Slope Ratios of Differential Thermal Curves of Dickite

Abstract

The relationships between the shapes of kaolinite and dickite differential lhermal curves are described. The slope ratios of dickite are distinctly greater than those of kaolinite of equivalent particle size for the endothermic peak at about 550~ and the same effect is observed for the exothermic peak at about 980~ especially in diluted samples. The packing density of dickite does not differ markedly from that of kaolinite and the curve obtained for weight vs. area of the endothermic peak is in agreement with this. The dickite samples, however, give slightly lower areas than those of kaolinite, an effect that is dependent on the shape of the particles. The exothermic peak is shown to be influenced considerably in shape and size by thermal gradients. INTRODUCTION Although considerable research has been carried out on the less well-ordered kandites, not so much attention has been paid to dickite and nacrite, especially as regards the m a t h e m a t i c a l relationships and shapes of the curves resulting from differential thermal analysis. In view of the restricted natural occurrence of these members of the group, the smaller amount of attention is not surprising. In this paper comparison is made between dickite and kaolinite and the relative shapes of the curves are utilised to gain an insight into the actual mechanism of the dehydroxylation reactions occurring on heating. EXPERIMENTAL The apparatus used had a temperature rise of 10~ nickel block and cover, chromel-alumel therrnocouples, and recording was by means of electronic recorders. The samples investigated were those used as standard clay samples by Kerr 0951) and were as follows, the number in brackets referring to the A.P.I. number: dickite, on U.S.550, near Red Mountain Pass, south of Ouray, Colorado, U.S.A. (No. 14); dickite, Bull Valley, 40 miles north-west of St. George, Washington Co., Utah, U.S.A. (No. 16); kaolinite, from Twin Knobs, Murfreesboro, Pike Co., Arkansas, U.S.A. 17 18 R. J. W. McLAUGHLIN (No. 1); kaolinite, from O'Neal pit 8.8 miles south-east Macon railway station, Georgia, U.S.A. (No. 4). The 2-20/z equivalent spherical diameter (e.s.d.) fraction was separated from the samples by centrifugal means, this fraction comprising almost the entire sample in the case of the dickite samples. X-ray examination and chemical analysis of these fractions revealed no foreign minerals and f rom the figures for the latter (Table 1) it will be seen that chemically the specimens are almost identical, so that any differences in behaviour must be largely due to morphological effects. The general chemical relationships between these and other kandites have been reported elsewhere (McLaughlin, 1959). TABLE 1--Chemical analyses of 2-20~ e.s.d, fraction of various kandites (in .per cent.)