Abstract: Quantitative X-ray diffraction analysis of minerals can most readily be performed on dry powdered samples. Current methods for determination of expandable clays require either wet oriented samples treated with glycerine or glycol or dry, randomly oriented samples treated with amines. Trimethylamine has been used to expand the montmorillonite structure to a 13.3 Å basal spacing. Unfortunately, this spacing is subject to considerable peak interferences.
We therefore undertook a review of amine-montmorillonite complexes for more suitable reagents. The formate and acetate salts of tri-n-hexylamine and of dimethyl-benzylamine meet our requirements. The peak positions are the same as for glycerine complexes, while the physical properties are the same as for trimethylamine.
Detailed studies of tri-n-hexylammonium ion absorption suggest single layers of ammonium ions lying parallel to the crystal basal plane, double layers in similar orientation, and a single layer with molecules on end. The most useful orientation is the latter, which occurs in the presence of an excess of the ammonium salt and gives a montmorillonite spacing 18 Å in dry powdered samples.
Linear calibration curves with a standard deviation of about 5 per cent of the amount measured can be prepared using the conventional internal standard techniques. Minimum detectable amount of montmorillonite with rate-meter techniques is 0.4 per cent and 0.2 per cent with scalar techniques.
Several thousand analyses using tri-n-hexylammonium ion have demonstrated its effectiveness for expanding a wide variety of sediments.