Polymorphs, micas

Amylin fibrils growing on mica were rather straight and exhibited various heights. They were compatible with the protofibril hypothesis of amyloid fibril polymorphism (Fig. 3), but no multistranded cables were present (Goldsbury et al, 1999). In contrast, coiled fibrils were often observed by SFM for fibrils assembled in solution prior to being adsorbed to mica (Jansen et al, 2005 Kad et al, 2003 Relini et al, 2004). In the case of... 

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

The main method used to distinguish the relative quantities of neoformed illite is by the polymorph or structure of the material. Using the criteria that 2M and 3T polymorphs of dioctahedral potassic mica are high temperature forms (Velde, 1965a), the determination of the relative quantities of lMd, and 1M vs. 2M, 3T polymorphs permits a semi-quantitative estimation of the proportion of neo-formed or low temperature illite present in a specimen. A method commonly used is a determination of relative intensities of X-ray diffraction peaks of non-oriented mica (Velde and Hower, 1963 Maxwell and Hower, 1967). Usually only 2M and lMd polymorphs are present in illite specimens which simplifies the problem. The 1M polymorph is typical of ferric illites and celadonite-glauconites, the more tetrasilicic types. 

Arguments have been made by Velde and Hower (1963) against the idea that montmorillonites are simply potassium-stripped micas, this on the basis of mica polymorphs in sedimentary rocks which should reflect the origin of the layer-silicate material. 

Figure 29. Possible general phase relations for illite and associated phyllosilicates as a function of varying P-T conditions. Ill = illite, either predominantly IMd or 2M in polymorph I = illite, 2M mica ID = k layer ordered mixed layered phase MLSS = mixed layered 3 or 2 layer ordering giving a superstructure reflection ML0 = mixed layered, ordered structure with no superstructure MLr = mixed layered non-ordered M, = fully expandable montmorillonite Chi = chlorite Kaol = kaolinite Exp 3 " expanding chlorite and/or corrensite.

Dioctahedral micas (alkali interlayer ions having total charge near +1 per formula weight) which have either lMd or 1M polymorphs are either metastable muscovite forms or are micas with a composition differing from muscovite, e.g., glauconite, celadonite, and illite. ... 

Stephens, H. A. (2001). MICA and MICB genes Can the enigma of their polymorphism be resolved Trends Immunol. 22(7), 378-385. 

A subclass of polymorphism known as polytypism is found for one-dimensional close-packed and layered structures such as SiC, Cdl2, GaSe, micas and clay... 

Besides classical MHC class I genes, other polymorphic genes of the MHC I region may also be susceptibility markers, such as the MHC class I chain related gene A (MICA). MICA-A9 has been described as a strong genetic susceptibility marker for psoriatic arthritis (Gonzalez et al., 2002). 

Abstract. Structural properties of rubrene thin films on cleaved mica (001) surfaces were investigated by optical microscopy and x-ray diffraction. Optical microscopy shows, that the crystallization of rubrene results in formation of spherulites. X-ray specular diffraction reveals polycrystalline and polymorphic nature of rubrene. The pole figure measurements of films prepared at low deposition rates reveal orthorhombic structure and indicate fiber textures with crystallographic planes (121), (131) and (141) preferentially oriented parallel to the substrate surface. High deposition rate thin films in addition show polymorphism, corroborating the existence of the orthorhombic and the triclinic phase. 

Smith DC, Kechid S-A (1983) Three rare Al- and Na-rich micas in the Liset eclogite pod, Norway Mg-Fe-margarite, preiswerkite and Na-eastonite (abstr). Terra Cognita 3 191 Smith JV, Yoder HS Jr (1956) Experimental and theorethical studies of the mica polymorphs. Mineral Mag 31 209-235... 

Takeda H, Haga N, Sadanaga R (1971) Stractmal investigation of a polymorphic transition between 2M2-, lA/-lepidohte and 2Mi-muscovite. Mineral J 6 203-215 Takeda H, Ross M (1975) Mica polytypism Dissimilarities in the crystal structures of coexisting IM and 2M biotite. Am Mineral 60 1030-1040 Takeuchi Y (1965) Stractrrres of brittle micas. Clays Clay Minerals 13 1-25... 

Velde B (1980) Cell dimension, polymorph type, and infrared spectra of synthetic white micas the importance of ordering. Am Mineral 65 1277-1282... 

As mentioned in the Introduction, the wide variety of micas (Rieder et al. 1998) derives not only from chemical composition but also from structural features such as the many (infinite, in principle) possibilities of stacking the M layer, particularly the special type of polymorphism known as polytypism, discussed by Nespolo and Durovic (this volume). 

Nespolo M, Takeda H, Kograe T, Ferraris G (1999c) Periodic interrsity distribntion (PID) of mica polytypes Symbohsm, stmctural model orierrtation and axial settings. Acta Crystallogr A55 659-676 Newrrham RE (1961) A refinement of the dickite stmcture and some remarks on polymorphism of the kaolin minerals. Minerd Mag 32 683-704... 

Smith JV, Yoder HS (1956) Experimental and theoretical studies of the mica polymorphs. Mineral Mag 31 209-235... 

Brigatti MF, Poppi L (1993) Crystal chemistry of Ba-rich trioctahedral micas-IM Etrr J Mineral 5 857-871 Brigatti MF, Lalonde AE, Medici L (1997) Crystal chemistry of Fe3+-rich phlogopites A combined singlecrystal X-ray and Mossbauer study. Proc. 11 Int. Clay Conf. Ottawa, Canada, 317-327 Brindley GW, Oughton BM, Robinson K (1950) Polymorphism of the chlorites. 1. Ordered stmctrues. Acta Crystallogr 3 408-416... 

Hendricks SB, Jefferson ME (1939) Polymorphism of the micas with optical measurements. Am Mineral 24 729-771... 

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