When silicate tetrahedra in a mineral are replaced by aluminum-containing tetrahedra, concomitant changes occur in the size of the tetrahedron (usual Si—O bond length = 0.160 nm. A1—O bond length = 0.178 nm) and in the cations or protons that balance the tetrahedral unit charge. Regular substitutions with distinct chemistries and structures lead to the formation of groups of discrete minerals called aluminosilicates. 

Fig. 11.6. Silicate networks, (a) silicate tetrahedra linked to form a framework, (b) a simplified four-connected network representing the silicate framework.

In agreement with this rule, it is observed that silicon tetrahedra tend to share only corners with other silicon tetrahedra or other poly-hedra. No crystal is known in which two silicon tetrahedra Bhare an edge or a face, and in most of the silicate structures only corners are shared between silicate tetrahedra and other polyhedra also. This rule... 

The microscopic and macroscopic properties of asbestos fibers stem from their intrinsic, and sometimes unique, crystalline features. As with all silicate minerals, the basic building blocks of asbestos fibers are the silicate tetrahedra which may occur as double chains (SiO)6l 14, as in the amphiboles, or in sheets (SiO)i-104. as in chrysotile. 

Chrysotile. In the case of chrysotile, an octahedral hrncite layer having the formula (Mg04(0H)g)4 6 is intercalated between each silicate tetrahedra sheet. 

Amphiboles. The crystalline structure common to amphibole minerals consists of two ribbons of silicate tetrahedra placed back to back. 

Experimental trends in Si shielding observed experimentally arise from variations in the coordination number (i.e. the number of atoms in the 1st coordination sphere), the extent of polymerization of the silicate tetrahedra, the degree of replacement of one net-work forming cation by another (e.g. coupled Na+, Al+3 for Si+4 substitution), the size of the rings of tetrahedra present and the Si-O-Si angles (1,2). Similar trends are seen in gas-phase molecules, species in aqueous solution and in both crystalline and amorphous solids. Polarized double-zeta basis set Hartree-Fock level calculations using small molecular cluster models reproduce these trends semiquantitatively, as we will show. 

The disparity in size of the aluminate and the silicate tetrahedra must be the reason why, at least for some frameworks, the range of Si/Al ratios, and therefore the extent of the post-synthesis isomorphous substitution of Al for Si is limited (27). For boron, with the ionic radius of 0.23 A as compared with 0.51 A for aluminium, the disparity in size is even greater (2). Quantum chemical calculations predict that the tetrahedral coordination of aluminium is favoured in comparison with BO4 groupings (32.33). An attempt to insert boron into the framework of ferrierite (34). a structurally related zeolite, was unsuccessful. 

Figure 5.3 The amount of order in silicates can vary dramatically. A. The crystalline backbone structures for olivine, pyroxene, and quartz. The charge of the silicon tetrahedra is neutralized by metal cations in olivine and pyroxene. B. Silicate melts contain a mix of unaligned crystalline structures with metal cations randomly distributed in the melt. C. Chaotic condensates have not formed silicate tetrahedra rather, they appear more like a frozen gas state. These materials are typically under-oxygenated and contain more metals than a glass. Annealing supplies the chaotic silicate with the energy needed to rearrange into the more stable silicate tetrahedra. D. The gas phase largely consists of SiO. Metals are typically present as atoms or simple monoxides while excess oxygen can be found as OH (Nuth et al. 2002).

A Si NMR study (BlOO) showed that for cement, as for CjS pastes, the content of Q° silicate tetrahedra decreases with time and that those of Q and later of tetrahedra increase. After 180 days, the degree of hydration, estimated from the intensities of the NMR peaks, was approximately 90%. These results are consistent with those obtained by the TMS method, and suggest that the hydration products present after 180 days contain at most only a small proportion of monomer. The possible effects on the silicate anion structure of drying, whether during hydration as a result of localized water shortage or subsequently, were considered in Section 5.3.2. 

Figure 2.53. The crystal structure of tobermorite, viewed along the be plane. The amorphous gel produced during cement formation likely contains defects such as missing/disordered silicate tetrahedra and/or water sites. Reprinted from Chem. Geol. 2000, 167,129, Copyright 2000, with permission from Elsevier.

Rings containing three silicate tetrahedra are observed in cyclosilicates (e.g., benitoite). The relative prevalence of three-, four-, and higher-membered rings has been discussed by Chakoumakos et al. (1981), and we will return to this topic in Chapter 5. 

FIG. 31-4. A portion of an infinite layer of silicate tetrahedra, as present in talc and other minerals with layer structures. 

FiG. 31-6. A double chain of silicate tetrahedra, extending from one end of an asbestos crystal to the other end. This double chain, called the tremolite chain, has the composition (Si40j )3j,. 

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