Brucite stacking

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.

This compound is isostructural with brucite (Mg(OH)2) and Cdl2. The unit cell is hexagonal with a = 0.3258 nm and c = 0.4605 nm. The structure consists of sheets of corner-sharing, trigonally distorted Fe(OH)6 octahedra stacked along the [001] direction. The Fe" ions occupy only half the available octahedral interstices and this results in a structure in which each filled layer of sites alternates with an empty layer of sites. The OH radical behaves as a single entity. Amakinite is a rare mineral of the composition (Fe,Mg,Mn)(OH)2, also with brucite structure. Fe(OH)2 is readily oxidized by air and even by water, upon which the colour changes from white to brownish. The structure can be maintained up to a replacement of one tenth Fe" by Fe " (Bernal et al., 1959). 

On the other hand, LDH structure consists of the stacking of brucite-like layers, Mg(OH)2, where some divalent cations are substituted by trivalent ones, giving rise to a positive residual charge. To neutralise such charges an appropriate number of hydrated anions is placed between the layers. The general formula that can represent LDHs is ... 

Layered double hydroxides (LDHs), with a hydrotalcite-like structure, are a class of materials which have received considerable attention in the last decade. The structure of LDHs is based on the stacking of metal cation hydroxide (brucite-like) layers, with a positive charge on the layers resulting from the isomorphous substitution of some of the bivalent... 

The structure of HT closely resembles that of brucite, Mg(OH)2. In the latter structure, Mg cations are octahedrally coordinated by hydroxyl groups, which are edge shared to form stacked layers. Compared to brucite, in HT some of the Mg cations have been replaced by AP, resulting in positively charged cation layers the charge is compensated by anions situated together with water molecules in between the brucite-like layers, as schematically represented in Figure 2.39 [212]. 

If ehloritization occurs in disorder-free biotite polytypes other than IM, mechanisms 1 and 2 must result in different stacking sequences. If brucite-like sheets are formed by mechanism 1, the original stacking sequences in the biotite polytype are preserved because all 2 1 layers are preserved. In the case of mechanism 2, original stacking sequences must be altered owing to the removal of some 2 1 layers. Thus, it is possible to determine the number of biotite layers consumed by the formation of a chlorite unit cell and hence, its formation mechanism. 

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. 

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). 

Phyllosilicates are a family of silicates with a structure based on the stacking of interconnected Si20s tetrahedral layers (T) with octahedral layers (O) of either Brucite [Mg(OH)3] or Gibbsite [Al(OH)3] type (Figure 8.1). 

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