Clay minerals in mixtures: sample preparation, analysis, and statistical interpretation: Clays

Abstract--A method of clay mineral sample preparation for electron microprobe analysis has been devel-oped in which a film of clay plus 10-12 wt. % colloidal graphite isdeposited on a porous ceramic disc using a specially designed suction device. Correction procedures are used to obtain quantitative elemental nal-yses representing the average chemical composition ofthe prepared sample. A statistical technique isem-ployed to estimate the most likely proportions ofclay minerals representing the known composition. Chem-ical compositions ofclay minerals are presented in terms of five coordinates ("Si, " "Al, " "Mg, " "K," and "Fe"). Using literature data, the chemical compositions of 13 different clay mineral groupings were defined statistically by their multivariate means and variance-covariance matrices. A correlation param-eter, X ~, was calculated to compare the chemical composition ofa sample with that of any mixture of the defined clay mineral groupings, the minimum X ~ indicating the best-fit mixture. From chemical analyses of artificial mixtures only approximate day mineral proportions could be deter-mined when the various clay mineral groupings had been defined statistically from literature analyses. The best results were obtained when the actual compositions ofthe end-members forming the artificial mixtures replaced the statistical definitions. Tests of the estimation procedure on day mineral mixtures for which chemical compositions and corresponding clay mineral proportions were found in the literature, indicate that the technique has appreciable merit