Sheet structures silicates

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

Figure 1.45 Top view of a silicate sheet structure resulting from sharing three corners of the Si04 tetrahedra. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Plan As shown in Figure 22.33(c), the silicate sheet structure has the simplest formula Si205 . We first add Mg to give the proper Mg Si ratio. We then add OH ions to obtain a neutral compound... 

Figure 12.13 Schematic representation of the two-dimensional silicate sheet structure having a repeat unit formula of (81205) .

One of the most common clay minerals, kaolinite, has a relatively simple two-layer silicate sheet structure. Kaolinite clay has the formula Al2(Si205)(0H)4 in which the silica tetrahedral layer, represented by (81205) ", is made electrically neutral by an adjacent Al2(OH)4 layer. A single sheet of this structure is shown in Figure 12.14, which is exploded in the vertical direction to provide a better perspective on the ion positions the two distinct layers are indicated in the figure. The midplane of anions consists of... 

These silicate sheet structures are not confined to the clays other minerals also in this group are talc [Mg3(Si205)2(0H)2] and the micas [e.g., muscovite, KAl3Si30io(OH)2], which are important ceramic raw materials. As might be deduced from the chemical formulas, the structures for some silicates are among the most complex of all the inorganic materials. 

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

The crystal structures of all the minerals in the serpentine group contain the same basic building blocks. The basic unit is composed of a silicate sheet of composition (Si205) ", in which three of the O atoms in each tetrahedron are shared with adjacent tetrahedra (Fig. 2.2A), and a nonsilicate sheet of... 

The micas are characterized by extended silicate sheets rather than chains. Their structures resemble the serpentine mineral group in that they are dom-... 

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... 

In the structures cited in Table 12.3, except for pure silicon dioxide, metal ions are required for overall electrical neutrality. These metal ions are positioned in tetrahedral, octahedral, etc. positions in the silicate-like lattice. Sometimes they replace the silicon atom. Kaolinite asbestos has aluminum substituted for silicon in the Gibbosite sheet. Further, sites for additional anions, such as the hydroxyl anion, are available. In ring, chain, and sheet structures neighboring rings. 

Many ceramics are partially polymeric in structure. These include the new superconductive materials that exist as polymeric sheets connected by metal ions similar to many of the silicate sheets. 

Several insulating inorganic solids possessing sheet structures, for example, silicates belonging to the pyrophyllite family (Thomas, 1982), and acid phosphates (Alberti Constantino, 1982 Clearfield, 1981) of some tetravalent metals form intercalation compounds with a variety of donor molecules in these cases, intercalation does not involve a redox process, unlike in the cases of transition metal chalcogenides and... 

The micas have layer structures in which silicate sheets are combined with aluminate units the aluminum ions can be octahedrally as well as tetrahedrally coordinated. For example, the mica muscovite contains both octahedral and tetrahedral Al3+ ... 

Fluorosilicates. Compared to the simple silicates, these crystals have more complex chain and sheet structures. Examples from nature include hydrous micas and amphiboles, including hornblende and nephrite jade. In glass-ceramics, fluorine replaces the hydroxyl ion fluorine is much easier to incorporate in glass and also makes the crystals more refractory. Four commercial fluorosilicate glass-ceramic compositions and their properties are listed in Table 2. 

Low-temperature sheet structure silicates with a high Fe2+ content seem to be restricted to the 1 1 minerals. The Fe2+ content of the octahedral sheet of the 2 1 clays is seldom larger than 0.6 per 3.0 positions (less than 0.3 for most samples). Few low-temperature 2 1 clays have enough A1 substitution in the tetrahedral sheet to adjust its size to that of the large octahedral sheet. Substitutions of this magnitude, 1 A1 per four tetrahedral positions, at low temperatures, are favored more by the 1 1 than the 2 1 arrangement. Presumably, the lack of a layer charge and, therefore, the need for the tetrahedra to rotate to accommodate an interlayer K, and the fact that the tetrahedral sheet is sandwiched between two octahedral sheets allow interlayer size adjustments to be made more easily in the 1 1 than in the 2 1 clays. 

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

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. 

Figure 7.3 Sheet silicate anion structure idealized.

In the case of the crystalline silicates an approach which takes account of the partly covalent character of the Si-O bond is helpful. The [SiCL]4- tetrahedron is taken as a basic building unit, and in most of the silicates these tetrahedra are linked together in an ordered fashion to form strings as in diopside (MgCa(Si03)2), sheet structures as in clay minerals, or three-dimensional frameworks as in quartz and the feldspars. Within these frameworks isomorphic replacement of one cation type for another is extensive. For example, the replacement of Si4+ by Al3+ is common, with the necessary lattice charge balance being maintained either by the incorporation of interstitial cations such as Na+... 

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