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Crystalline and Amorphous Soil Minerals of the Mississippi Coastal Terrace¹

ABSTRACT

Vernniculite and "allophane" are predominant clays of poorly drained Gulf coastal soils, while vermiculite and gibbsite are found in the better drained soil studied. Vermiculite present in both situations is interlayered with alumina, somewhat similar to that described by Rich in Virginia soils. Removal of the interlayer alumina allows collapse of the mineral, but no expansion beyond 14Å. The ability to fix large amounts of K was also induced by the treatment, indicating the vermiculitic nature of the mineral.

The presence of an amorphous, aluminous mineral was evident from the nature of the X-ray diffraction patterns, chemical analyses and from electron photographs. The most direct means for its determination, however, was the use of an infrared absorption technique. A very strong 2.78 micron hydroxyl absorption maximum was observed in the colloid containing large amounts of alumina. The intensity of the absorption maximum was lessened as amount of the substance decreased.

Presence of these clays in soils derived from Recent or late-Pleistocene marine sands may be explained by either of two alternate hypotheses: (a) glauconite weathering or (b) deposition of weathered micas in the presence of aluminous precipitate in semimarine environment. Either or both processes may have been operative.

¹ Contribution from Mississippi Agr. Exp. Sta., State College, publication No. 801. Presented before Div. II, Soil Science Society of America, Nov. 19, 1959, at Cincinnati, Ohio.

² Formerly Associate Professor, Mississippi Agr. Exp. Sta. Present address is ARS, USDA, Beltsville, Md.

Received for publication October 26, 1959. Accepted for publication January 5, 1960.