Brucite layer

Figure 2. The brucite structure of Ni(OH), (a) hexagonal brucite layer, in which the small circles are the Ni atoms and the large circles the O atoms and alternate O atoms are below and above the plane of the Ni atoms (b) stacking of the planes showing the orientation of the O-H bonds.

In the pyroaurite structure the brucite layers are cationic. However, on oxidation the resultant brucite layers in y - NiOOH are anionic. To preserve electroneutrality, cations and anions are exchanged in the intercalated layer during the oxidation-reduction process. This is illustrated in Fig. 4. In the case of Mn-substituted materials, some Mn can be reduced to Mn(II). This neutralizes the charge in the brucite layer this part of the structure reverts to the P - Ni(OH)2 structure and the intercalated water and anions are expelled from the lattice. With this there is a concomitant irreversible contraction of the interlayer spacing from 7.80 to 4.65A [72]. 

Fig. 3. A tetrahedral layer in which all the tetrahedra point in the same direction. Fig. 4. A complete layer of octahedra (brucite layer).

Hydrotalcite clays, for example, are built up of positively charged brucite layers, for reviews see Cavani et al. (1991). Upon calcination they become active as solid bases in e.g. aldol and Knoevenagel condensations (see Fig. 2.26) (Fgueras et al., 1998 Corma and Martin-Aranda, 1993 Climent e/a/., 1995). 

In terms of local order about the cations, it is generally argued that according to Pauhng s rules [257], M cations should not occupy adjacent sites (unless accompanied by vacant cation sites, i.e. a gibbsite unit within a brucite layer [56]) the corollaries of this are that the minimum possible f jg 2 and that when is exactly equal to 2 there is also... 

Three-layer alternating with brucite Brucite layer positively charged [some Al(lll) replacing M(ll)], partially balances negative charge on Td-Oh-Td (mica) layer Low CEC, nonswelling... 

The CEC of clay minerals is partly the result of adsorption in the interlayer space between repeating layer units. This effect is greatest in the three-layer clays. In the case of montmorillonite, the interlayer space can expand to accommodate a variety of cations and water. This causes montmorillonite to have a very high CEC and to swell when wetted. This process is reversible the removal of the water molecules causes these clays to contract. In illite, some exchangeable potassium is present in the interlayer space. Because the interlayer potassium ions are rather tightly held, the CEC of this illite is similar to that of kaolinite, which has no interlayer space. Chlorite s CEC is similar to that of kaolinite and illite because the brucite layer restricts adsorption between the three-layer sandwiches. 

If layers of silicon tetrahedra are condensed on both sides of a hydrargillite layer, a substance is obtained having the composition AU3i Oio(OH)t. This is the clay mineral pyrophyllite. The substance MgaSi40io(OH) obtained similarly from a brucite layer (Fig. 7-10) is the mineral talc. Both of these substances, involving the loose superposition of neutral layers, are very soft, with extreme basal cleavage.70... 

Kruissink et al.56 confirmed the earlier observation of Beecroft et al.s5 that the decomposition of the precipitate occurs in two steps. In the first, between 130 and 230 UC, depending on the anion, the molecular water is removed from the interlayer and there is a slight decrease in the interlayer spacing. The brucite layer is destroyed at a higher temperature ( 350°C) with the evolution of water (from the OH-ions) and for example, C02 or N02, depending on the anions in the coprecipitate. 

Chloritoid. Optical spectra of chloritoids, again studied mainly on account of the Fe2+ —> Fe3+ IVCT band at 16,300 cm-1 (Faye et al., 1968 H lenius et al., 1981), also contain features assignable to CF transitions in Fe2+ ions. These cations are located in the M1B positions in brucite layers which are surrounded by four OH" ions and two trans- non-bridging oxygens belonging to isolated [Si04] tetrahedra in silicate layers in the chloritoid structure. The two absorption bands at 10,900 cm-1 and 8,000 cm-1 yield approximate values of A0 = 9,000 cm-1 and CFSE = 4,050 cm-1, respectively, for the Fe2+ ions. 

