beforehand. The Gartly clay mineral is also unusual in that Mg-saturation, using ... lonites with similar swelling characteristics (Walker, 1958). Chlorite may be.
Clay Minerals (1969) 8, 39.
E F F E C T S OF P R E T R E A T M E N T ON A 14.~ SWELLING MINERAL FROM GARTLY, ABERDEENSHIRE J. L. P E R E Z - R O D R I G U E Z *
AND M. J. W I L S O N
The Macaulay Institute for Soil Research, Aberdeen
(Received 10 September 1968)
ABSTRACT: A 14 A swelling mineral, interlayered with what is probably an organometallic complex, occurs in the surface horizon of a gleyed soil near Gartly, Aberdeenshire. The interlayer material is thermally stable at 300r C and is easily removed by oxidation with H202 and NaOC1. Treatment with HC1 and Na2EDTA destroys the ability of the mineral to expand with glycerol, although not if the clay is H202 treated beforehand. The Gartly clay mineral is also unusual in that Mg-saturation, using magnesium acetate at pH 7, leads to the rapid formation of brucite-like interlayers. It is concluded that in the investigation of soil clays it is best to avoid all pretreatments.
The positive identification of the 14 A minerals commonly found in soil clays has long been a subject of study. In general, montmorillonite and vermiculite may be distinguished by observing the effect of a polar organic liquid such as glycerol on the X-ray diffraction pattern, although under certain circumstances presaturation with Mg 2+ m a y be necessary to distinguish between vermiculites and montmorillonites with similar swelling characteristics (Walker, 1958). Chlorite m a y be differentiated from vermiculite by observing the response of the 14 A reflection after heating at 300 ~ C; at this temperature the chlorite structure remains stable whilst vermiculite collapses to 10 A, although precautions against rehydration during cooling must be taken. If interlayer contaminants are present this structural contraction is impeded and usually spacings intermediate between 10 and 14 A are recorded. Swelling chlorite m a y be detected by a combination of glycerol and heat treatment. However, some soil clays, such as those described by Beutelspacher & Fiedler (1964), are not amenable to standard diagnostic procedures. These clays yielded a broad diffraction band between 10 and 14 A which varied in position and intensity according to various pretreatments, thereby precluding unequivocal identification. The writers encountered similar difficulties during an investigation on some Aberdeenshire soil clays. It was found that several commonly used pretreatments affected the dominant 14 A mineral to such an extent that a positive identification was rendered * Present Address: Centro de Edafologia y Biologia Vegetal, Cortijo de Cuarto, Seville. Spain.
40
J.L. Perez-Rodriguez and M. J. Wilson
extremely difficult. Especially important were the effects observed after removal of the organic matter by H202, an observation that may be of appreciable significance should these effects be general in soils rich in organic matter. EXPERIMENTAL
Material The soil studied in this investigation is located near the village of Gartly, Aberdeenshire (NJ/522323). It is a gley, developed on glacial drift derived from Devonian andesitic lavas. The profile is about 112 cm deep, the lower 80 cm of which show the rusty brown mottling typical of impeded drainage. The surface horizon is rich in organic matter (23%) and it was from this part of the profile that the clay used in this study was separated. Methods Approximately 5 g of soil were dispersed in water using an ultrasonic probe at 20 KHz for 5 min and the clay was separated into two fractions---
