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Accueil du site → Doctorat → États-Unis → 1973 → The pedogenic alteration of acicular clay minerals on the semiarid Texas High Plains

Texas Tech University (1973)

The pedogenic alteration of acicular clay minerals on the semiarid Texas High Plains

Bigham, Jerry M.

Titre : The pedogenic alteration of acicular clay minerals on the semiarid Texas High Plains

Auteur : Bigham, Jerry M. 

Université de soutenance : Texas Tech University

Grade : Doctor of Philosophy (PhD) 1973

Présentation
Sepiolite and attapulgite are widespread on the High Plains of West Texas and eastern New Mexico. They appear to be authigenic minerals that have formed as coprecipitates with lacustrine dolomites and calcites ranging in age from early Pleistocene to Holocene. Where soils have subsequently formed in these sediments, or in materials derived from these sediments, the inherited acicular clay minerals are an integral part of the soil mineralogy. Their condition within the soil environment appears to be metastable, however, and when sufficiently intense pedogenesis occurs they are altered and ultimately destroyed. Both the mechanism and products of this alteration process are somewhat unclear. In addition, the data accumulated in this study are insufficient to ascertain the similitude of the behavior of sepiolite and attapulgite under non-equilibrium conditions. Because of their structural and chemical similarity, it is assumed that they also react similarly within the soil environment. Numerous factors, which undoubtedly vary with both time and place, are probably important in determining the alteration process. At best, only generalizations can be made concerning the forces involved and the products of acicular clay mineral weathering.
Initially, the matrix carbonates must have an important effect upon the stability of sepiolite and attapulgite. The exact role of the carbonates is uncertain, but there is little doubt that their removal by leaching processes from the soil zone must be initiated before degradation of the fibrous minerals can occur. Carbonates are commonly the dominant constituents of High Plains lacustrine sediments and their preservative effect on the acicular clay minerals is probably a result of both physical and chemical phenomena. Physically, their most important effect is probably in the engulfment and subsequent isolation of acicular particles from the soil solution and the activities of soil fauna and flora. Chemically, carbonates produce an environment that 2+ 2+ is saturated with Ca , Mg , or both. The importance of 2-f Ca is somewhat doubtful since alteration of sepiolite and attapulgite has clearly occurred in the soils investigated even though free CaCO^ is observed in most horizons. Ion activity measurements strongly suggest, however, that when Mg activities decrease below a critical, and as yet undetermined, level the stability of the acicular minerals is adversely affected. These findings are not unexpected since sepiolite and attapulgite contain relatively high amounts of Mg, Coincident with the removal of carbonates there appears to be a physical breakdown of the acicular particles. Physical degradation is probably a result of natural turbations common to an active soil zone and is manifested by a dimunition in individual fiber length and particle aggregation. There can be little doubt, however, that chemical forces are responsible for the ultimate pedogenic destruction of sepiolite and attapulgite.

Annonce

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