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Resumen:
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This paper correlates the mineralogical characteristics of twinning and exsolutions of a microcline giant crystal with macroscopic twin-domains and perthitic texture, taken from the Golconda III granitic pegmatite ( Minas Gerais, Brazil), with the spatial distribution of the blue luminescence from radiation-induced O- defects. The twinning macrostructure and the Na distribution of the microcline have been studied by optical microscopy, backscattered electron scanning electron microscopy (SEM), chemical mapping and quantitative chemical analyses with electron microprobe. Spectra luminescence data were recorded by high sensitivity thermoluminescence, X-ray radioluminescence, cathodoluminescence ( CL) and also CL-imaging under optical microscope at room temperature, and under SEM at liquid nitrogen temperature. The crystal recorded three different stages of re-equilibration during the geological cooling: ( 1) building up of regular macrostructures, formed mainly by only two Albite-twin law orientation variants, by solid-state domain self-assembly; ( 2) coarsening of one orientation variant at the expenses of the other by dissolution - reprecipitation reactions and ( 3) later degradation of regular twins into irregular domains. The spatial distribution of the blue luminescence in microcline can be used for two practical applications: ( 1) to mark water-rich fluid-flow channelways at low temperatures in which these light emissions are absent and ( 2) to visualize elastic strain fields across twin patterns, with maxima emission from the Na-rich ( 0 1 0) boundaries and remnants Pericline twin cores and minima from the Na-poor ( 0 1 0) twin cores and residual Pericline boundaries. Elastic strain must be the physical factor determining the radiation sensitivity of the lattice, i.e. radiation damage is not random in twinned lattices and defect formation occurs preferentially at the sites with high local elastic strain. Environmental ionizing radiation could involve a twofold effect during subsolidus microcline re-equilibration, i.e. Si/Al re-ordering, mainly acting on coherent twin boundaries, and also producing reactive H center dot and OH center dot radicals as radiolytic products of water.
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