The mica module

Oai Plane of the lower apical (a) oxygen atoms of the tetrahedra these oxygen atoms are shared between one tetrahedral and one octahedral sheet. The 0 / plane contains also hydroxyl (OH) groups (and their substitutions) which belong only to the octahedral sheet. 

Y Plane of the octahedral cations Y which are often indicated by M (here this letter is used to indicate an octahedral site M and the M layer the symbol O is used to indicate the sheet containing the M sites). 

Oau Plane of the upper (m) apical oxygen atoms (0 ) of the tetrahedra (cf. Oai). 

Obu Upper (m) plane of the basal b) oxygen atoms Ob) belonging to the tetrahedra (cf Ob). 

Planes are combined to form three types of sheets Oj, + Z + 0 form two tetrahedral sheets (T/ and T ) 0 + Y + 0 form one octahedral (O) sheet. The whole M mica layer corresponds to the T/-0-T (also termed 2 1) sequence this layer is also called the conventional mica layer and is often designated as the TOT layer. The interlayer cations are located between two successive M layers in the I plane and their coordination is discussed below. The separation between two I planes is about 10 A 

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The mica module, consisting of an M (TOT or 2 1) layer plus an interlayer cation, is conveniently considered to be built by eight atomic planes in the following sequence, starting from the bottom in Figure 2. 

Figure 13. Decreasing trend of the mica module thickness (top) and ditrigonal rotation a vs the increasing content of Si in 7Mi (rhombi and crosses) and 3T (open triangles and circles) natural phengitic micas. To obtain a the knowledge of the crystal stracture is necessary. The values corresponding to samples with known stracture are indicated by rhombi (2Mj) and circles (37). The values at 3.81Si represent the only two synthetic phen-gites which are included because their crystal stractures are known (Sn th et al. 2000). R represents the correlation coefficients of the shown regression lines.

Probably features other than crystal chemistry concur to explain the wide distribution and persistence of variously sliced mica modules. The high symmetry of the mica modules could be a key feature, in the sense that it favors different stacking and connections with other modules both of the same kind and different nature. Mica polytypes and twins are clear examples of symmetry-assisted structures. The flexibility of... 

The well defined contact geometry and the ionic structure of the mica surface favours observation of structural and solvation forces. Besides a monotonic entropic repulsion one may observe superimposed periodic force modulations. It is commonly believed that these modulations are due to a metastable layering at surface separations below some 3-10 molecular diameters. These diflftise layers are very difficult to observe with other teclmiques [92]. The periodicity of these oscillatory forces is regularly found to correspond to the characteristic molecular diameter. Figure Bl.20.7 shows a typical measurement of solvation forces in the case of ethanol between mica. 

Fig. 23 a, b. a An amplitude micrograph of PMMA brushes adsorbed on mica from a dilute solution in chloroform measured by tapping mode SFM. b Three-dimensional SFM micrograph of single brush-molecules indicated by the arrow in (a). The axial modulations have been explained by phase segregation of the side chains into nodules (insert) [82]... 

The stmctural model of nafertisite was obtained on doubling the mica-like module... 

As mentioned at the beginning of this chapter, a mica module is intended to consist of an M layer plus the interlayer cation. The use of the module thickness tm = csinp/n n is the number of M layers in a unit cell) allows the comparison of data from different polytypes. 

The M mica module occurs not only in biopyriboles, chlorites and interstratified clay minerals as mentioned above, but also in some other polysomatic series. Because these series represent different possibilities for the presence of mica-like structures in minerals, it seems useful to shortly describe some of them. 

Guggenheim and Eggleton (1987, 1988) described some modular 2 1 layer silicates in terms of fragments of the M (TOT) mica module, with or without interlayer cations. The modularity of these silicates originates by the inversion of part of the tetrahedral linkage. On the basis of the inversion fragments, the basic TOT layers may form either islands (e.g., stilpnomelane and zussmanite) or strips (where the octahedral sheets remains continuous as in ganophyllite and minnesotaite, or discontinuous as in the above mentioned palysepioles). 

Figure 18.11 A 3D-SFM image obtained at an interface between a cleaved mica surface and PBS solution (A = 0.62 nm). The 3D-SFM image (4 x 4 x 0.78 nm ] was constructed from the approaching Z profiles and has 64 x 64 X 155 pixels m XYZ. The frequency and amplitude of the Z modulation during 3D-SFM imaging were 200 Hz and 0.78 nm, respectively. The lateral scan speed was 12.2 nm/sec. Each XZ cross-sectional image was obtained in 0.32 sec, while the entire 3D image was obtained in 53 sec. ...

Figure 12.5 AFM cantilever height deflection signal ith the tip fixed on a mica substrate and irradiated with the beam modulated by an interferometer. (Reprinted with permission from Ref. [11]. Copyright 2000, Society for Applied Spectroscopy.)...

Early laminates tended to be small because available presses were small, and their original uses were to replace small parts such as the natural mica insulator boards used in radio chasses. As decorative laminates evolved from industrial laminates and the size grew to serve markets such as tabletops, countertops, and wall paneling, laminate dimensions tended to fall into the typical building module ratio of about two length to one cross, such as 2 x 4s,... 

Figure 17-2. Projection along [100] of the crystal structure of an amphibole and its modular slicing as modules of mica and pyroxene...

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

Astpllit-like ( HOHI layer. With respect to the bafertisite-like layer, a second one-chain-wide mica-like module M is present between two bafertisite-like modules. 

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