Mica family

In each of the three mica families, the packet pairs P2yq2y+i and q2y+iP2/+2 are geometrically equivalent through a p-operation of the Oc2y+i OD layer and of the Tet2/+2 OD layer, respectively. These operations are denoted as 2y,2y+i[p ] and 2y+i,2y+2[p ] respectively. The resulting polytype depends on the kind of these operations (they follow from the Z-symmetry of Oc or Tet) and on their sequence in the polytype. Since p.p = i and, particularly for OD structures, a product like ki[p ] mn[p ]is allowed only if l=m, each even number of such products, e.g.,... 

When three oxygens of each tetrahedron are shared, the chains or rings turn into sheets, with each silicon containing one unshared oxygen that carries a negative charge. A small section of an essentially infinite sheet of these phyllosilicates (from the Greek phyllo for sheet ) is shown below. These anions are responsible for the layered structure of the mica family of minerals. 

Talc [14807-96-6], a naturally occurring mineral of the general chemical composition Mg2Si40 Q(0H)2, is a crystalline hydrous magnesium siUc ate belonging to the general mineral family of the layered siUcates. Other layered siUcates are kaolin, mica, and pyrophyUite (1). 

In studies of amphiboles (44), isolated strips of triplechain silicates were discovered embedded in the double-chain parent structure. It was later realized that new types of silicate structures, composed of recurrent triple chains, existed in nature. The part that HREM played in the identification of this new family of triple-chain silicates, which constitute a further step in the progression pyroxene, amphibole,. .. mica, was crucial. 

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). 

The more concentrated is the system, the better statistics must be involved. Here (for higher electrolyte concentrations) a family of HNC approximation approximations may be mentioned, which demonstrated very good agreement with the Monte Carlo results. They have also been checked with experiments on the interaction between two mica sheets in electrolytes (for CaCb electrolyte see [37]), and found a variety of applications. For example, a proper account of the ion distributions allows to explain such phenomenon as a clay swelling in the presence of an electrolyte, while the standard DLYO description fails [36]. Also these ideas have been utilized when studying different biological systems [38-40]. 

Access to information was not enough by itself, however. The gathered facts also had to be put together in the right way to form an answer. In 1928, Pauling applied himself to solving the complicated structures of an important family of minerals called the silicates, which include topaz, talc, and mica. He knew it was likely that these minerals... 

Aluminosilicates form an extensive family of compounds that include layered compounds (such as clays, talc, and micas), 3-D compounds, (e.g. feldspars, such as granite), and microporous solids known as molecular sieves. The structural diversity of these materials is contributed to by aluminum s ability to occupy both tetrahedral and octahedral holes as it also does in y-Al203. Thus, aluminum substitution for silicon in silicate minerals may lead to replacement of silicon in tetrahedral sites or the aluminum can occupy an octahedral environment external to the silicate lattice. Replacement of Si with Al requires the presence of an additional cation such as H+, Na+, or 0.5 Ca + to balance the charge. These additional cations have a profound effect on the properties of the aluminosilicates. This accounts for the many types of layered and 3-D structures (see Silicon Inorganic Chemistry). 

Families of micas accodin gtb symmep ofb O sbet. The type of occupancy... 

Table 1. Families of micas based on the symmetry of the octahedral sheet. Comparison with dioctahedral and trioctahedral classification is given. (Modified after Durovic 1994).

In the homo-octahedral family, the three M sites are by definition identical in content and size. Any difference in one of the M sites violates the H centering, lowering the symmetry of the O sheet to that of the meso-octahedral family. A difference between the other two M sites destroys also the inversion center and lowers the symmetry of the O sheet to that of the hetero-octahedral family. From the practical viewpoint, differences among the M sites are often small and must be evaluated on statistical grounds. As discussed by Bailey (1984c) for the specific case of micas [cf. an application in Amisano-Canesi et al. (1994)], if o/ is taken as the estimated standard deviation (esd) of an individual quantity and o = Oiln is the esd of the mean of n values, the esd of a difference (A) between two mean values is given by (3 = Two quantities are... 

Table 2. -symmetry S) and type of layer (Z) in the three families of mica polytypes within the Trigonal model. 5 indicates the electron density of the octahedral site (site occupancy). 

X-pp ellipes. The intensity distribution in the 1st ellipse (as well as other X-type ellipses) is typical of each mica polytype. Knowing the symmetry principle (subfamily A or B, or mixed-rotation, revealed by the 2nd ellipse) helps to obtain the stacking sequence from the intensity distribution in the 1st ellipse. Because the X rows are non-family rows in both the Pauling and trigonal model, the computation of the intensities in the X-type ellipses, to be compared with those experimentally measured, can be performed even in the simplest Pauling model. 

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