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The kinetics of crystallization of strontium carbonate (strontianite) from strontium bicarbonate solutions were examined. CO2 was stripped from a slightly acidic solution of Sr(HCO3)2 by stirring resulting in critical supersaturation and precipitation of strontianite. The reduction of the Sr2+ concentration was recorded as a function of time by measuring the electrolytic conductivity and the pH value.
Homogeneous primary nucleation is dominant at high supersaturations, whereas heterogeneous primary nucleation prevails at low supersaturations. The crystal growth rate increases with increasing supersaturation. This effect is less pronounced at higher supersaturations. The growth rate is mostly transport-controlled at high supersaturation. At lower supersaturation the crystal growth is mainly determined by integration of ions into the crystal lattice. These results may be used to explain the deposition of strontianite in natural systems. 相似文献
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Solutions of gum rosin and acetone reproduce the volatile- and temperature-dependent viscosity, together with the phase behaviour, of hydrated magmas. A range of experimental exsolution conditions was investigated, including the variation of supersaturation, rate of decompression, solution temperature and volatile content. Degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid. The end-products ranged from a mildly degassed liquid to a solid foam, which preserved strained vesicles. Solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes. 相似文献
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Morphological instabilities in periodic patterns occurring both in precipitation and crystallization processes (Liesegang rings and crystal zoning) are investigated and compared with similar patterns in geological samples (zebra rocks and mud bands in snow sediments). In classical Liesegang systems, undisturbed parallel or concentric precipitation bands are emanated from even or concentric diffusion sources in homogeneous diffusion matrices of gelatine or other gels. In the case of superposing diffusion sources, sources with undulatory curvatures or local diffusion barriers there may occur several types of instabilities within the sequence of regular patterns: (a) gaps within the bands forming radial alleys free of precipitate, (b) transition from broken bands to speckled patterns and (c) apparent branching of bands linked together by so-called anastomoses. Calculations with a competitive particle growth (CPG) model show that lateral instabilities in Liesegang bands (gaps and radial alleys of gaps) are the result of Ostwald ripening effects taking place after precipitation. Apparent branching of bands or formation of anastomoses can be simulated with a prenucleation model according to Ostwald's supersaturation theory. Similar irregularities can be observed in zebra rocks (e.g. banded siderite) whose bandings are commonly explained by sequential sedimentation processes. A very different mechanism is assumed to be responsible for the origin of mud bands in snow sediments. An initially homogeneous distribution of intrinsic mud in snow sediments can be arranged into parallel bands according to a crystal zoning mechanism which is based on repeated thawing and freezing of the snow sediment due to the daily alternation of sun and darkness. 相似文献
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An isothermal haze chamber (IHC) has been designed, incorporating results from the tests of earlier instruments held at the 1981 Reno (NV) workshop. This instrument, named isothermal haze chamber with discontinuous flow (IHCDF), periodically discontinues the flow through the haze chamber in order to form a well-defined region of 100% RH in the center of the chamber. Indeed, tests performed with our conventional continuous flow IHC showed a poorly defined 100% RH field in the chamber, mainly due to an unsteady laminar air flow through the chamber during measurement.Experiments with polydispersed NaCl aerosol showed that a supersaturation spectrum could be obtained within a growth time of 300 s, over a supersaturation range of 0.01–0.1%.The instrument is suitable for field use. It is contained in a package 1.4 m high and weighs 5 kg. Three minutes is required to collect the air sample and 5 min for drop growth, thus permitting the analysis of seven samples per hour. 相似文献
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