Effect of water on the α-β phase transition in a surface quartz layer
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The temperature dependence of the α-phase concentration in surface layers and in the bulk of quartz plates cut out at a distance of ∼2 mm from the natural growth surface of druses extracted at the Dodo deposit in the Polar Urals has been studied using infrared and Raman spectroscopy. It has been found that, in the bulk of the sample, the temperature dependence behaves as expected for a first-order phase transition; more specifically, below 800 K, it remains unchanged and, at high temperatures, approaches zero. In surface layers with thicknesses of ∼0.15 and ∼0.8 μm, the α-phase concentration decreases monotonically by approximately 10% with an increase in the temperature to 780 K. The temperature dependence of the α-phase concentration in the layer at a depth of ∼6 μm passes through two minima, namely, at ∼370 and ∼570 K, at which the concentration of this phase decreases by about one half. This is accompanied by an increase in the concentration of the β-phase. The revealed behavior of the α-phase concentration with an increase in the temperature has been assigned to the influence of water on crystal lattice distortions near growth dislocations. At 370 K, free water evaporates from grain boundaries, and at 570 K, the water bound by hydrogen bonds to the SiOH groups. The evaporation of water affects stresses at grain boundaries, and it is this factor that brings about a change of the α-phase concentration. It has been demonstrated that tensile stresses generated with increasing temperature in a near-surface quartz layer to ∼0.8 μm thick can reach ∼170 MPa. The stresses create microcracks, which culminate in destruction of the sample. The generation of the tensile stresses is explained by an increase in the volume of the microcrystal layer located at a depth from ∼1 to ∼8 μm from its surface as a result of the increase in the β-phase concentration in it.
KeywordsQuartz Crystal Phase Concentration Natural Quartz SiOH Group Crystal Lattice Distortion
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