Brucite-type layers

It was found that 23% of Co " was oxidized to Co ". The conversion of a brucite-like material to an LDH structure was observed in response to an increase in O2 partial pressure. Anionic species, such as NO3, were intercalated between the brucite-type layers of LDHs and the extent of anion intercalation increased as a function of O2 partial pressure. 

TG analyses confirm the absence of side phases [18,26] and show the presence of only two weight losses the first, at about S03K, is attributable to the elimination of the water molecules firom the interlayers, while the second, at about 673K, is due to the dehydroxylation of the brucite-type layers and to the elimination of the carbonate anions from the interlayers [7], As magnesium content increases, this latter loss is displaced towards the higher temperatures and is only a partial one, on account of the higher affinity of the Mg " " ions for CO2 [32-34]. This phenomenon is also observed for the samples calcined at 923K for 14h the FTIR spectra of these samples confirm the presence of residual carbonates. 

Incomplete oxidation and carbonation or a lower iron concentration may lead to coalingite-K in which three brucite-type layers occur between each carbonate/water layer. The idealized formula for the fully oxidized mineral thus is... 

Fig. 142. The monoclinic structure of lithiophorite (Li, Al)(0H)2 Mn02- The OH ions of the brucite-type layer (Li, Al) (OH)2 are stippled.

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

Makovicky Hyde (1981) have reviewed incommensurate misfit structures in graphite intercalation compounds, brucite-type compounds, sulphides and related layered systems. A simple two-dimensional incommensurate system is provided by graphite with adsorbed rare gas monolayers. At low densities and high temperatures. 

The hydrotalcite clays and their synthetic analogues are a large class of anion exchangers [3,87-90], The structure of hydrotalcite (see Section 2.6.2) can be envisaged as being made of brucite-type octahedral layers, where the M(III) cations partially substitute for M(II) cations [88] this replacement... 

Fig. S. Types of octahedral layers observed in non-commensurate layer structures A, a brucite-like layer with undifferentiated octahedra B, the brudte-Uke layer in tochilinites, with the octahedra preferred by Fe indicated by stippling C, the SnS2-like layer in cylindrite, with the octahedra preferentially occupied by Sb and/or Fe indicated by stippling D, the FeCls layers (occupied octahedra stippled) and, E, the M0CI5 layers (empty octahedra stippled) in the metal chloride-graphite intercalates F, the two orientations (to be superimposed) of the octahedral layers in Phase 1 of Organova et al. ...

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. 

Allmann, R. (1970). Double layer structures with layer ions Mefllq, Mefllly (OH)2]- + of brucite type. Chimia 24, 99-108. 

An additional structural variant for clay minerals is the chlorite-type structure. Chlorites are similar to the pyrophyllite-type structures with two tetrahedral sheets and an octahedral sheet making up each layer. Instead of alkali or alkaline earth interlayer cations, chlorites contain a brucite (Al-Mg hydroxide) layer between successive pyrophyllite-type layers [18]. 

Mostly focused on cationic clays, and particularly on montmorillonite and hectorite, smectite-type layered silicates and clay-based nanofillers have recently been extended to the family of LDH. Hydrotalcite-like LDH materials are described according to the ideal formula, [M1/ xM"l(OH)2]frl(lra [A H20]inter, where Mn and Mm are metallic cations, A the anions, and intra and inter denote the intralayer and interlayer domain, respectively. The structure consists of brucite-like layers constituted of edge-sharing octahedra. The presence of trivalent cations induces positive charges in the layers that are counterbalanced by interlamellar anions (Scheme 15.16). 

Natural talc is not pure, but contains Al, Ca and Fe. As already described (see section on montmorillonites) talc is a hydrated aluminium silicate having the formula Mg3Si40io(OH)2. Structurally it is composed of a layer of brucite, Mg(OH)2, sandwiched between two silica-type layers (see section on montmorillonites). 

Many other reactions could be illustrated, including the formation of ordered phases with n, stacking of perovskite slabs, ordered stacking of perovskite slabs and brucite-type double hydroxide layers or perovskite blocks with no interlayer cations present. For further information on the stmctural chemistry of these complex reactions, see Further Reading. 

Co-precipitation is also applied in the synthesis of hydrotalcite-like precursors (HTlcs) for catalysts e.g. based on Cu-Zn-Al, or Co-Al catalysts. Synthetic HTlcs, with a general formula (OHjg] [A ] nHgO, are hydrated hydroxycarbonates of a random lamellar structure. These materials can be visualized as brucite-type octahedral layers, in which cations partially substitute for cations (x = The cations are located in the center of... 

The phenomenon of neoformation of a mixed compound has been extensively studied in the case of the Ni/SiOj system, which forms nickel phyllosiUcate of 1 1 type, also referred to as nickel hydrosilicate. 1 1 nickel phyllosilicate exhibits a stacked structure, each layer consisting of a brucite-type sheet containing Ni(II) in octahedral coordination and a sheet containing linked tetrahedral Si04 units (Figure 14.3a). 

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