Date: Wed, 30 May 2001 22:03:54 -0300 (BRST) From: "Marcelo E. Alves" [mealves@carpa.ciagri.usp.br] To: rietveld_l@ill.fr Subject: Information Dear colleagues I would like to ask some basic material on Rietveld Method. I would like to learn about the applications of this method in Soil Mineralogy. Here in Brazil, the soil clay fraction is mainly constituted by kaolinite, gibbsite, goethite and hematite. Can anyone help me? Many thanks in advance. Marcelo |
From: Steve Hillier [s.hillier@mluri.sari.ac.uk] To: rietveld_l@ill.fr Subject: Quantification of soil clay minerals Dear Marcelo, There is no simple answer to your question about using the Rietveld method to quantify clay minerals in soils, or clay minerals by the Rietveld method in any sample for that matter. If your clay mineral assemblage is simple, and the clay minerals well crystallised with no 2D diffraction effects you may be able to get by. If on the other hand, the clay mineral assemblage is complex and the clay minerals poorly ordered you will face many more problems. Here at Macaulay Institite we tend to prefer the classic RIR method where clay minerals are concerned. Although the RIR method has its own drawbacks and fullpattern approaches seem much more elegant; when it comes to clay minerals, soil clay minerals especially, it can be considerably easier to measure a single peak that can be unequivocally assigned to one clay mineral or another than to try and model the contribution of a combination of poorly ordered clay minerals to a full pattern, invariably including other minerals in addtion to clays. At then end of the day the accuracy of which ever method you use will also depend on how much you know about the clays in your sample. Which ever method you decide upon I would always advise on taking the effort to add an internal standard. Even if you use the Rietveld method the addittion of a standard is about the only way to have some confidence that the quantification of an unknown is reasonable. An age old problem you will also face with clay minerals is preferred orientation, At the Macaulay Institute we spray dry all our samples to be sure that we eliminate preferred orientation before we start. http://www.mluri.sari.ac.uk/newcommercialservices/spray/index.html This makes single/several peak RIR methods much more precise and accurate it also means you have at least one less varaible to refine if you use the Rietveld method. If you want some literature on this subject the chapters by Bish and by Hughes et al on XRPD in the following reference are a good starting point. Amonette, J.E. & Zelazny, L.W. (1994) Quantitative methods in soil mineralogy. Soil Science Society of America Inc. Madison, Wisconsin, USA. Sincerely, Steve |