Micas identification

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 identification of size, shape, and axial ratio can also be done by direct observation in the electron microscope (EM). This is accomplished by depositing single molecules (if they can be obtained) directly on polymer-coated copper grids and then shadowing them with heavy metals or making a replica of the molecular surface on mica. The sample can then be viewed in the transmission EM and photographs can then be taken after calibration of the magnification factor. 

Behind the column mica solid state detectors were positioned. They were kept at lower temperatures in order to adsorb the RfCl4 molecules. It was assumed that the produced isotopes of Rf decay at least partly by spontaneous fission. Mica is known to be well suited for identification of... 

Some glauconites have been identified in hydrothermally altered basalts together with celadonites (Alt et al., 1992 Clayton and Pearce, 2000). This material appears to form a mixed-layer mica-ferric smectite series. The formation of glauconite mixed-layer minerals is therefore not restricted to peloids under shallow-ocean-bottom conditions. However, the identification of glauconite as distinct from ferric Ulite is difficult. Perhaps these mineral occurrences should be given another name. 

Drits VA, Weber F, Salyn AL, Tsipursky SI (1993) X-ray identification of one-layer illite varieties Application to the study of illites around uranium deposits of Canada. Clays Clay Minerals 41 389-398 Duit W, Jansen JBH, Breemen AV, Bos A (1986) Anunonium micas in metamorphic rocks as exemplified by Dome de L Agout (France). Am J Sci 286 702-732... 

Weiss Z, Wiewio ra A (1986) Polytypism of micas. Ill X-ray diffraction identification Clays Clay Minerals 34 53-68... 

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 . 

In micas (as well as in many other phyllosilicates) the Pauling model and also the homo-octahedral approximation are abstractions which are very useful, among others, for didactic purposes to gain first knowledge, but also for the calculation of identification diagrams of MDO polytypes, and for the calculation of PID functions, described in sections about experimental identification of mica polytypes below. A better approximation, but still an abstraction, is the Trigonal model, which is important for the explanation of subfamilies and for some features in the diffraction patterns. Also, when speaking of a specific polytype, a characteristic sequence of abstract mica layers is intended rather than deviations from stoichiometry, distribution of cations within octahedral sheets, distortion of coordination polyhedra, etc. 

The real layers building micas deviate from their archetypes by several distortions, and the shifts between successive layers are in general not exactly rational. The intensities, but not the geometry, of the family reflections differ from polytype to polytype of the same family, and the divergence increases with the deviation of the real layers from their archetypes. Notwithstanding, the concepts of family structure and family reflections are useful in the identification of twins and polytypes, as shown below. 

Owing to the efficiency of atomic scattering factors as a function of sind/X, the diffractions close to the origin of the reciprocal lattice are best suited for identification purposes. Moreover, any family structure in micas is trigonal or hexagonal and from Friedel s law it follows that the reciprocal lattice rows 20/, 13/, 13/, 20/, 13/ and 13/... 

EXPERIMENTAL INVESTIGATION OF MICA SINGLE CRYSTALS FOR TWIN / POLYTYPE IDENTIFICATION... 

Nespolo M (1999) Analysis of family reflections of OD-mica polytypes, and its application to twin identification. Mineral J 21 53-85... 

Nespolo M, Takeda H, Ferraris G (1997a) Crystallography of mica polytypes. In Modular aspects of minerals / EMU Notes in Mineralogy, vol. 1. SMerlino (ed) Ebtvbs University press, Budapest, p 81-118 Nespolo M, Takeda H, Ferraris G, Kogure T (1997b) Composite twins of IM mica derivation and Identification. Mineral J 19 173-186... 

Ross M, Takeda H, Wones DR (1966) Mica polytypes systematic description and identification. Science 151 191-193... 

Takeda H, Ross M (1995) Mica polytypism identification and origin. Am Mineral 80 715-724 Takeda H, Sadanaga R (1969) New unit layers for nucas. Mineral J 5 434-449... 

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