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In order to characterize the [micro]m-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica in both 2D and 3D and compare our evidences with theoretical dissolution precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz-white mica and quartz-K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes, and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite. Amorphous leached layers with kaolinite composition were observed at the boundaries of K-feldspar-quartz and K-feldspar-white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution. We suggest that the nanoscale observations of the quartz-white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stockhert (Tectonophysics 608:1-29, 2013 (See CR35)), while K-feldspar-quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution reprecipitation as described by Hellmann et al. (Chem Geol 294-295:203-216, 2012 (See CR13)). Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1186-8) contains supplementary material, which is available to authorized users., Author(s): Zita Bukovska[sup.1] [sup.2] , Richard Wirth[sup.3] , Luiz F. G. Morales[sup.4] Author Affiliations: (1) Faculty of Science, Institute of Petrology and Structural Geology, Charles University in Prague, Albertov 6, [...]

Transmission electron microscopy and 3D focused ion beam/scanning electron microscope nanotomography are applied to grain and phase boundaries between quartz, plagioclase, K‐feldspar, clinopyroxene, amphibole, and calcite. The samples come from metamorphic, plutonic and volcanic rocks, and hydrothermal quartz, and experienced cooling and decompression after highly variable P–T peak conditions. Most of the boundaries are partially open in the range of up to several hundred nanometres and partly to totally filled with secondary minerals, such as actinolite, biotite, chlorite, sheet silicates, and quartz, as well as with amorphous matter. Cracking and opening of boundaries are suggested to be related to anisotropic thermoelastic response of crystals to cooling. It starts below the brittle–ductile transition of the involved minerals. The partially open grain and phase boundaries, together with dissolution‐generated cavities, can form porosity of more than 2 vol.% and permeability under conditions of at least lowermost greenschist facies. Such networks of partially open or partially refilled boundaries potentially affect properties of crystalline rocks and processes in the upper crust, such as metasomatism, weathering, migration of radionuclides through bedrock of geological repositories of nuclear waste, and deformation in nature and in experiment. [ABSTRACT FROM AUTHOR]

Interphase boundaries are planar defects that separate two different minerals, which in rocks in general have different compositions and/or crystalline structures, and may play an important role as a pathway for fluids in rocks. Interphase boundaries can be classified coherent, semicoherent incoherent boundaries, depending on the degree of atomic structure matching between the two minerals. In coherent boundaries, the crystals match perfectly at the interface plane and the interfacial plane has the same atomic configuration in both phases. Small differences in lattice mismatch can be accommodated by elastic strain (coherency strain), which reduces the interface energy at the expense of increasing the energy on the adjacent phases. When the coherency strain energy becomes too large, semicoherent interfaces are then formed, where the elastic strain is replaced with localized strain due to dislocations. Incoherent interphase boundaries have a disordered structure between the two phases or large lattice mismatch at the interface. Although the determination of the degree of coherency of interphase boundaries and possible orientation relationships between the two adjacent phases is routinely performed by selected area electron diffraction in the transmission electron microscope, this method lacks of statistical representativeness. With the advent of techniques like EBSD, it is possible to calculate orientation relationships not only in single pairs of crystals, but in full thin sections, and not limited to single phases, but also between different minerals. The interphase misorientation is calculated from two orientations of two neighbour crystals of different phases. A set of single misorientations is then used to calculate the misorientation distribution function (MDF), from where we identify a maximum, and their crystallographic interpretation. If we then know the misorientation and the unit cell parameters of the individual phases, the crystallographic relationshipsbetween them can be described with the pairs of parallel crystallographic planes and the pairs of crystallographic directions. We present examples of the use of interphase misorientation analysis on the transformation of olivine-antigorite from serpentinites from Val Malenco, on the calcite-aragonite transition from experiments performed in a Griggs apparatus, from spinel lherzolites from the Horoman peridotite complex and from the Atlantis Bank mid-ocean ridges gabbros (ODP Hole 735). [ABSTRACT FROM AUTHOR]

Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.