Chapter 10 - Gypsic Features

Abstract

Gypsic features, composed of gypsum (Ca(SO4)·2H2O), are most abundant in soils in arid or semiarid climates. Gypsum occurrences in soils are generally pedogenic, formed through progressive mineral precipitation from saturated solutions. Some hypergypsic horizons developed on gypsiferous parent materials. Lenticular gypsum is the most common morphological type of gypsum in soils. Four main types of gypseous materials are distinguished: powdery gypsic materials, massive accumulations of microcrystalline gypsum, rupture-resistant materials composed of interlocking crystals and eolian gypsarenites. Gypsic pedofeatures include infillings, coatings, nodules, and crystals and crystal intergrowths. Their nature is partly related to the distance from the water table, with microcrystalline gypsum at the top of the profile and coarse-grained gypsum close to the water table. Infillings are composed of lenticular or tabular crystals, which can evolve to dense aggregates with a mosaic-like fabric. Structures with a crescent fabric are commonly formed by biological activity. Gypsum nodules are formed by local gypsum precipitation and can be polyphasic. Scattered gypsum crystals and crystal intergrowths are common in gypsiferous soils, sometimes with thin bands (dust lines) outlining earlier growth stages. Dissolution features, which are often observed for irrigated soils, include corrosion of the crystal faces, dissolution along cleavage planes and rounding. Pseudomorphic voids and calcite pseudomorphs after gypsum crystals are quite frequent and record changes in soil conditions through time. The most widely used soil classification systems define gypsic and petrogypsic horizons on the basis of the gypsum content and origin, which can be studied using micromorphological techniques. In addition, micromorphology can also help to understand and predict the behaviour of gypsum-bearing soils for agricultural, environmental and civil engineering land use.