Gypsum weathering

Then we calculate the proportion of Ca originating from evaporite (gypsum) weathering (CaeVap(GYp)) as follows ... 

Tortosa, where dissolved exportation derived from gypsum weathering is about 1.5 times derived from carbonate weathering. 

Hydration is the incorporation of water mole-cule(s) into a mineral, which results in a structural as well as chemical change. This can drastically weaken the stability of a mineral, and make it very susceptible to other forms of chemical weathering. For example, hydration of anhydrite results in the formation of gypsum ... 

The action of carbonic acid on limestone produces a calcium bicarbonate solution that is exceedingly soluble in water. (For comparison, at 20°C the solubility of calcium carbonate in water is only 0.0145 g per liter while the solubility of calcium bicarbonate is 166 g per literJ ) Magnesium ions from dolomite are also released into aqueous solution according to the same mechanism. The weathering of gypsum, calcium sulfate, also releases calcium ions into natural water supplies. 

Coarse-sized particles dominate the particle size distribution of arid soils. Some soils are also quite gravelly. The subsurface horizons commonly exhibit accumulation zones of carbonates, gypsum and more soluble salts. Many arid zone soils are shallow and gravelly, some are alkaline. Their structure is weak. From most soils, clay accumulation horizons (argillic horizons) are absent, or are only weakly developed, and so are minerals that indicate an advanced degree of weathering. 

Fig. 9 TDS derived from carbonate weathering (TDScarb) and from evaporite (gypsum) dissolution ("l DSevaplGY,.,) versus total TDS for all the monitoring stations. All expressed in mg L 1...

In contrast to the fate of silicates, a catchment exhibiting a small area of so-called Biindner Schiefer, a sandy-marly schist containing soluble anhydrite or gypsum, will produce a remarkably high weathering rate for the entire basin. This effect arises in the alpine catchments of the Ticino, Rhine, and Rhone. The occurrence of Biindner Schiefer also causes sulfate concentrations in the range of 0.5-1 mmol in rivers. Natural and anthropogenic atmospheric sulfur... 

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