Fe 6 octahedra

Figure 20. The crystal structures of childrenite and jahnsite (a) childrenite projected onto (100) (b) childrenite projected onto (001) (A e) octahedra are shadow-shaded, Mn cations are shown as circles (c) jahnsite projected onto (001) (d) jahnsite projected onto (010) (Fe ( )6) octahedra are P-net-shaded, Ca atoms are shown as circles, (Mn ( )6) and (Mg( )6) octahedra are shadow-shaded.

Whitmoreite, Fe (H20)4[Fe (P04)(0H)]2, consists of a fairly densely packed sheet of (PO4) tetrahedra and (Fe c e) octahedra parallel to (100) (Fig. 32d). Pairs of Fe ( )6 octahedra condense to form edge-sharing [Fe 2(t)io] dimers that occupy the vertices of a 4" plane net and link by sharing comers. This results in an interrupted sheet of octahedra, the interstices of which are occupied by (PO4) tetrahedra (Fig. 32d). These sheets stack in the a-direction, and are linked by interstitial (Fe 02 H20 4) octahedra and by hydrogen bonds (Fig. 32e). 

Figure 33. The crystal structure of mitridatite projected onto (100) (Fe ( )6) octahedra are shadow-shaded.

Like goethite and akaganeite, lepidocrocite consists of double chains of Fe(0,0H)6 octahedra running parallel to the c-axis. The double chains share edges with adjacent double chains and each chain is displaced by half, with respect to its neighbour, thus forming corrugated sheets of octahedra (Fig. 2.5 d). These sheets are stacked perpendicular to the [100] direction and are separated by double rows of empty octahedral sites. 

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). 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

We saw in Section 13.6 that hydrolysis and subsequent polymerization of aqueous metal cations can lead to the precipitation of gels. In the case of Fe(H20)63+ in mildly acidic solutions, the polymerization sequence of Eqs. 13.25 and 13.26 and Fig. 13.6 first reversibly forms cationic colloidal spherules, 2-4 nm in diameter, with the structure of 7-Fe0(0H) [double chains of Fe(0,0H)6 octahedra] on a timescale of about 100 s. These lose H+ and harden over several hours and then, over several days, form aged polymer rods, then rafts, and ultimately, after several months, needles of solid goethite [cc—FeO(OH)].1,2 Thus, aging is an important feature of hydrolytic polymerization. 

Comparison of the properties of metal alkoxides with their structures permits a conclusion that the polymeric nature does not always lead to chemical inertness. The major role appears to be played by the nature of the M-OR bonding. Solubility in alcohols and liquid ammonia of the methoxides of alkaline and alkaline earth metals and that in hydrocarbons ofthe isopropoxides of K, Rb, Cs (isostructural with the corresponding methoxides), and also M(OC2H4OMe)n, M = Pb, Bi indicates the easy oligomerization due to solvation or chelation. At the same time the methoxides and ethoxides of Al, Cr, Fe, and so on, forming the strongest covalent bonds in the [MOs/6] octahedra (and not prone to solvation in alcohols), appear almost inert. They can be dissolved only due to complexation or partial destruction with formation ofoxobridges. 

SCHEME 7.2 The Mc>72Fe3o cluster containing 72 Mo(VI)-oxide polyhedra (purple) and 30 Fe(III) as Fe(0)6 octahedra (brown). (See the color version of this figure in Color Plate section.)... 

Clays are mainly hydrous layer silicates of the phyllosilicate family in which the basic building blocks are Si(0,0H)4 tetrahedra and M(0,0H)6 octahedra (M = Mg, Fe, Fe, etc), which polymerize to form two-dimensional... 

The minerals of the burangaite, Na[Fe Al5(P04)4(OH)6(H20)2], group contain a trimer of face-sharing octahedra that is a feature of several basic iron-phosphate minerals (Moore 1970). An (Fe ( )6) octahedron shares two trans faces with (Al( )6) octahedron to form a trimer of the form (the h cluster of Moore 1970). This trimer is corner... 

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