Apical oxygens

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

The late transition metals, either Co or Ni, bond to four apical oxygen atoms of two vertex-shared 5 units (Fig. 17.9a), two each to layers both above and below, and also bond to two bridging pyrazine ligands in a trans fashion. The al-... 

Fig. 17.9 Sections of the V4O10 layers in Co(pyz)V40,o (a) and (Me4N)V40,o (b). Repeat distances of the apical oxygen atoms are labeled as dashed lines, which are a...

O O Basal oxygen atoms Og Apical oxygen atoms Oa Si atoms... 

Fig. 2.2A Schematic representation of the structural components of the serpentines. The tetrahedral (T) sheet. The composition can be expressed as [(Si205) ] , is formed when three of the four oxygens in the tetrahedon are shared. The large open Circles represent oxygen ions the small solid circles are silicon ions. The open circles concentric with the silicon atoms represent oxygen atoms located vertically, above the silicon (apical oxygens).

Fig. 2.2B The octahedral (O) sheet. The composition can be expressed as f(Mg3-x-YR xR y)02(OH)4ln. The dashed circles represent the apical oxygens of the underlying T sheet. These ions replace the hydroxyls in the octahedral sheet and are common to the T and O sheets. The cross-hatched circles represent the remaining hydroxyls. The circles with crosses represent hydroxyls at the upper portion of the octahedral sheet. The small circles represent magnesium ions.

Fig. 2.12 Structural components and variations in the micas. (A) Plan view of the continuous aluminosilicate sheet (T), [Si,Al205] , a portion of the mica structure. (B) Stereographic representation of an idealized mica. The structure is composed of continuous layers containing two tetrahedral aluminosilicate sheets (T) that enclose octahedrally coordinated cations, or Mg (O). This layer or sandwich," the T-O-T or 2 1 aggregate, is held together by or Na ions. (C) The two possible positions (I and II) of octahedral cations in the micas. Sets of three locations for each are superimposed on the tetrahedral hexagonal aluminosilicate sheet. (D) The three possible directions of intralayer shift when octahedral set I (upper) or II (lower) are occupied. The dashed lines and circles represent ions below the plane of the paper. (E) Distorted hexagonal rings of apical oxygens in the tetrahedral sheet of dioctahedral micas compared with the undistorted positions of the apical oxygens in the tetrahedral sheet of trioctahedral micas.

Very little X-ray data is available for zirconium or hafnium complexes that contain oxo ligands. The only X-ray evidence for the existence of the zirconyl or hafnyl group, M=Oz+, comes from a structural study of K2Zr03.157 This compound contains chains of Zr05 square pyramids that share basal edges. The Zr—O distance to the apical oxygen atom (1.92 A) is appreciably shorter than the sum of ionic radii (2.20 A), the sum of the covalent radii (2.22 A)... 

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