Silicates, Silica, and Aluminosilicates

Silica, Si02, is an involatile solid and occurs in many different forms, nearly all of which possess lattice structures constructed of tetrahedral Si04 building blocks, often represented as in structure 13.17. Each unit is connected to the next by sharing an oxygen atom to give Si—O—Si bridges. At atmospheric pressure, three polymorphs of silica exist each is stable within a characteristic temperature 

A piezoelectric crystal is one that generates an electric field (i.e. develops charges on opposite crystal faces when subjected to mechanical stress) or that undergoes some change to atomic positions when an electric field is applied to it such crystals must lack a centre of symmetry (e.g. contain tetrahedral arrangements of atoms). Their ability to transform electrical oscillations into mechanical vibration, and vice versa, is the basis of their use in, e.g., crystal oscillators. 

6-coordinate Si is formed in which the Si—O bond length is 179 pm (compare with the sum of rco.j,(Si)= 118 pm and / cov(O) = 73 pm). This form of silica is more dense and less reactive than ordinary forms. Sihca is not attacked by acids other than HF, with which it forms [SiFg]. Fusion of Si02 with ahcah leads to the formation of silicates. 

Although esters of type Si(OR)4 (equation 13.58) are known, no weh-defined sihcic acid (H4Si04) has been established. 

Normal sihca is only very slowly attacked by alkali, but silicates are readily formed by fusion of Si02 and metal hydroxides, oxides or carbonates. The range of known silicates is large and they, and the aluminosilicates (see later), are extremely important, both in nature and for commercial and industrial purposes. 

When silica glass is heated to 1750K, it becomes plastic and can be worked in an oxy-hydrogen flame. Silica glass apparatus is highly insensitive to thermal shock owing to the low coefficient of thermal expansion of silica. Borosilicate 

The hydration of cement powder is the final step in which it is transformed into a hard material which is insoluble in water. The hydration process is exothermic, and is exemph-lied for 3Ca0 Si02 as follows  

2(3Ca0 Si02) + 6H2O 3Ca0-2Si02-3H20 + 3Ca(OH)2 or, in the notation form used in the cement industry  

For C3S , the enthalpy of hydration is 500kJkg , compared with 250, 850 and 330kJkg for C2S , C3A and C4AF , respectively. The setting period consists of several stages. The addition of water to dry cement initially causes a rapid, and highly exothermic, dissolution of ions and formation of hydrated species. This is followed by a dormant period (different explanations for which have been put forward), and then a final exothermic event which 

Sodium sihcates of variable composition are made by heating sand (which is impure quartz containing, e.g.. 

The Earth s crust is largely composed of silica and silicate minerals, which form the principal constituents of aU rocks and of the sands, clays and soils that result from degradation of rocks. Most inorganic building materials are based on silicate minerals and include natural silicates such as sandstone, granite and slate, as well as manufactured materials such as cement, concrete (see Box 14.8) and ordinary glass (see above). Clays (see Box 14.10) are used in the ceramics industry and mica is used as an electrical insulator. 

an extra singly charged cation must be present to maintain electrical neutrality. Thus, in the feldspar orthoclase, KAlSisOg, the anion [AlSi308] is related to 

---

The object of this study was to apply mid-infrared spectroscopy to zeolite structural problems with the ultimate hope of using infrared, a relatively rapid and readily available analytical method, as a tool to characterize the framework structure and perhaps to detect the presence of the polyhedral building units present in zeolite frameworks. The mid-infrared region of the spectrum was used (1300 to 200 cm"1) since that region contains the fundamental vibrations of the framework (Si,Al) 04 tetrahedra and should reflect the framework structure. Infrared data in similar spectral regions have been published for many mineral zeolites (30) and a few synthetic zeolites (23, 49, 50). There is an extensive literature on infrared spectra of silica, silicates, and aluminosilicates (17). However, no systematic study of the infrared characteristics of zeolite frameworks as related to their crystal structure has appeared. 

For mesoporous silica (pore 0>2O A), successful results were obtained by the synthesis of the MCM family of silicates and aluminosilicates (of Section II.B). Their preparation is achieved by liquid crystal templating and micellar phases (hexagonal MCM-41, cubic MCM-48, laminar MCM-50)36,37 and further developments are currently reviewed38. 

As for the acidity of Al containing MCM-41, ammonia TPD data indicate that it is comparable to that of amorphous silica-alumina, and much lower than the acidity of zeolites such as USY or H-mordenite [115,120,134]. This is consistent with Raman, FTlK and Si NMR data which indicate that despite their long range order, M41S mesoporous silicates and aluminosilicates exhibit essentially amorphous walls [48,49,115]. 

As hydrolysis of the silicates and aluminosilicates continued (equation 1), dissolved sodium ions were continually produced, and acid was consumed. The pH of the water gradually increased. Formation of kaolinite was replaced by formation of montmoril-lonite and finally by production of illite. Silica was more soluble than alumina and as aluminosilicates were attacked by the water a protective coating of A1(0H) formed around the particles. This coating helped transport mineral particulates to the sediment. The reactions indicated by equation 1 were replaced by the family of reactions indicated in equation 2, written using the hydrolysis of albite as an example. 

As noted above, a major fraction of the silica released to natural waters from the weathering of silicate and aluminosilicate minerals, such as the feldspars, remains in solution, where it occurs chiefly... 

As the examples of non-crystalline condensation disperse structures, one can name silicates and aluminosilicates (silica and aluminosilica gels, both hydrated and dehydrated). Silica gels form in the course of chemical reaction between sodium silicate and acid, namely [16,33] ... 

Water diffusion and solubility have been measured for a wide range of glass compositions, including a number of studies for vitreous silica, alkali silicates and aluminosilicates, soda-lime-silicates, and alkali borates and germanates. Early models for diffusion suggested that the diffusivity of water in melts was controlled entirely by the viscosity of the melt, as predicted by the Stokes-Einstein equation. This equation predicts that the diffusivity of a spherical particle in a liquid is inversely proportional to the viscosity. This hypothesis has been shown to be quantitatively incorrect by 4 orders of magnitude, to predict a much... 

Silicates and aluminosilicates, including hydrated and dehydrated silica and aluminosilicate gels, represent other examples of noncrystalline condensation structures. Silica gels form in the course of a sol-gel transition in which the new amorphous phase is formed due to the reaction of sodium silicate with an acid ... 

The rocks in the crust are predominantly (more than 90%) made of silicates (including silica SiOj) and aluminosilicates. A question may be asked why are silicates and aluminum derivatives the predominant components of rocks on the Earth There are two reasons. One is that silicon and aluminum are among the most abundant elements on the Earth (and also in the universe). The second is that silicates form a variety of stable solid structures. 

Hydroxymethyl-2,5-diphenyloxazole used as a liquid scintillator was incorporated via covalentbonding into a porous silica glass (Hamerton, 2000). The sol-gel glasses were found to be highly efficient detectors of 8-radioactivity. Chemiluminescence of luminol was observed in silicate and aluminosilicate sol-gel glass (Akbarian, 1993). 

---