MORPHOLOGY & MINERALOGY OF
WATER DISPERSIBLE SAND & SILTS
Abstract
As a part of the regional investigation of water dispersible clays, sand and silt fractions of 33 regional samples which had been dispersed in distilled water were examined to determine the morphology of the undispersed clays, organic material and interesting features of the fractions. Mineralogy of grains was determined using a combination of energy dispersive fluorescent X-ray spectra and a broad knowledge of soil composition. Many of the soils had aggregates of clay-sized material. In most cases, the aggregates were compact much like pieces of whole soil suggesting that dispersion and fractionation processes had failed to separate individual particles. In other samples the aggregates had an open structure, often nearly monomineralic, suggesting that dispersion of the mineral was selective. Other clay material stayed in the sand fraction as coatings of sand particles or as lithorelicts. Organic material appeared in several morphologies. Many soils contained organic material that exhibited cellular features of the plants from which they were formed. Other organic material appeared as thin sheets or filaments. Several samples contained pollen grains. The organic material did not exclusively appear as a part of aggregates and in most cases where it did appear mixed with mineral grains it did not appear to be the cement holding the aggregate together.
Other interesting features of the soils also were observed. The Cecil Ap has a high concentration of Al2SiO5 (sillimanite or kyanite). The quartz in the sand from the Decatur soil had two very different morphological types. Much of the sand was very slightly altered, angular quartz while many of the remaining grains were highly etched almost to the point of falling apart. The strongly contrasting morphologies suggest two parent materials, probably fresh material from the Piedmont and heavily preweathered Coastal Plain sediments or chert. The Heiden C2 contained many interesting forms of CaCO3 including snail shells with recrystallized calcite on the surface, coccoliths, rhombohedrons, bladed varieties, botryoidal forms, and apparent limestone lithorelicts along with weathered apatite. The Maury and Memphis soils also contained apatite. The Maury soil Bt had sand-sized crystals of Fe oxide, probably hematite. The Humatas soil from Puerto Rico and the Alachua Bh from Florida had kaolinite vermiforms. The Wayah contained weathered, zoned plagioclase grains.
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Last Modified: November 24, 1998
Document Prepared by:
North Carolina Agricultural Research Service
North Carolina State University