The application of a new microdiffraction technique to the study of clay minerals

Powder X-ray diffraction techniques typically provide a bulk analysis of a crushed sample. In order to analyse a specific phase the material has to be selectively sampled, and then crushed to prepare it for analysis. This can be time-consuming, and effectively destroys the fabric of the sample. However, with the use of the latest microdiffraction instrumentation (the General Area Detector Diffraction System, or GADDS), it is now possible to obtain powder diffraction analyses from uncrushed, unprepared, and uneven specimens, from areas as small as 50 microns in diameter. The GADDS is produced by Bruker AXS, and is basically new technology applied to the oldest powder diffraction technique, with a two-dimensional position-sensitive area detector used instead of photographic film. Unprepared samples (up to 2 kg) may be mounted on the stage. The spot to be analysed is focused using a laser-video alignment system, and the analysis is collected over a period of time ranging from a few seconds to a few minutes, depending on the diameter of the analysed area. Integration of the diffraction data produces a conventional pattern which is analysed in the same manner as for normal powder XRD. In order to correlate the XRD data with chemical data from the smectites, polished specimens of the laterite clays were analysed by microprobe using high resolution element X-ray mapping. The X-ray mapping defined areas of different smectite composition and provided quantitative analyses. Smectite compositions obtained from these analyses could be grouped into three main compositional variants, supporting the microdiffraction results. These smectite compositional groups comprised two groups with Si from 3.5 to 3.8 and octahedral Mg from 0.4 to 0.7: one with Fe occupancies from 1 to 1.4, and octahedral Al less than 0.3, the other with Fe from 0.8 to 1.0, and octahedral Al from 0.4 to 0.6. The third group comprised smectites with Si ranging from 3.2 to 3.4, Fe from 0.65 to 1, Mg from 1.2 to 1.55, and Al less than 0.1. We have tentatively correlated these three compositional groups with the three (06,33) diffraction maxima, and additional investigations to further clarify the relationships are currently underway