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Book: Acid Sulfate Weathering
Published by: Soil Science Society of America



  1.  p. 147-168
    SSSA Special Publication 10.
    Acid Sulfate Weathering

    J.A. Kittrick, D.S. Fanning and L.R. Hossner (ed.)

    ISBN: 978-0-89118-905-3


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Gypsiferous Soils in the Western United States

  1. W. D. Nettleton,
  2. R. E. Nelson,
  3. B. R. Brasher and
  4. P. S. Derr1


Gypsum (CaSO4·2H2O) is the only pedogenic calcium sulfate mineral that has been found in soils with ustic, xeric, and aridic moisture regimes. It has been found in soils in 14 of the 17 conterminous western states by the National Soil Survey Laboratory and likely will be found in the other three. In these arid to subhumid soils, parent material differences in large part control the occurrence of gypsum. But gypsum in soils can be from other sources too. For example, drainage of coastal wetlands oxidizes sulfides to acid sulfates, and in their reclamation, if not before, the acid sulfates are neutralized by carbonate to form gypsum. Gypsum is also formed in minesoils by neutralization of the acid sulfates released by oxidation of sulfides. Pedogenic gypsum, in contrast to allogenic gypsum, accumulates in subsurface horizons relative to surface and underlying horizons mostly as euhedral to subhedral spindle-shaped crystals in pores and veins. As the s-matrix in a developing gypsic horizon becomes plugged, the pore volume decreases and the restricted hydraulic conductivity keeps the soil moist longer allowing the growing gypsum crystals to interlock and indurate the horizon. Subsidence of soils through solution and removal of gypsum can crack building foundations, break irrigation canals, and make roads uneven. Concrete in slab structures, irrigation canals, and building foundations deteriorates and cracks as extremely high pressures develop during formation of highly hydrated ettringite [Ca6·Al2(SO4)3(OH)12·26H2O] or during conversion of thenardite (Na2SO4) to mirabilite (Na2SO4·10H2O) if the temperature in the concrete and the soil drops low enough.

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