X-ray Analysis of Soil Colloids by a Modified Salted Paste Method

Isaac Barshad
University of California, Berkeley

Abstract: Glycerol-ethanol sodium salted clay pastes prepared from clays extracted directly from the soils and not allowed to dry were found to yield superior x-ray diffraction diagrams for the identification of the major clay minerals than pastes prepared from clays that suffered dehydration after extraction.

Prevention of drying of soil clays after their extraction from the soils is essential for proper identification of the clay minerals. This is particularly true for identification of hydrated halloysite and for differentiation between high exchange forms and low exchange forms of montmorillonite. This differentiation is based on the finding that glycerol-ethanol salted potassium-saturated montmorillonite pastes prepared from clay suspensions have a 001 spacing of 18.5 Å whenever the exchange capacity ranges between 92 and 95 meq/100 g oven-dry clay, and 14.5 Å whenever the exchange capacity ranges between 115 and 135 meq/100 g. Either both these spacings or a mixed-layer spacing appears in montmorillonites having exchange capacities between 95 and 115 meq/100 g.

Saturation of pure vermiculite minerals brings about contraction in the 001 spacing from 14.2 Å to 10.6 Å only after air drying in those forms with exchange capacities ranging between 200 and 207 meq/100 g water-free mineral, but in vermiculites with an exchange capacity of 250 meq/100 g this contraction occurs even without drying. Both of these forms of vermiculite are found in soils.

Several sets of “standardized” natural clay mineral mixtures are necessary for quantitative x-ray analysis of soil clays.

Clays and Clay Minerals; 1958 v. 7; no. 1; p. 350-364; DOI: 10.1346/CCMN.1958.0070125
© 1958, The Clay Minerals Society
Clay Minerals Society (www.clays.org)