A reconnaissance laser Raman and Fourier transform infrared survey of silcretes from the Kalahari Desert, Botswana

This paper documents the first application of laser Raman and Fourier transform infrared vibrational spectroscopic techniques (Raman/FT-IR) to the analysis of silcrete. Analyses are undertaken at a reconnaissance level on bulk silcrete samples of varying ages, drawn from a range of geomorphological settings across the Kalahari Desert of Botswana. Results suggest that Raman/FT-IR, combined with inspection of petrographic thin-sections, provides an effective tool for establishing the range and type of silica polymorphs present within samples. Analyses of Raman and mid-infrared spectra indicate that the majority of silcretes are dominated by α-quartz with subordinate moganite. There is, however, considerable inter-sample variability, ranging from silcretes consisting of almost pure quartz to those containing complex admixtures of disordered microcrystalline or non-crystalline silica in association with quartz and clays. Taken as a whole, the results are supportive of a similar environment of formation for all samples, with development taking place under broadly semi-arid conditions. Raman/FTIR is also shown to have a number of benefits over other conventional techniques for silcrete analysis, such as X-ray diffraction. Firstly, inspection of the OH region of the infrared spectrum of bulk samples is shown to have potential for establishing the provenance of quartz clasts within silcrete host sediments, through the identification of high-temperature conditions of crystal growth in detrital quartz grains. Raman and mid-infrared analyses also permit clay identification within silcrete samples where such minerals are not readily resolvable in thin-section. The technique further allows the relative maturity of silcrete cements to be established, particularly for recently formed silcretes. These are found typically to contain less mature silica polymorphs and incorporate larger quantities of molecular water than α-quartz dominated silcretes. Comparisons of the range of silica polymorphs identified using Raman/FT-IR with those established by petrographic investigation can also reveal evidence of silica paragenesis. This is the case in two stratigraphically older samples from the Kalahari margin in which less stable silica polymorphs appear to have transformed over time to chalcedony and microquartz. Whilst the ‘whole-rock’ approach used in this survey generates some problems of interpretation, the overall results suggest that the use of micro-Raman and micro-FT-IR techniques should allow even more precise information about the environments of formation and developmental histories of silcrete profiles to be produced in the future.