Characterization of clay mineral fractions in tuffisitic kimberlite breccias by X-ray diffraction

Kimberlite is described as a diverse and complex hybrid rock in the literature. It is also described as volatile rich, potassic, ultrabasic, igneous rocks that occur as volcanic pipes, dykes and sills1. The minerals present in kimberlites vary widely and can include from trace to predominance any of the following minerals: apatite, calcite, chlorite, diopside (pyroxene), magnetite, monticellite, olivine, phlogopite (mica), quartz, serpentine, smectite, talc and vermiculite. Tuffisitic kimberlite breccia (TKB) is one of several types of kimberlite that occur in both Cullinan (previously Premier) and Venetia mines. The formation of tuffisitic kimberlite breccia is discussed by Clement2. Bartlett8 gives an account of the kimberlite geology and related aspects of the Cullinan diamond pipe. The volcanological and structural aspects of the Venetia tuffisitic kimberlite breccia are described by Kurszlaukis and Barnett3. These two types of kimberlite contain different clay minerals in various quantities, apart from their specific assemblage of other kimberlitic minerals. The presence of more than one type of clay mineral in a kimberlite (or members from different clay mineral groups) can cause overlapping of peaks on X-ray diffractograms and results in complications when characterizing kimberlite mineralogy by X-ray diffraction (XRD). This problem can be resolved only by additional treatment of the sample to change the diffraction properties of the clays present. Different treatments of clay-rich ores to enable accurate identification of the peaks have been discussed by a few authors4–7. The use of strongly orientated specimens is suggested by Bühmann4 and Moore and Reynolds7 as this enhances the basal (001) reflections which are the most diagnostic. The clay fraction of the ore can also be treated with ethylene glycol, glycerol and/or hydrazine before being heat-treated at 500°C. The XRD scans of these differently treated clay preparations are then compared to review the movement, collapsing or enhancement of peaks as discussed by Bühmann4. In this paper these sub treatments when utilizing XRD are applied to kimberlite material. During an investigation of the engineering properties of a number of kimberlites it was realized that the type and quantity of clay minerals in kimberlites relate to certain of these properties, especially weathering characteristics. Tuffisitic kimberlite brecccia can contain a significant clay component, which influences its physical properties. Samples from the Cullinan and Venetia TKBs were selected as both contain various clay minerals in different quantities. The purpose of this investigation is to establish the presence, identity, and relative abundance of clay minerals from both sample locations by XRDtechniques, especially after the abovementioned preparation techniques were carried out to alter the diffraction properties. Characterization of clay mineral fractions in tuffisitic kimberlite breccias by X-ray diffraction