Sheet silicate network

Clay minerals consist of hexagonal networks of Si04 tetrahedra. The basal planes of the tetrahedra are in the plane of the sheet silicate network, and their tips point... 

Although many polymers are based on the ability of carbon to form stable long-chain molecules with various functional groups attached, carbon is not unique in this ability. Recall from Chapter 22 the chains, sheets, and networks found in natural silicates, in which the elements silicon and oxygen join together to form extended structures. This chapter focuses on organic polymers, whose... 

Each tetrahedron in the phyllosilicates (sheet silicates) is connected to three other tetrahedrons through an oxide linkage. The phyllosilicates have essentially the same structure as a double chain inosilicate, except that the chains extend into a two-dimensional, sheet-like network, as shown in Figure 12.21. Several important examples of sheet silicates include micas and clays, such as talc and kaolinite. In the clays, such as talc, water molecules are intercalated between the sheets. The water molecules are held in the host by hydrogen bonding and dipole—dipole forces. There are a large number of sites where water can adhere within the sheets, which explains why these materials are so absorbent. While hydrated, clay can be molded into virtually any shape that one desires. When the molecule is heated in a furnace, the intercalated water can often be removed to leave behind a hard and rigid structure. 

The insertion of PEO into mica-like sheet silicates in the reactions of a melt of the polymer with the host Na - or NH4 -exchanged lattice is one of tiie few examples of direct polymer intercalation. Poly(ethylene oxide) is also inserted into the lamellar networks of V205-nH20, MoOs," " - MnPSj, CdPSj, " etc. Thus an aqueous solution of PEO (M = 10 ) reacts with an aqueous gel of V2O5 nH20 to form a composite after removal of water. In this case, the interlayer distance increases from 1.155 to 1.32nm. Alkali metal ions react with PEO to form inclusion compounds. These can also be inserted into layers of ionic silicates, for example, into MMN. The distance between the layers of the PEO-Li salt complexes intercalated into MMN is 0.8 nm, the PEO chain adopts a slightly strained helicoid conformation in these locations. 

In a similar way, the sheets of the two-dimensional network silicates, shown in Figure... 

Silicates also exist in which each silicon atom bonds to one outer oxygen and to three inner oxygen atoms. The result is a linked network in which every silicon atom forms three Si—O—Si links, giving a planar, sheet-like structure. The empirical formula of this silicate is S12 O5. In many minerals, aluminum atoms replace some of the silicon atoms to give aluminosilicates. The micas—one has the chemical formula... 

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

The crystalline form of carbon known as graphite, is composed of stacked hexagonal networks of C atoms. The generalized structure of such a network is identical to that presented in Fig. 2.1 as the ideal array for silicate sheets. The graphite sheet is simpler in that only one atom, C, is located at the... 

As noted in Section 3.1, Si—0 bonds gain high strength from the large electronegativity difference between Si and 0, while remaining covalent and thus directional. Consequently, —Si—0—Si—0— (polysiloxane) chains are very stable indeed. Furthermore, since Si is almost always tetravalent, branched Si—0 chains and, indeed, sheets and three-dimensional networks are common both in naturally occurring minerals and in synthetic silicates, many of which are of primary economic importance (Chapter 7). 

The basic structural units in layer silicates are silica sheets and brucite or gibbsite sheets. The former consist of SiO tetrahedra connected at three corners in the same plane forming a hexagonal network. The tips of the tetrahedra all point in the same direction. This unit is called the tetrahedral sheet. The bijucite or gibbsite sheet consists of two planes of hydroxyl ions between which lies a plane of magnesium or aluminum ions which is octahedrally coordinated by the hydroxyls. This unit is known as the octahedral sheet. These sheets are combined so that the oxygens at the tips of the tetrahedra project into a plane of hydroxyls in the octahedral sheet and replace two-thirds of the hydroxyls. This combination of sheets forms a layer. 

When Si04 tetrahedra are linked into infinite two-dimensional networks as shown in figure 7.3, the empirical formula for the anions is (Si052 )n. Many silicates have sheet structures with sheets bound together by cations that lie between them. 

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