Experience has shown that X-ray powder diffraction (XRPD) is the best primary method for quantitative analysis of clay bearing samples. The following recommendations assume XRPD is the primary method employed, although the Reynolds Cup allows for any method(s) to be used.
1) Correct qualitative analysis i.e. mineral identification is a critical prerequisite to quantitative analysis. Therefore DO NOT rely solely on automated search/match routines. Misidentification of phases that are not present in the mixtures is THE biggest source of bias. Also, do not just rely on bulk sample analyses for mineral identification. Clay minerals in particular are more readily identified by separating clay size fractions (i.e. <2 µm) and applying diagnostic treatments and intercalation procedures, e.g. see Brown & Brindley (1980) and Moore & Reynolds (1997). Finally, do not just accept your identifications without some consideration of the sample type and system; is the assemblage of identified minerals realistic or are there more logical or common alternatives to be checked?
2) Quantitative analysis may be biased by numerous factors, so to avoid the most critical ones – ensure particle size is adequate (nominally < 10 μm) by wet grinding. Hand grinding alone is unlikely to achieve adequate particle size reduction. Try to avoid texture in your powder specimens, as preferred orientation effects are difficult to compensate for. Preferably, use full-pattern methods (e.g. Rietveld or experimental pattern summation) over single peak techniques for quantification.
3) Cross-check and validate your quantitative data where possible by independent measurements For example – Calculate the bulk chemical composition from your mineralogical analysis and compare with measured XRF or other chemical analysis data (any mineralogical analysis must be compatible with a chemical analysis) etc, etc.
Basic reading:
Bish, D.L. and Reynolds, R.C., Jr. (1989) Sample preparation for X-ray diffraction. Pp. 73-99 in: Modern Powder Diffraction (D.L. Bish and J.E. Post, editors). Reviews in Mineralogy 20, Mineralogical Society of America, Washington, D.C.
Brown, G. and Brindley, G.W. (1980) X-ray diffraction procedures for clay mineral identification. Pp. 305-359 in: Crystal Structures of Clay Minerals and their X- ray Identification. (Brindley, G. W. and Brown, G. editors). Mineralogical Society Monograph No. 5, Mineralogical Society, London.
Moore, D.M. and Reynolds, R.C., Jr. (1997) X-ray diffraction and the identification and analysis of clay minerals, 2nd edition, Oxford University Press, New York.