Subsequently, in 1999 the same group showed that the activity of the ruthenium hydrotalcite was significantly enhanced by the introduction of cobalt(II), in addition to ruthenium(III),in the Brucite layer [115]. For example, cinnamyl alcohol underwent complete conversion in 40 min in toluene at 60 °C, in the presence of ruthenium/cobalt hydrotalcite, compared with 31% conversion under the same conditions with ruthenium hydrotalcite. A secondary aliphatic alcohol, 2-octanol, was smoothly converted into the corresponding ketone but primary aliphatic alcohols, for example, 1-octanol, exhibited extremely low activity. The authors suggested that the introduction of cobalt induced the formation of higher oxidation states of ruthenium, for example, Ru(IV) to Ru( VI), leading to a more active oxidation catalyst. However, on the basis of the reported results it is not possible to rule out low-valent ruthenium species as the active catalyst in a hydridometal pathway. The results obtained in the oxidation of representative alcohols with ruthenium hydrotalcite and ruthe-nium-cobalt-hydrotalcite are compared in Table 5. 

In a private communication (1979) Drits has described a (basically) II variety, tochilinite in, which occurs in the tochilinite specimens together with the other tochilinite types. The mutual orientation of the sulphide and brucite layers in this variety is the same as in tochilinite I. Besides having a chess-board distribution of partly occupied Fe te-trahedra in the sulphide layers, tochilinite III displays statistical disorder of octahedral cations in the brudte-like layers. The two lattices have dimensions hsuiphide = CsuipUde =... 

The ideal trioctahedial ddorites without oidering in the brucite layers are SS structures with stackiiig disorder... 

Al " ions fill 2/3 of octahedral sites in the structure of bayerite layer composed of two layers of hydroxyl ions the coupling of layers is provided so that each OH" group of one layer is located opposite to the OH group of the next layer. cations occupy all the octahedral sites in brucite layer each layer adjoins the other one due to OH" groups being located in triangular sites of adjacent layer. Therefore, the thickness of layer packing of brucite type is less than that of bayerite type. 

The formation conditions, the composition (Ni0 Al203 = 4-14) and the characteristic basal distance d = 0.8 nm of the phase agree with another model of layer packing a continuous brucite layer is a leading layer while bayerite layer possesses discrete structure, i.e., it is not tightly packed with Al cations. 

Tomilov N.P., Ivashina V.S., Berger A.S. About double hydroxides of nickel and aluminium of layered stmcture with leading bayerite or brucite layers. 11. Double hydroxides of nickel and aluminium at sotphon equilibrium in the system sodium hydroalumocarbonate - nickel nitrate solution. Izvestiya SO AN SSSR, ser. khim. nauk 1984 1 48-52. 

Its structure is three-layered, i.e., it is formed from two hexagonal silica layers and one central brucite layer between them. There are four types of chemical bonds in talc structure ionic Mg-0 bonds, covalent, partially ionic Si-0 and H-0 bonds hydrogen bonds between OH and O-Si, and van der Waals bonds between the layers. Van der Waals bonds are the weakest among all bonds. Therefore, early stages of grinding are accompanied by mechanochemical dehydration which can be represented by the equation ... 

Mg(OH)g octahedra FIGURE 14.5 The structure of LDHs (a) brucite layer (b) LDH layer. 

The composition of talc varies depending on its source. The most important factor is the amount of tremolite present. In the USA, for instance, Montana talcs are considered to be asbestos and tremolite free. The California plate-like talcs contain minor amounts of tremolite (less than 3%), whereas hard talcs contain between 5 to 25% tremolite. Some industrial talcs mined in upper New York State contain 25 to 50% tremolite. The other important component in its composition is water which is chemically combined in the magnesium oxide or brucite layer. Figure 2.62 shows the molecular structure of talc. Talc may lose this water only on... 

If the octahedral layers are made up of three Mg + coordinated with six or OH, then all the cation sites are occupied by Mg. The layers are then termed trioctahedral, or brucite layers, because they are compositionally equivalent to the mineral brucite, Mg(OH)2. 

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