Structural state and differusion of impurities in natural quartz of different genesis |
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| Authors: | N G Stenina LSh Bazarov MYa Shcherbakova R I Mashkovtsev |
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| Institution: | 1. Institute of Geology and Geophysics of the Siberian Branch of the Academy of Sciences of the USSR, 630090, Novosibirsk 90, USSR
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| Abstract: | Impurity inhomogeneities and other structural defects have been studied by means of transmission electron microscopy (TEM), X-ray microanalysis and electron paramagnetic resonance (EPR) in untreated and heat-treated quartz samples of three genetic types: hydrothermal, pegmatitic and magmatic. The impurities present are Al, Na and H2O, which occupy tetrahedral (Al3+) or interstitial (Na+, H2O) positions in the quartz lattice. Impurities form imperfections of various degrees of segregation: from point defects to micropores with a gas-liquid content. Their size, form, density and distribution in the lattice depend on the formation conditions of the quartz, the presence of dislocations and plane defects serving as sinks for the impurity atoms, and the heat treatment regime. Experimental data indicate that gas-liquid inclusions of dimensions up to some microns are the result of impurity segregation during postcrystallizational cooling. Crystalline quartz amorphizes upon electron irradiation. A model of structural water explaining experimentally observed features of this phenomenon is proposed whereby the water molecule, represented as a dipole, enters microregions of the silica lattice with a high impurity content and there forms a bond between ‘defective’ SiO3]2? and AlO4]5? tetrahedra. On irradiation, the Si---O donor-acceptor bonds trap nonelastically scattered electrons and are ruptured as a result. The water released by this lattice discontinuity forms microbubbles that diffuse along sinks into the larger micropores thus further increasing their volume. |
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