Rony
Moderator
Abstract
Dissolution of sulfate rocks proceeds by different mechanisms and at
different rates compared to those associated with the dissolution of
carbonate rocks. Gypsum dissolves by a simple two-phase dissociation.
Anhydrite, when dissolved, forms a solution of calcium sulfate, which at
common temperatures and pressures is in equilibrium with the solid
phase of gypsum, but not with anhydrite.
The solubility of gypsum at 20°C is 2.531 g/L or 14 mM/L, which is
roughly 140 times lower than the solubility of common salt, but two
orders of magnitude greater than the solubility of CaCO3 in pure water.
The difference in solubility between gypsum and calcite decreases to
10-30 times if the latter is dissolved in the presence of CO2.
The dependence of the solubility of gypsum on temperature is
nonlinear, reaching a maximum at 43°C. Hydrostatic pressure does not
substantially affect the solubility of gypsum, but the solubility increases
sharply with pressure applied to the gypsum rock. The solubility differs
for grains of varying sizes. The solubility of gypsum is boosted with the
presence of other salts (foreign ions) in solution, up to three times, for
example, in the presence of NaCl, and up to six times in the presence of
Mg(NO3)2.
These effects are important for speleogenesis, as gypsum is commonly
associated with other salts within evaporitic formations. The presence of
common ions does not decrease the gypsum solubility considerably. The
bacterial or thermal reduction of sulfates rejuvenates the dissolution
capacity of water by consuming sulfate ions. Dedolomitization has a
similar effect. Both processes have very important consequences for
speleogenesis in deep-seated settings.
The dissolution of gypsum follows a fifirst-order rate law, whereas the
dissolution rate of anhydrite obeys a second-order law. Rates of gypsum
dissolution are very high in the region far from saturation, but they
decrease abruptly when the solution approaches high saturation levels.
Because of transport-controlled dissolution kinetics, rates are strongly
dependent on boundary-layer conditions within the flflowing solution.
They are affected by velocity, the ionic strength of the solution, and its
temperature.
Conversion of gypsum to anhydrite and back to gypsum is common
due to the thermodynamic instability of these minerals within the
physicochemical range of common geologic environments. The
mechanisms and rates of conversion of anhydrite to gypsum are still
poorly understood. They display complex dependencies on the tectonic
regime, on the water-bearing properties of surrounding formations, and
on both the regional and local flflow regimes. Contrary to a common
view, little or no expansion of volume occurs during hydration of
anhydrite in most underground environments.
Dissolution of sulfate rocks proceeds by different mechanisms and at
different rates compared to those associated with the dissolution of
carbonate rocks. Gypsum dissolves by a simple two-phase dissociation.
Anhydrite, when dissolved, forms a solution of calcium sulfate, which at
common temperatures and pressures is in equilibrium with the solid
phase of gypsum, but not with anhydrite.
The solubility of gypsum at 20°C is 2.531 g/L or 14 mM/L, which is
roughly 140 times lower than the solubility of common salt, but two
orders of magnitude greater than the solubility of CaCO3 in pure water.
The difference in solubility between gypsum and calcite decreases to
10-30 times if the latter is dissolved in the presence of CO2.
The dependence of the solubility of gypsum on temperature is
nonlinear, reaching a maximum at 43°C. Hydrostatic pressure does not
substantially affect the solubility of gypsum, but the solubility increases
sharply with pressure applied to the gypsum rock. The solubility differs
for grains of varying sizes. The solubility of gypsum is boosted with the
presence of other salts (foreign ions) in solution, up to three times, for
example, in the presence of NaCl, and up to six times in the presence of
Mg(NO3)2.
These effects are important for speleogenesis, as gypsum is commonly
associated with other salts within evaporitic formations. The presence of
common ions does not decrease the gypsum solubility considerably. The
bacterial or thermal reduction of sulfates rejuvenates the dissolution
capacity of water by consuming sulfate ions. Dedolomitization has a
similar effect. Both processes have very important consequences for
speleogenesis in deep-seated settings.
The dissolution of gypsum follows a fifirst-order rate law, whereas the
dissolution rate of anhydrite obeys a second-order law. Rates of gypsum
dissolution are very high in the region far from saturation, but they
decrease abruptly when the solution approaches high saturation levels.
Because of transport-controlled dissolution kinetics, rates are strongly
dependent on boundary-layer conditions within the flflowing solution.
They are affected by velocity, the ionic strength of the solution, and its
temperature.
Conversion of gypsum to anhydrite and back to gypsum is common
due to the thermodynamic instability of these minerals within the
physicochemical range of common geologic environments. The
mechanisms and rates of conversion of anhydrite to gypsum are still
poorly understood. They display complex dependencies on the tectonic
regime, on the water-bearing properties of surrounding formations, and
on both the regional and local flflow regimes. Contrary to a common
view, little or no expansion of volume occurs during hydration of
anhydrite in most underground environments.