Mica structure

The mica structure provides an interesting variation of the types of close-packing observed for large ions in crystals. The two central layers of 0 , OH-, and F- ions form close-packed planes with three spheres in a hexagonal unit of edge 5.2 A, at positions 00, l/i2/i, and VsVa relative... 

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

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.

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

Figure 18. Schematic representation of several possible types of solid solution. Shaded and blank layers represent expanding and mica-like units (2 1 structures). Solid and unfilled circles represent two species of interlayer ions, a totally random in all aspects b = interlayer ion ordering, single phase montmorillonite c = ordered interlayer ions which result in a two-phase mica structure, two phases present d = randomly interstratified mineral, one phase e = regular interstratification of the 2 1 layers giving an ordered mixed layered mineral, one phase present f = ordered mixed layered mineral in both the interlayer ion sites and the 2 1 interlayering. This would probably be called a single phase mineral.

We have thus far discussed only the most simple and best known types of interlayering, those between expandable and mica structures. It is possible however, that several types of 2 1 lattice can coexist in the same structure. For example di- and tri-octahedral forms of various types of each species. Because of their similarity under X-ray investigation, it is almost impossible to detect their presence in a mixed layered structure. 

Radoslovich (1963b) has shown that when Na+ ions replace K+ ions in muscovite, the dimension of the b-axis is increased. This requires additional flattening and rotation of the silica and alumina tetrahedra. This suggests that the amount of Na+ that can be tolerated by the mica structure increases with temperature the increased thermal motions would allow the structure to accommodate local strains more readily. Thus, little Na+ would be expected in low-temperature illites. In addition, Na would leach out more readily than the K and any illite that had been through the weathering stage would not retain much of its Na. 

Micas. If the double-chain amphibole structure diagrammed in Figure 2.8a is extended in two dimensions by the bonding of all three basal 0 atoms of each tetrahedron with Si atoms of other tetrahedra, a sheet silicate (phyllosilicate) is formed with the structure shown in Figure 2.9a. This polymer, extended infinitely in two dimensions, has the formula (Si40to) and is the basis of the mica structure (as well as the layer silicate clays, as will be discussed later in this chapter). 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

In some weathering environments, Fe and Mg are ejected from the mica structure to counter the excess positive charge built up in the trioctahedral sheet as Fe " is oxidized. In the process, Mg may form a Mg(OH>2 sheet between the 2 1 layers, forming chlorite as an intermediate weathering product. 

High fluoride content in trioctahedral micas impedes release. This is actually a special case and an exception to the first rule. Since F", which can proxy for OH" in mica structures, is not a dipole, it attracts electrostatically regardless of whether the mica is trioctahedral or dioctahedral. As a result, trioctahedral micas in which F isomorphously substitutes for much of the structural OH" release with difficulty. That is, this particular type of trioctahedral mica behaves much like a dioctahedral mica with respect to removal. 

Chlorites in soil occur as primary minerals derived from mafic rocks and as secondary minerals from the weathering of biotite, hornblende, and other amphiboles and minerals (Bamhisel, 1977). Chlorites are 2 1 1 minerals consisting of 2 1 mica structure in addition to an interlayer hydroxide sheet. Chlorites have low CEC and surface areas. 

The 2 1 mica layer is composed of two opposing tetrahedral (T) sheets with an octahedral (M) sheet between to form a TMT layer (Fig. la). The mica structure has a general formula of A M2.3 T4 Oio X2 [in natural micas A = interlayer cations, usually K,... 

Tetrahedral cation atomic coordinates, taken from the original reference, were transformed from fractional to Cartesian to calculate the Layer Offset, the Intralayer Shift, and the Overall Shift. The Layer Offset is based on the displacement of the tetrahedral sheet across the interlayer from one 2 1 layer to the next, which should be equal to zero in the ideal mica structure. The Intralayer Shift is the over-shift of the upper tetrahedral sheet relative to the lower tetrahedral sheet of the same 2 1 layer. The Overall Shift relates to both effects. 

The effects of temperature on the mica structure, based on the single-crystal refinements of muscovite at 650° C (Guggenheimet al. 1987) and of paragonite at 600° C (Comodi and Zanazzi 2000), can be summarized as follows ... 

The large number of mica species (end-members) and varieties is based on chemical variability and peculiar structural features like polytypism, local and global symmetry. In addition, mainly because of an inherent misfit between the constituent tetrahedral and octahedral sheets, in the specific mica structures several structural parameters undergo adjustments relative to their ideal values. Consequently, the mechanisms ruling distortions from ideal models must be considered when investigating a mica behavior under geological conditions. 

Figure 2. Cross-section perpendicular to the M layer of the mica structure seen along [110]. Sequence and labeling of eight distinct building atomic planes are shown. Hydroxyl (OH)" groups are represented by black circles (see text for explanation of labeling).

Mainly because of a dimensional misfit between the T and O sheets (cf below), in real mica structures the Pauling model (in which there are no structural distortions) is too abstract and must be replaced at least by a model which takes into account a rotation of the tetrahedra within the (001) plane. This ditrigonal rotation is discussed below the resulting model has been called the trigonal model by Nespolo et al. (1999c). 

As already mentioned, in real mica structures the Pauling model is too abstract and must be replaced at least by the trigonal model, which considers a rotation of the tetrahedra around the perpendicular to (001). In fact, h being about 9.4, 8.6 and 9.3 Ain brucite, gibbsite and T sheet (with Si Al = 3 1), respectively, the dimensions of the T and of the O sheets do not match. Consequently, as discussed below, some structural distortions are needed to overcome the misfit and to form these two sheets into a layer. 

MICA STRUCTURE AND POLYSOMATIC SERIES Layer silicates as members of modular series ... 

The T and O sheets occurring in the mica structure are present in all layer silicates (phyllosilicates) the entire M (TOT) mica layer is present in 2 1 layer silicates only. The description of layer silicates is often given by emphasizing different stacking of T and O sheets, even if an explicit discussion in terms of modular series is absent from the literature. 

The recognition of such relations is also of practical importance. For instance, if during the refinement of a mica structure the homo-octahedral model fails, only the choice between the related meso- or hetero-octahedral models has to be made. All such polytypes have the same framework of all atoms except those octahedrally coordinated. Therefore, they have identical or very similar basis vectors, and the space-group type of the homo-octahedral polytype is their common supergroup. Also their diffraction patterns are closer to one another than to those of other polytypes the geometry in reciprocal space is virtually the same and also the distribution of intensities is very similar owing to the fact that the framework of non-octahedral atoms in an average mica represents about 70 % of the total diffraction power. 

The relations of homomorphy in mica structures are summarized in Table 7. Full symbols are given for homo-and meso-octahedral polytypes, shortened symbols (the line of orientational characters) - for hetero-octahedral polytypes. The reason for the somewhat unusual layout of this table is related to the fact that two out of the six homo-octahedral MDO polytypes, IMand 20, have the same projection normal to [010] (YZ projection). Thus, for the framework of the non-octahedral atoms in the homo-octahedral MDO polytypes (and also for the corresponding homo-octahedral approximations), there exist jive different YZ projections labeled by Roman numbers I to V in the first column of Table 7. The significance of the YZ projections will be explained below in the section Identification of MDO polytypes . 

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