Grain-sizereductionmechanisms and rheologicalconsequences in high-temperaturegabbromylonites of Hidaka, Japan

International audienceThe study of microstructures and crystallographic fabrics in a granulite-facies shear zone of the Hidaka Metamorphic Belt showed that the strong shearing localized within the mylonite resulted in the asymmetrical elongation of the inherited orthopyroxene porphyroclasts and the generation of fine-grained plagioclase and orthopyroxene layers as asymmetric tails of orthopyroxene porphyroclasts. The orthopyroxene porphyroclasts and the coarse plagioclase matrix surrounding them have a strong crystallographic preferred orientation acquired through deformation by dislocation creep. In contrast, the small orthopyroxene and plagioclase grains located in the tails have equant shapes and random fabric that are interpreted as the result of deformation by grain-boundary sliding. The small orthopyroxene grains are generated on the sheared rims of the orthopyroxene porphyroclasts by subgrain rotation, inheriting the orientation of the porphyroclasts before deforming by grain-boundary sliding (GBS) and losing this fabric. Additional mechanism of grain-sizereduction is the disruption of orthopyroxene porphyroclasts by synthetic shear zones localized on clinopyroxene exsolutions. The switch in deformation mechanism from dislocation creep to GBS, associated with the grain-sizereduction, yielded estimates of deviatoric stress one order smaller than lithostatic pressure. Besides, such rheological evolution attests of the mechanical softening during deformation, which contributed to the localization of the strain within the mylonite

Lewis-Acid Sites of TiO₂ Surface for Adsorption of Organic Dye Having Pyridyl Group as Anchoring Unit

Adsorption of an organic dye having a pyridyl group as an anchoring unit (<b>NI4</b>) is studied on nanocrystalline TiO₂ surface by being compared with a similar dye having a carboxyl anchor (<b>NI2</b>). Adsorption of both dyes followed a Langmuir isotherm, and analysis of the isotherms showed that an adsorption equilibrium constant of <b>NI4</b> is (1.1 ± 0.1) × 10³ M^–1, much smaller than (0.6 ± 0.05) × 10⁵ M^–1 for <b>NI2</b>, and amounts of adsorbed dyes at saturation are similar to each other: [(1.5 ± 0.2) and (1.7 ± 0.3)] × 10¹⁴ cm^–2 for <b>NI2</b> and <b>NI4</b>, respectively. Coadsorption experiments with 4-carboxy TEMPO (<b>4CT</b>) and either <b>NI2</b> and <b>NI4</b> revealed that the adsorption sites of <b>NI2</b> and <b>NI4</b> are different from each other. This was supported by measurements of average nearest-neighbor interspin distances of <b>4CT</b> radicals coadsorbed on TiO₂ by a spin-probe ESR technique. The above findings and elaborate FT-IR studies demonstrated that <b>NI2</b> and <b>NI4</b> adsorb, respectively, on Brønsted- and Lewis-acid sites of TiO₂ surface. The number of Lewis-acid sites evaluated from the temperature-programmed desorption experiments of TiO₂ was in good agreement with the amount of <b>NI4</b> adsorbed on TiO₂ at saturation, providing a further confirmation for Lewis-acid sites acting as a predominant adsorption site of <b>NI4</b>