Colors of Acid Sulfate Soils
- D.S. Fanning,
- M.C. Rabenhorst and
- J.M. Bigham
The colors of acid sulfate soils reflect various stages in the process of sulfuricization. Sulfidic mineral materials associated with potential acid sulfate conditions typically have chroma ≤ 1 and values ≤ 4. Hues (when present) range from 10YR through 2.5Y and 5GY, to 5G and 5B. Black (N2) materials often contain Fe monosulfides occurring as very fine, x-ray amorphous particles. Monosulfides evolve H2S when exposed to HCl. Pyrite, the most common Fe sulfide mineral, is also black when powdered, but does not pigment soil materials as much as the monosulfides because of coarser grain size and does not evolve H2S with HCl. Sulfidic organic materials commonly have higher chromas (normally 2 or more) and browner hues than sulfidic mineral materials. The oxidation of sulfides may produce a variety of relatively soluble sulfate minerals, including melanterite, rozenite, and copiapite, that have different colors but that are transient phases in the formation of jarosite and insoluble Fe oxides. The latter may include ferrihydrite, goethite, hematite, lepidocrocite, and an akaganéite-like mineral (schwertmannite) that is especially abundant in acid drainage waters. All these minerals possess distinctive colors and may occur in active or postactive acid sulfate soils. In particular, jarosite and Fe oxides are often found as bright mottles in channels and on ped faces in sulfuric horizons with low chroma (≤ 2) colors (reflecting less-oxidized conditions) in ped interiors. Jarosite may have lower chromas than was specified for the sulfuric horizon in Soil Taxonomy. Revised sulfuric horizon criteria in the 1992 Keys to Soil Taxonomy do not specify the color of jarosite and permit jarosite to be absent from sulfuric horizons in some cases. In postactive acid sulfate soils, jarosite is commonly lost by dissolution from the surficial and upper B horizons. In tropical areas, the Fe released by jarosite decomposition appears to form hematite. Other sulfate minerals that are white or colorless, such as gypsum and barite, may also appear in some postactive acid sulfate soils. The identification of acid sulfate soils by color or mineralogy at various development stages is an obvious aid to understanding their genesis. The recognition of potential acid sulfate conditions is also important for preventing undesirable environmental situations that may result when sulfidic materials are drained or inadvertently exposed at the earth's surface.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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