Soils Whose Main Component Is Gypsum.

Soils whose main component is gypsum (CaSO₄●2H₂O), or whose behavior is governed by this mineral, occur in dry areas having gyprock outcrops or other sources of gypsum. Gypseous soils, often occurring in sparsely populated areas, went unnoticed for years even eluding talented scientists. Now, scientists have confirmed that gypseous soils occur on all Continents.

Many gypseous soil horizons fit in the Soil Taxonomy definitions of Gypsic or Petrogypsic, and in similar categories of other soil classification systems. The predominance of gypsum, whose contents can surpass 90%, makes this mineral dominate soil chemical and physical properties, that are governed by the size, shape, and arrangement of the gypsum particles, their solubility, and other attributes; moreover the presentation of the gypsum differs from most fresh gyprocks. Gypseous horizons are often made by microcrystalline gypsum, with a flour-like touch and a white or pinkish-white color. When coarser crystals are abundant, the touch is gritty and the white color less prominent. These materials are poorly structured, except for blocks of metric sizes, and their gypsum grains appear well separated under the microscope. Another gypseous materials are the travertinic variants, with hard bread-crumb touch and white color, a vughy appearance, and interpenetrating crystals under the microscope. The rheology of gypseous soil horizons is not well known, and a physical model of the interaction of gypsum particles under different moisture conditions is needed to refine the current field criteria for distinguishing Gypsic from Petrogypsic diagnostic horizons and to unveil their genesis.