Stacks, aluminosilicate layers

Pentasils constitute a well known family of porotectosilicates. The framework structure of the members of this family, based on five membered rings of tetrahedra, can be described in terms of two different stackings of layer pairs related by an inversion center (i-type) and mirror simmetry (o-type), respectively ( 1 ). ZSM-5 aluminosilicate, the parent borosilicate BOR-C ( 2 ), and the pure silica analog Silicalite-1 ( 3 ), represent the most important... 

Clay minerals are hydrous layered aluminosilicates. The aluminosilicate layer is composed of two types of sheets octahedral (0) and tetrahedral (T). Each sheet is composed of parallel planes of atoms, either oxygen or aluminum (silicon) in an alternating sequence. The large variability in the properties of clay minerals is related to the stacking sequence of the two basic types of sheets, T and 0, and to the uncompensated charge that develops by isomorphous substitution of unevenly charged cations. 

The state of research on MMT, which among the family of layered aluminosilicates is the most common mineral used in polymer/nanoclay hybrid materials, will be reviewed. MMT is constituted of stacks of hydrated aluminosilicate layers, whose crystal structure is composed of an octahedral alumina sheet sandwiched between two tetrahedral silica sheets (Figure 6A) The layers are separated by galleries where cations (e.g., Na+, K+) are present to balance the negative charge of the aluminosilicate sheets arising from isomorphic substitution of A1 or Si with other metals. 

TEM microphotographs of the so-obtained samples show the typical morphology of intercalated nanocomposites. Furthermore with organo-modified fillers, in addition to small stacks of intercalated clay particles, a limited amount of completely exfoliated/delaminated silicate sheets coexists with the intercalated aluminosilicates layers. 

Figure 4.6 Layer structure of kaolinite. Edge view showing the aluminosilicate sheets, and the stacking arrangement of these sheets. (After Evans, 1966 Figure 11.11, by permission of Cambridge University Press.)...

FAU Faujasite M29 [Al58Si134 0384] Aluminosilicate 240H2O (M = Na2, Ca, Mg) High silica AlP04-based 3D 12-rings 7.4 A fau sod d6R ABC stacking of puckered sodalite cage layers... 

Pyrophyllite is the simplest layer aluminosilicate in which two tetrahedral Si04 layers are condensed on to the octahedral A106 layer to produce a three-sheet layer, the composition of the unit cell being [Al2(OH)2(Si205)2]2. Another ideal structure in which A1 is replaced by Mg is that of talc. In both cases the three-sheet layer is electrically neutral and the layers are stacked in the ABAB... sequence. Because of the cohesive strength of this ideal structure, neither pyrophyllite nor talc occurs in the form of the very fine particles which generally characterize clay minerals. 

Layered aluminosilicates include the micas and clay minerals they share the characteristic of having alternating layers of a gibbsite-like aluminol sheet and a silicon oxide (silanol) sheet, but differ in the stacking of these sheets, the presence of additional cations in various structural locations, and the amount of structural water and hydroxyl groups. By virtue of analogy with amorphous silica and quartz, the silanol sheets are not... 

A detailed analysis of the X-ray powder diffraction pattern of two borosilicates was used to develop a model for their structures (31). The material called BOR-C is reported to have the MFI structure containing regular stackings of pentasil layers related by inversion centers. On the other hand, the BOR-D material was found to have the MEL structure. It was structurally disordered, consisting mainly of pentasil layers related by inversion centers as well as layers related by mirror planes. The disordered structure model for BOR-D also was proposed for the aluminosilicate ZSM-11 and the silicalite-2 analog (32). 

A woodpile structure, a similar layered structure with an unidentified space group similar to the 070 space group (Fddd), was observed in a BCP/aluminosilicate assembly [9]. In contrast to the perforated lamellar structure, zig-zag cylinders are stacked layer-by-layer (see Fig. 2). Although a zig-zag cylinder is usually energetically less stable than a parallel cylinder due to larger interfacial energy, the relaxed chain conformation of polymer chains, leading to a reduced entropic penalty, may allow the zig-zag cylinder formation. 

---