Structural hectorite

This particle is naturally occurring and found around the world. It is easily mined and purified. The reactor for the particle was a volcano. The ash from many volcanoes was spread around the earth during an intense period of activity many millions of years ago. This ash was transformed into clay (montmorillonoids or smectites) by natural processes, into uncharged species (talc and pyrophyllite) and charged species through isomorphic substitution of the crystal structure (hectorite, montmorillon-ite, saponite, suconite, volchonskoite, vermiculite, and nontronite). 

Many varieties of clay are aluminosilicates with a layered structure which consists of silica (SiOa" ) tetrahedral sheets bonded to alumina (AlOg ) octahedral ones. These sheets can be arranged in a variety of ways in smectite clays, a 2 1 ratio of the tetrahedral to the octahedral is observed. MMT and hectorite are the most common of smectite clays. 

There are many other forms of clay including synthetic clays such as laponite, a synthetic hectorite. Suitable tuning of the properties of these systems can produce similar structures. These systems have the advantage of small particle size and a relatively improved level of particle monodispersity over their naturally occurring counterparts and are being increasingly used as rheology modifiers. 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

Chemical analyses and structural formulas of stevcnsite and hectorite samples... 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

Clay minerals are hydrous layer silicates of colloidal dimensions, with most if not all of the individual platy particles in the colloidal range of c. 1 nm-1 pm (van Olphen, 1976 Van Damme et al., 1985). The term phyllosilicate (phyllo = leaf like) is applied to the broad group of hydrous silicates with layer structures. The essentia] components of the phyllosilicate structure are two-dimensional tetrahedra and octahedra of oxygen atoms (or ions). The coordinating atoms (or cations) in the centre of the tetrahedra are for the most part Si, but Al3 or Fe3+ may also be present. The coordinating cations in the octahedra are usually Al3, Mg2+, Fe3 or Fe2. Some clay structures (e.g. hectorite) can be synthesized in a reproducible and relatively homogeneous form. 

The 2 1 layer clays (i.e. with three-sheet layers) include a group of expanding or swelling clays, which comprise the smectites (e.g. montmorillonite, saponite and hectorite) and the vermiculites. The basic structure of a smectite is shown in Figure... 

Two-dimensional structures Clays and layered silicates Kaolinite Al2Si205 (OH)4 Hectorite Naj(Mg3 j Lij )Si40io(OH)2-mH20 Montmorillonite Nuj (Al2 j MGj )Si40io(OH)2-mH20 Niobates and tantalates... 

Figure 3 The layered structure of smectite clays such as montmorillonite or hectorite is represented. The aliuniniun sites are octahedral and the silicon sites are tetrahedral. The interlayer space is filled with water and metal cations...

Hectorite is a natural mineral clay, obtained from altered volcanic ash with a high silica content. It is composed of two tetrahedral layers formed by phyllosilicate sheets and one octahedral layer. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. It is structurally similar to talc but differs by substitution, mainly in the octahedral layer. Common impurities include aluminum, calcium, chlorine, iron, potassium, and titanium. 

Hectorite is an aluminum-free mineral of the smectite type. Isomorphous substitution could occur at tetrahedral silicon sites as well as at the octahedral sites originally occupied by lithium and magnesium. Monitoring the x-ray powder diffraction patterns as a frmction of crystallization time, it was found that the hydrothermal crystallization was complete after 12h at 200°C, independent of the alumina content of the reaction mixture. However, NMR spectroscopy proves that some structural change still occurs after this time period. 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

Figure 1. The hectorite structure, an idealized, unit cell formula being Na0.tr [Lit., Mg,.st](Sig. )0,c(0H,F)t. Key O, oxygen , OH and occasionally fluoride. The tetrahedral sites are occupied mainly by silicon magnesium and lithium occupy the octahedral sites.

---