Clay mineral associations in melanic soils of South Africa

Abstract

The melanic horizon is 1 of 5 diagnostic topsoil horizons distinguished in the South African soil classification system. Melanic soils span a wide spectrum, ranging from those that intergrade with a vertic to those that intergrade with a humic horizon. Melanic soils are therefore expected to vary considerably with respect to a variety of physical, chemical, and clay mineralogical properties. The objective of the present study was to determine the clay mineral compositions of melanic horizons from 58 modal profiles and to establish to what extent melanic soil properties are related to clay mineralogy. Special emphasis was placed on the characterisation of the clay fraction in terms of group and species identification. X-ray diffractometry was employed almost exclusively as the investigative technique in mineral identification and quantification. Melanic A horizons showed a large degree of variation with regard to their clay mineral associations. More than half of the soils were dominated by smectite, 30% by kaolinite, and the rest by an association of about equal proportions of mica, kaolinite, and smectite. Talc and hydroxy-interlayered vermiculite occurred in a number of soils while one horizon was dominated by an illite/smectite interstratification. The smectite component was identified as belonging to either beidellite or vermiculite species, depending on the method employed. About a quarter of the smectitic soils contained montmorillonite as well but not as the dominant swelling phase.

mollisols, clay mineralogy, layer charge.