Determination of Gypsum Content in Dryland Soils Exploiting the Gypsum–Bassanite Phase Change

The presence of gypsum in soil, even in small amounts, is relevant from the genetic, taxonomic, and applied points of view. Moreover, those soils having gypsum as the main component, for example gypseous soils, host a number of rare and endangered organisms. Determining gypsum content in soils is crucial to understanding their behavior; however, current methods of determination are cumbersome or imprecise. This study was conducted to develop an accurate new method to determine soil gypsum content by reducing the inaccuracies or poor assumptions in previous methods. We determined gypsum content in samples using quartz sand and clay minerals mixed with gypsum reagent powder to 0 and approximately 2, 5, and 50% gypsum content; we also studied seven gypseous soil samples. Gypsum (CaSO₄·2H₂O), after heating at 105°C, dehydrates, losing the two water molecules of the gypsum crystal cell and becoming mostly anhydrite (CaSO₄). Anhydrite is very hygroscopic, absorbing atmospheric water very quickly; this process has been found to occur even in desiccators where the relative humidity (RH) is between 12 and 20% depending on the desiccant. Anhydrite rehydrates under laboratory conditions to bassanite (CaSO₄·0.5H₂O), which is stable at RH values <100%. We propose exploiting this difference in mass between gypsum and bassanite to quantify the gypsum content of soils. We found that by equilibrating the soils at constant RH, e.g., 40%, before and after heating the samples, the clay water content is constant, eliminating a major error with other thermogravimetric methods. This simple, yet effective method determined gypsum content to within 1% in all the mineralogies studied, whereas other oven-based methods overestimated gypsum content by as much as 36% in a pure Ca-montmorillonite sample.