CRITERIA FOR THE CHARACTERIZATION OF KAOLINITE, HALLOYSITE, AND A RELATED MINERAL IN CLAYS AND SOILS

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Abstract
The methods of differential thermal analysis, powder X-ray diffraction, and electron microscopy provide criteria for identification of kaolinite and halloysite in clays. These are evaluated with regard to soil clays. When the shape of the main endothermic peak on the differential thermal analysis curve of a kaolin mineral is expressed as the ratio of the tangents of the angles of slope of the 2 sides of the peak, the values of the slope ratios for halloysites are greater than 2.5 and for kaolinites are generally less than 1.8. Slope ratio can be used positively to identify kaolinite. The presence of (hkl) X-ray reflections with h, k and 1 not equal to zero, and (001) reflections having interplanar spacings from 7.15 to 7.20 A are also definite kaolinite criteria. Electron microscopical observation of hexagonal plates of kaolinite and cylindrical forms of halloysite offer further definite criteria. Results of the 3 methods are entirely consistent for kaolin mineral specimens. In many soil clays containing kaolin, both kaolinite and halloysite were present as well as small particles of a more poorly crystallized material having certain structural properties of both kaolinite and halloysite, suggesting a separate kaolin mineral rather than an intermediate form.

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