As very little detailed X-ray diffraction investigations have been carried out in South Africa on 2:1 phyllosi1icates in soils, the aim of the present study was to contribute to the knowledge of soil genesis, as well as K-fixation and swelling, by investigation of the clay fraction of selected soils known to be rich in these minerals. X-ray diffraction analysis has been used almost exclusively as the investigative technique. In Chapter 1 a literature review is presented on the reasons for X-ray diffraction peak broadening and the problems encountered in the identification of swelling clay minerals. For interstratifications, the concept of an ABAB layer sequence, considered as having suggested an abab that the inter1ayer space, X-ray diffraction is questioned. data from which It is the ABAB arrangement is inferred can as well be explained in terms of an alternative AAAB layer sequence, having an aabb interlayer arrangement. Chapters 2, 3 and 4 deal with layer silicate formation/alteration in the course of soil development in dolerite and shale-derived profiles. Dolerite-derived pedons could· be characterized by one of the following layer silicate suites : suite i : discrete smectite (Fe-containing beidellite-montmorillonite) with or without traces of kaolinite and talc (Vertisol) suite ii : smectite-kaolinite interstratification (Vertisol) suite iii : 14 ft minerals (vermiculite, beidellite, montmorillonite, chlorite) and 7 ft minerals (halloysite, kaolinite ) in about equal proportions (Vertisol and Mollisol) suite iv : kaolinite with subordinate chlorite and traces of talc (Oxisol, Ultisol). Eccashale-derived Vertisols are dominated by mica-smectite interstratifications. The occurrence of an iron-rich pedogenic talc is discussed in Chapter 4. X-ray and chemical data suggest 30 - 50 mole percent substitutions of iron for magnesium. The mineralogical basis for K-fixation has been established in Chapter 5. Two K-fixing components could be identified : dioctahedral high-charge vermiculite as a discrete mineral and random mica-smectite interstratifications with 20 - 60% mica. In Chapter 6, some of the most expansive soils in South Africa have been investigated. They can be subdivided into two groups denoted by the swelling component as follows : (a) smectite-dominated (the smectite species involved being most probably beidellite with a heterogeneous charge distribution); (b) mica-smectite interstratification with random or ordered stacking arrangement. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1986.