Unit cells, dimensions micas

It was not until 1927 that the first X-ray diffraction photograph of a mica was taken by Mauguin (Fig. 1). In the following year, Mauguin (1928) measured the unit cell dimensions of five species of mica. However, he was unable to resolve the problem of the micas, although he foresaw its solution. By means of the weight of the unit cell... 

Typical results of specific surface area determinations on phyllosilicates by nitrogen gas/water vapor or nitrogen gas/CPB adsorption are listed in Table 1.7. For Mg-vermiculite and Na-montmorillonite, the measured adsorption specific surface area is close to that calculated from the unit cell dimensions and structural formula. For illitic mica, the area is about 14 per cent of the ideal crystallographic value, indicating that this mineral forms particles containing about seven phyllosilicate layers that cannot be penetrated by water vapor or CPB. 

It has been recognized for some years that the unit cell dimensions of micas are dependent on their chemistry i.e., when isomorphous substitutions occur in micas, the cell dimensions change in ways that depend upon ionic radii and should, therefore, be predictable. A number of attempts have been made to develop cell-dimension formulas or algebraic relations between cell parameters and the ionic proportions as expressed in the structural chemical formulas. 

One can thus subdivide a crystal, whether in practice or in theory, into ever smaller units an unusually thick crystal of mica is often referred to as a book of mica because we know in advance that it can be cleaved into innumerable sheets, but not indefinitely. There is a certain limit when a single repeat unit, or unit cell, is reached within which the structure is not periodic, just like the page of a book, which does not cleave. Crystallog-raphers have long since established a series of international conventions to define the unit cell of a crystal species because there are a multitude of correct ways of doing this. This is because, for example, in a one-dimensional repetition of different patterns of A, B, and C, like ABCABCABCABC etc., one can consider the repeat unit to be ABC, or BCA, or CAB, or even ABCABC this situation arises in all three dimensions in a crystal. Hence, in order to establish a crystallographical convention, it is frequently necessary to make a choice among several equally valid options. 

This stylized mica structure corresponds to a unit cell with approximate dimensions a = SA, b = 9A, c = lOA (Figure 4), which is monoclinic with = 95° but has a face-centered a-b plane. This model has hexagonal surface symmetry, whereas in almost all real micas, the surfaces will show only pseudohexagonal symmetry at best, or more probably trigonal symmetry. This unit cell contains 2[r(Si3Al)Z2 Oio(OH)2] units chemically, and is called a one-layer mica structure. 

In 1934, the first insights into the structure of vermicuUte were obtained by two independent workers using X-ray powder methods. Kazantzev, on the one hand, reported that the unit cell is analogous to that of biotite, but of slightly larger dimensions, with K partly replaced by H and Fe by Mg. The other, Gruner, showed that the structure consists of silicate layers resembling those of mica or talc, with double sheets of water molecules between them. These so-called interlayer water molecules occupy a space very nearly equal to that occupied by a brucite layer in the chlorite structure, with the result that the X-ray diffraction effects obtained from vermiculites and chlorites have certain similarities. 

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