Silicates with framework structures

The structural complexity of the 3D framework aluminosilicates precludes a detailed treatment here, but many of the minerals are of paramount importance. The group includes the feldspars (which are the most abundant of all minerals, and comprise 60% of the earth s crust), the zeolites (which find major applications as molecular sieves, desiccants, ion exchangers and water softeners), and the ultramarines which, as their name implies, often have an intense blue colour. All are constructed from Si04 units in which each O atom is shared by 2 tetrahedra (as in the various forms of Si02 itself), but up to one-half of the Si 

Gays are an essential component of soils, to which we owe our survival, and they are also the raw materials for some of mankind s most ancient and essential artefacts pottery, bricks, tiles, etc. Clays are formed by the weathering and decomposition of igneous rocks and occur typically as very fine panicles e.g. kaolinite is formed as hexagonal plates of edge. 1-3 p m by the weathering of alkaline feldspar 

China clay or kmlin, which is predominantly kaolinite, is particularly valuable because it is essentially free from iron impurities (and therefore colourless). World production in 1991 was 24.7M1 (USA 39%, UK 13%, Colombia, Korea and USSR 7% each). In the USA over half of this vast tonnage is used for paper filling or paper coating and only 130000 tonnes was used for china, crockery, and earthenware, which is now usually made from ball clay, a particularly fine-grained, highly plastic material which is predominantly kaolinite together with clay-mica and quartz. Some 800000 tonnes of ball clay is used annually in the USA for white ware, table ware, wall and floor tiles, sanitary ware, and electrical porcelain. 

unlike Che micas, consists of electrically neutral layers without the interleaving cations. It is valued for its softness, smoothrtess and dry lubricating properties, and for its whitertess, chermcal inertness and foliated structure. Its most important abdications are in ceramics, insecticides, paints and paper manufacture. The more familiar use in cosmetics and toilet preparations accounts for only 3% of world production which is about 5 Mt per annum. Half of this comes from Japan and the USA. and other major producers are Korea, the former Soviet Union, France and China. Talc and its more massive mineral form soapstone or steatite arc widely distributed throughout the world and many countries produce it for domestic consumption either by open-cast or underground mining. 

Most feldspars can be cla.ssified chemically as members of the ternary system NaAlSi30s-KAlSi30s-CaAl2Si20 . This is illustrated in Fig. 9.14, which also indicates the names of the mineral phases. Particularly notable 

Minerals Yearbook Vol. I, 1991, US Dept of the Interior, Bureau of Mines. Washington DC. pp, 403-4.5 (1991). 

The possible types of 3D structure have been noted earlier in this chapter, namely  

We shall be concerned here exclusively with frameworks in which tetrahedra share all four vertices. In all these structures some (often about one-half) of the tetrahedral positions are occupied by A1 (rarely by Be), and positive ions are present to neutralize the negative charge of the (Si,Al)02 framework. These 

Layeis of tetiahedia which, by fuithei linking through O atoms, form the framework structures of (a) paracelsian and (b) the common felspars. Open and shaded triangles represent tetrahedra the fourth vertices of which project either above or below the plane of the paper. 

We noted earlier the change of a synthetic sanidine (KAlSisOs) under high pressure (120 kbar, 900°C) to the hollandite structure, in which Si is 6-coordi-nated.  

A characteristic feature of zeolites is the existence of tunnels (or systems of interconnected polyhedral cavities) through the structures, and we may therefore expect three main types in which the tunnels are parallel to (a) one line, giving the crystals a fibrous character, (b) two lines and arranged in planes, so that the crystals have a lamellar nature, or (c) three non-coplanar lines, such as cubic axes, when the crystals have no pronounced fibrous or lamellar structure. The most symmetrical 

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Silicates with framework structures—felspars and zeolites. 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

Langasite is a kind of materials with framework structure without inversion symmetry i. quartz and BeO are also framework structure which is formed mainly by a covalent bond such as Si04, AIO4, Zn04. Especially, silicates including lanj ite make many framework structures by connection of Si04 tetrahedra as cydo-, ino-, phyllo-, and tecto-silicates. These framework structures are noticed recently for appHcations on many kinds of properties such as zeolite for optical properties by absorption of special compwimds. 

Fig. 25. (a) Nepheline structure as an example of alumino-silicates with framework, (b) Crystal structure of aluminate calcium C12A7. New-type superior properties such as superconductor were designed from this structure. 

B. Baumgartner, G. Muller Framework distortions by large ions in MAlSi20e alumino-silicates with keatite structure , Eur. J. Mineral. 2, 155-162 (1990)... 

We saw that zeolite A has a Si/Al ratio of 1. Some zeolites have quite high Si/Al ratios zeolite ZK-4 (LTA), with the same framework structure as zeolite A, has a ratio of 2.5. Many of the new synthetic zeolites that have been developed for catalysis are highly siliceous ZSM-5 (MFI) can have a Si/Al ratio which lies between 20 and oo (the latter, called silicalite (see Section 7.2.2) being virtually pure Si02) this far outstrips the ratio of... 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Coordination polymers, open-framework materials, and hybrid compounds built up with various anions have been described in the literature. The most common anions employed in open-framework structures are silicates and phosphates.1-3 Metal carboxylates with a variety of structures and dimensionalities have also been described in the recent literature.4 5 In recent years, other oxyanions such as sulfate, selenate, selenite, and tellurite have also been employed to design these structures.6,7 Surprisingly, coordination polymers... 

In the previous three sections, we have not attempted to differentiate between dense and open hybrid framework structures. As with inorganic silicates and aluminosilicates, there is no fundamental chemical difference between the dense and open hybrid structures, though their properties and applications are often quite distinct. There are also differences in the synthetic strategies that are needed to synthesize them, and in section 4 we shall discuss some of the reaction... 

Zeolites, sometimes called porotectosilicates, are the most important framework silicates. A zeolite may be defined as an aluminosilicate with a framework structure... 

Zeolitic catalysts are constituted of [TO4] tetrahedrons which are assembled by sharing oxygens to form one of several topologically distinct framework structures. In the case of alumino-silicates, T=Si with some A1 substitutions, while in silico-alumino-phosphates (SAPO), T sites are occupied by alternating A1 and P atoms with some Si substitutions. The substitutions destroy the electro-neutrality of the framework and necessitate the presence of compensating cations. While mono- or bivalent metallic cations usually occupy crys-tallographically well-characterized sites in the cavities, protons are chemically bonded to framework oxygens. 

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