The Occurrence of Sepiolite and Attapulgite in the Calcareous Zone of a Soil Near Las Cruces, New Mexico

Abstract
Sepiolite and attapulgite were identified by X-ray diffraction, elemental, and thermal analyses in the calcareous zone of a soil formed on a relict basin-fill plain in Dona Ana County, New Mexico. Sepiolite occurs in the lower part of the zone of calcium carbonate accumulation; attapulgite occurs above, in, and below the sepiolite zone. The sepiolite was observed as whitish aggregates remaining after the carbonate was dissolved with sodium acetate buffer (pH 5). Extended treatment with this buffer dissolved a significant amount of the sepiolite but did not appear to dissolve much attapulgite. Sepiolite was concentrated for analysis without significant solution by separating the <100 mesh material from the crushed soil, treating it with pH 5 buffer for a short time and separating the carbonate-free clay. The sepiolite is relatively high in aluminum compared to most sepiolites. The sequence of dominant clay mineral with depth in the profile is montmorillonite, attapulgite, sepiolite, attapulgite, montmorillonite. No sepiolite or attapulgite was found above the calcareous zone; a little attapulgite was found at the base of the profile, which was not totally free of carbonates. Thin sections showed sepiolite and attapulgite occurring as aggregates, as coatings on mineral grains and as small fibers. The distribution of these minerals in the profile and the arrangement of the small fibers in the calcrete suggest that these minerals crystallized during the period of caliche formation. As will be seen (Table 5) the sepiolite is present mainly in the nodular calcareous zone; the attapulgite, mainly in the blocky calcareous zone and in the transition zone below the calcareous zone (C6).

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