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Book Chapter

Evaluation of the Structural Formula and Alkylammonium Methods of Determining Layer Charge

By
David A. Laird
David A. Laird
US DA -Agricultural Research Service National Soil Tilth Laboratory 2150 Pammel Drive Ames
IA 50011
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Published:
January 01, 1994

Abstract

The structural formula and alkylammonium methods are completely independent analytical techniques for determining the layer charge of 2:1 expanding phyllosilicates (i.e., smectites and vermiculites). Other methods of determining layer charge are empirical and rely on calibrations with values of layer charge determined by either the alkylammonium or structural formula methods. The intent of this chapter is to provide a critical evaluation of both the alkylammonium and structural formula methods. Theoretical bases for these methods are discussed as are the theoretical and practical problems inherent to each method. A direct comparison of values of layer charge determined by the structural formula and alkylammonium methods is presented.

Within the context of this review, "tetrahedral charge" and "octahedral charge" are defined as the net negative charge due to isomorphous substitution in the tetrahedral and octahedral sheets of 2:1 phyllosilicates, respectively. "Permanent charge" is the sum of thetetrahedral and octahedral charges. "Variable charge" is the net negative charge, which is pH dependent, associated with broken bonds on lateral edges of 2:1 phyllosilicates, and "total charge" is defined as the sum of the permanent and variable charges. Although, the term "layer charge" is widely used in discussions of 2:1 phyllosilicates it is rarely defined, In some instances layer charge is used synonymously with permanent charge, whereas in others layer charge is identified with total charge. Most commonly, however, layer charge is operationally defined by a particular analytical method, and that is the sense in which the term is used in this chapter.

Layer charge is

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Contents

Clay Minerals Society Workshop Lectures

Layer Charge Characteristics of 2:1 Silicate Clay Minerals

A. R. Mermut
A. R. Mermut
University of Saskatchewan S.I.P., Department of Soil Science Saskatoon SK., S7N 0W0 Canada
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S. A. Boyd
S. A. Boyd
Michigan state University, East Lansing, MI, USA
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W. J. Farmer
W. J. Farmer
University of california, Riverside, CA, USA
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W. F. Jaynes
W. F. Jaynes
Amercian Colloid Co,. Arlington Heights, IL, USA
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G. Lagaly
G. Lagaly
Univesity of Kiel, Kiel, Germany
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D. A. Laird
D. A. Laird
USDA-ARS, Ames, IA, USA
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A. R. Mermut
A. R. Mermut
University of Saskatchewan, saskatoon, Sk, Canada
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Clay Minerals Society
Volume
6
ISBN electronic:
9781881208266
Publication date:
January 01, 1994

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