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Quantitative Determination of Vermiculite in Soils¹

ABSTRACT

A chemical method based on potassium fixation was developed for quantitative determination of vermiculite in soils. The cation-exchange capacity (CEC) of a sample is determined by washing with CaCl₂ and replacement of Ca with MgCl₂. The sample is then washed with KCl, heated to 110C overnight to dehydrate and collapse the layers of vermiculite and to fix K. The K remaining exchangeable is determined by NH₄Cl washings. The difference between these two CEC values gives the interlayer charge of vermiculite. A number of vermiculite standards and soil samples of widely different clay mineral compositions give remarkably near 100% totals when the vermiculite content is based on the average value of interlayer charge of 154 meq/100 g of vermiculite and when the other minerals present are appropriately determined (mica by K₂O, chlorite by ignition loss, montmorillonite by CEC measured by non-fixed K, quartz and feldspar by Na₂S₂O₇ fusion, allophane, kaolinite, and halloysite by selective dissolution in NaOH). The total CEC of vermiculite is 159 meq/100 g when the allocated external surface charge of 5 meq is included.

¹ Contribution from the Department of Soil Science, University of Wisconsin, Madison. Supported in part by a grant from Fulbright-Hays Act and through the Committee on International Exchange of Persons, Conference Board of Associate Research Councils, in part by a grant through the Research Committee of the Graduate School from the Wisconsin Alumni Research Foundation and in part by National Science Foundation grants G13793 and GP-4144—Jackson.

² Associate Professor of Soil Science, University of Thessaloniki, Greece (on leave), and Professor of Soil Science, respectively.

Received for publication November 12, 1964. Accepted for publication June 18, 1965.