Structures of Layer Silicates

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. 1a⇓). The mica structure has a general formula of A M2–3 T4 O10 X2 [in natural micas: A = interlayer cations, usually K, Na, Ca, Ba, and rarely Rb, Cs, NH4, H3O, and Sr; M = octahedral cations, generally Mg, Fe2+, Al, and Fe3+, but other cations such as Li, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn can occur also in mica species; T = tetrahedral cations, generally Si, Al and Fe3+ and rarely B and Be; X = (OH), F, Cl, O, S]. Vacant positions (symbol:□) are also common in the mica structure (Rieder et al. 1998). In the tetrahedral sheet, individual TO4 tetrahedra are linked with neighboring TO4 by sharing three corners each (i.e., the basal oxygen atoms) to form an infinite two-dimensional “hexagonal” mesh pattern (Fig. 1b⇓). The fourth oxygen atom (i.e., the apical oxygen atom) forms a corner of the octahedral coordination unit around the M cations. Thus, each octahedral anion atom-coordination unit is comprised of four apical oxygen atoms (two from the upper and two from the lower tetrahedral sheet) and by two (OH) or F, Cl, O and S anions [the X anions, usually indicated as the OH or O(4) site]. The OH site is at the same level as the apical oxygen but not shared with tetrahedra. In the octahedral sheet, individual octahedra are linked laterally by sharing octahedral edges (Fig. 1c⇓). The smallest structural unit contains three octahedral sites. Structures with all three sites occupied are known as trioctahedral, whereas, if only two octahedra are occupied [usually M(2)] …

Mica Crystal Chemistry and the Influence of Pressure, Temperature, and Solid Solution on Atomistic Models

The use of mössbauer spectroscopy of iron in clay mineralogy

Recent crystal chemical research on rock-forming silicate minerals has focused on the accurate refinement of room temperature and high-temperature crystal structures. The data are of sufficient accuracy to allow detailed examination of structural topology and intracrystalline equilibria and to provide some insight into exsolution phenomena, chemical bonding, and phase transitions. This paper summarizes the results of recent research on olivines, humites, garnets, aluminosilicates, pyroxenes, pyroxenoids, amphiboles, micas, silica minerals, feldspars, feldspathoids, and scapolites.

Crystal Chemistry of Silicate Minerals of Geophysical Interest

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.

Micas are important rock-forming minerals and petrographers consider them mainly for their chemical aspects. The importance of the chemical composition is well known to all researchers dealing with minerals. To give emphasis to the chemical composition, the official classification of the micas (Rieder et al. 1998; Rieder 2001) allows exceptions (note the introduction of ‘species that are not end member’ in Rieder et al. 1998) to the rules which are normally used to define mineral species (Nickel and Grice 1998). However, as shown throughout this book, the role of structure features (including some aspects of polytypism) in determining fields of stability of micas and, therefore, in providing geological insights, is increasingly recognized as crucial. Thus, it seems justified that a chapter dedicated to the description of the general structural background of micas should be presented independently from specific cases, which are discussed in other chapters.

This chapter is an introduction to the symmetry and geometric aspects of micas. Nevertheless, some less conventional topics not covered in other chapters are reported in appendices. Appendix I concerns the wide presence of mica-like modules in the growing group of natural, layer titanosilicates (Khomyakov 1995; Ferraris et al. 2001b, d) and other more or less exotic structures belonging to the expanding field of modular crystallography (Merlino 1997). Important results have been obtained by the oblique-texture electron diffraction method …

Structural Features of Micas

The structure of a dioctahedral 2M2 mica was defined by high voltage electron diffraction. The cell parameters are: a = 8-965, b ~ 5-175, c = 20.31 A, fl = 100 ~ 40', Z = 4, space group C2/c. Despite the peculiar character of the layer dispogition (or 4 as ~r~ as) , the oxygens of the layers are packed according to the cubic law. Consequently, the interlayer cations K have a trigonal prismatic coordination. The angle of tetrahedral twist is 11 ~ 20'. The interatomic distances T-O indicate ordered replacements of AI for Si.

A. P. Zhoukhlistov

Papers

The Crystal Structure of the Dioctahedral Mica 2M2 Determined by High Voltage Electron Diffraction