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TEM in Geology. Basics and applications 35 space group. The low-temperature polymorph is a sub-group of symmetry of the hightemperature one. The loss in translational symmetry in order-disorder transformations can also produce antiphase domains (Brenker et al. 2003). The mean antiphase domain size and form depend on peak temperature, duration of peak metamorphism, cooling rate, and composition. They therefore provide very useful information about the geological history of the minerals. Retrograde intergrown phyllosilicate grains Phyllosilicate grains, showing optical characteristics identical to biotite, are frequent. They differ in mineral nature, composition, and origin, and may be grains corresponding to a single mineral or a combination of two or more mineral species. One of the most significant cases was described in the Alps by Chatterjee (1966), who gives it the name “oxidized chlorites” due to its basically chloritic composition. Grains with the same optical and chemical characteristic are rock-forming minerals in the graphite schists of the Nevado-Filabride Complex in the Internal Zones of the Betic Cordillera. Based on Chatterjee´s description in the Alps, the Nevado-Filabride grains were also considered as oxidized chlorites. Nevertheless, Mellini et al. (1991) recognised that most of the area was dominated by 14 Å lattice fringes, with chlorite composition, but smectite was also present as individual 10Å layers interlayered in the chlorite or as packets a few layers thick. In other areas, a poorly crystalline 16Å phyllosilicate with chlorite composition, interpreted as hydrated chlorite, was recognised in addition to semi-amorphous material showing hematite spacings and composition. All these supergenic-origin materials were interpreted as the result of fluidmediated processes in relation to the uplift of the Nevado-Filabride rocks, due to erosion and/or post-metamorphic extension. A similar process was later described in the overlying Malaguide Complex by Nieto et al. (1994). These two cases were some of the first retrograde alteration procceses, which, together with numerous other examples, were employed some years later (Nieto et al. 2005) to define retrograde diagenesis. Serpentine minerals The serpentine tetrahedral and octahedral layers present partially different dimensions along the a and b directions. The various serpentine minerals display
36 Fernando Nieto different mechanisms that provide solutions to their fitting. For example, a rolled microstructure producing cylinders is a characteristic in TEM for recognising chrysotile. In contrast, the antigorite structure results from a structural modulation of the serpentine layers along the a direction. The shorter tetrahedral layers change their polarity every given number of sub-cells, passing from upper to lower position in relation to the longer octahedral layers (Capitani and Mellini, 2004). A selected area electron diffraction (SAED) pattern consists of the main diffraction spots from the sub-cell, which are surrounded by satellite diffraction spots from the modulated structure of antigorite. The m-values, representing the number of tetrahedra in one modulation, range from 13 to 50 and can be determined from the respective spacings of the sub-cell and the modulated structure. Antigorite microstructures vary from highly ordered to lower periodic structures in the c direction (Auzende et al. 2006). TEM is a convenient tool for investigating and characterising antigorite variability at such a scale. Serpentine microstructures can potentially preserve information on metamorphic conditions. The deformation processes of serpentinites during subduction are complex. According to microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. With increasing metamorphic grade, the brittle behaviour gives way to pressure solution, which persists up to eclogite-facies conditions. Once the partial dehydration reaction is reached, antigorite may recrystallise, mainly by a pressure-solution mechanism. The required fluids would derive from the progressive dehydration of antigorite (Auzende et al. 2006). Pressure solution is an effective mechanism to accommodate deformation; according to these authors, serpentinites could localize the deformation within a subduction/exhumation channel, thus making it possible to preserve eclogites from depths of about 100 km below the Earth’s surface. Ammonium micas The common interlayer cations in low-grade metapelite micas are K and Na. In addition, since the late 1950s, ammonium has been recognised as an important interlayer constituent in white micas associated with organic matter. This third important interlayer component had not been thoroughly studied due, in part, to the intrinsic difficulty of analyzing NH 4 by in-situ methods and, in part, to the typically defective character and
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