Understanding Gypsum Soils.

Gypsum soils are perplexing, as they are both a problem and an interesting puzzle.  High gypsum (gypseous) soils present complications with corrosion of concrete, metal, and building materials, typically low water and nutrient retention, and dissolution piping and subsidence.  On the other hand, understanding and interpreting the formation, morphology, water movement, and function of gypsiferous and gypseous soils is intriguing and challenging. The CaSO₄–H₂O system in nature consists of three minerals depending on the degree of hydration - gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄).  Chemical names for these three hydration states are dehydrate, hemihydrate, and anhydrous or anhydrate, respectively. Dissolution and precipitation of these three minerals depends on temperature, vapor pressure, and water activity and is represented by the phase diagram for the CaSO₄–H₂O system. Gypsum is the stable phase at low temperatures (below 42 °C), anhydrite is stable at higher temperatures, and bassanite is metastable at all temperatures. The phase diagram for the CaSO₄–H₂O system is an important consideration when discussing mineral stability, along with calcium sulfate mineral persistence (both spatially and temporally) in the soil environment. Pedogenic stability of gypsum or anhydrite impacts taxonomic classification and interpretations. Both CaCO₃ and soluble salts (NaCl) can occur with gypsum in desert environments. Solubilities of gypsum and anhydrite increase substantially in the presence of NaCl. Conversely, the solubility of gypsum and anhydrite will be much lower in the presence of CaCO₃ due to the common ion effect. Gypsum content strongly affects the soil chemical and physical properties of gypseous soils.  Standard pedogenic concepts of illuviation and cementation have questionable applications to gypsum soils. Results from standard laboratory analyses for particle-size distribution on gypsiferous and (particularly) gypseous soils do not represent the “functional texture” of these soils and affect soil interpretations.