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The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels (similar to 320 degrees C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.

The distribution of octahedral cations in the two component layers of a 7 angstrom - 14 angstrom interstratified mineral with a bulk chemical composition (Fe4.122+Mg0.07Mn0.01Al1.69 +/- 0.11)(Si2.56Al1.44) O-10(OH)(8) was investigated using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in combination with the image simulations. In the 14 angstrom component layers, comparison between the observed and simulated images revealed that the M4 sites of the interlayer sheets were occupied preferentially by Al together with a small amount of Fe; the other M1, M2 and M3 sites were occupied by dominant Fe and residual Al in equal proportions. Two types of octahedral sheets with disordered and ordered cation distributions were recognized in the 7 angstrom component layers. The two types of sheets were similar to the octahedral sheet of the 2

The structures of Fe-rich chlorite and berthierine and the formation mechanisms of 7 angstrom-14 angstrom interstratified minerals were not previously fully understood owing to the difficulties in analyzing them by X-ray diffraction (XRD). The present study characterizes Fe-rich chlorites in quartz veins of epithermal to xenothermal vein-type ore deposits without later structural modifications, based on high-resolution transmission electron microscopy (HRTEM) along with XRD examination and chemical analysis. Samples have a wide range of Fe/(Fe+Mg) ratios from 0.38 to 0.98 and tetrahedral Al substitution for Si from 0.94 to 1.44 atoms per formula unit (apfu). The variation in Fe content nearly parallels the tetrahedral Al content. The formation temperatures estimated by chlorite geothermometry range. from 190 degrees C to 320 degrees C. In HRTEM, most of the samples showed interstratification between 7 angstrom, 14 angstrom, and/or (in some samples) smectite layers. Chlorites with relatively low Fe contents (Fe/(Fe+Mg) approximate to 0.4) were characterized by mostly 14 angstrom periodicity with the polytype IIbb. In contrast, interstratification of 7 angstrom and 14 angstrom layers predominated with increasing Fe content and the proportion of 7 angstrom layers exceeds 80% in Fe-rich samples with Fe/(Fe+Mg) > 0.9. The 7 angstrom component layer approximated Fe-rich berthierine based on the chemical composition. Layer stacking structures in the Fe-rich samples were complex, and characterized by disorder of 7 angstrom and 14 angstrom layers, differences in the polarity of the tetrahedral sheets, variations of the slant of the octahedral sheets, and positional disorder between octahedral and tetrahedral sheets involving the hydrogen bonding, as indicated from HRTEM observations along the Y, directions of the phyllosilicates. The complex stacking structures observed in Fe-rich samples suggest that irregularity was controlled by neither the Fe/(Fe+Mg) ratio nor the formation temperature; stacking was controlled by kinetic factors in the process of mineral precipitation under disequilibrium conditions.

This present study provides an overview of the clay-mineral reactions involved in the chloritization process in a mixed-layer mineral series, and focuses on the properties of the resulting low temperature chlorites (formed at <220 degrees C) in diagenetic and hydrothermal systems. According to the literature, most chlorite species occurring in low-temperature geological systems are derived from specific clay precursors except for direct precipitates from solution in veins. in addition, three main types of clay mineral series have been associated with these chloritization processes

Mixed-layer corrensite-chlorite in a glauconitic sandy-clayey rock has been investigated and the three-dimensional stacking structure of corrensite was determined for the first time using high-resolution transmission electron microscopy (HRTEM). We are also able to identify the corrensite-chlorite transition mechanism. The crystals consist of corrensite and chlorite packets excluding successive smectite layers, consistent with the result of XRD analysis previously reported for the same specimen. One-dimensional HRTEM imaging of corrensite with dark contrast corresponding to the cation sheets indicated two types of the smectite-like interlayers in corrensite, probably containing one atomic plane and without any distinct material, which results in the corrensite basal heights of ca. 26.5 and 24.4 angstrom, respectively, in TEM. Two-dimensional HRTEM imaging revealed that the polytypic stacking sequence in the chlorite-like layer [the two 2

Applications of electron backscatter diffraction (EBSD) to obtain crystallographic information of minerals in petrographic thin sections in mineralogy and petrology are increasing. However, platy phyllosilicates that mostly appear with their silicate layers terminated by the surface in the thin sections generally do not show sharp EBSD patterns in spite of gentle mechanical polishing. TEM examination indicated that this is due to crystal bending of phyllosilicates from the surface to a few micrometers in depth, caused by the polishing process. Ion beam etching commonly used to prepare TEM specimens was found to be applicable to remove the surface region with crystal bending. As a result, clear and sharp EBSD patterns were acquired from the phyllosiicates (micas, chlorite, etc.) in petrographic thin sections, by which their crystal orientations and polytypes were unambiguously determined.