QUANTITATIVE CLAY PETROLOGY:THE TREES BUT NOT THE FOREST?

Abstrac ~ Gravitational settling and centrifuge t chniques have been criticized in the preparation f X-ray specimen mounts for quantitative clay petrology because of the marked mineral segregations that can occur. Although the same factors can operate to produce rror in the separation of the traditional less-than-2-#m fraction, this potentially more important source of bias is seldom taken into consideration. A fair representation f the bulk sample deserves at least as much attention as a representative X-ray mount. The most accurate and precise X-ray analyses may be geologically misleading or meaningless otherwise and translate as exercises inprecision for its own sake. The principal reason for achieving reater precision in most quantitative X-ray diffraction studies of fine-grained sediments i to try to obtain meaningful infor-mation about the distribution of clay mineral species and for geologic interpretation of the observable trends. More than a few authors have directed their efforts at attempting to achieve a 'true ' quantification in clay petrology. Several major sources of error hav