Using X-Ray Fluorescence to Quantify Clay Content in Mudrock and Sandstone Outcrops

Brown, Alton A.; Davies, Russell K.

This study evaluates protocols for collecting X-ray fluorescence (XRF) data on outcrop and using these data to estimate clay content. These protocols were tested on a marine chert - mudrock section (Devonian Woodford Formation, southern Oklahoma) and fluvial sandstone - mudrock section (Castlegate Fm, Cretaceous, central Utah).

Because clay content and bulk mineralogy can be estimated from these major and minor element concentrations, counting times need not be as long and surface preparation need not be as meticulous as in laboratory chemostratigraphic trace element studies. Theoretical x-ray attenuation by a rough surface geometry should affect Al and Si the most and should not significantly affect heavy elements such as Fe and K. Theory was then tested by measuring changes of apparent elemental concentrations measured against sample geometry, surface preparation, and offset from the detector nose. The instrument used for this study showed no significant Al or Si concentration differences up to an offset of 1 mm between a mudrock reference sample and the detector nose and for surface roughness equivalent to a rough-sanded, scratched surface. Battery-powered angle grinders remove most weathering effects, flatten the surface, and leave the surface smooth enough to acquire reproducible data.

Local calibration is necessary to convert measured elemental ratios to mineralogy. Samples were analyzed by x-ray diffraction to calibrate directly the measured elemental ratios to mineralogy. The calibrated relationships can then be used to estimate mineralogy of the other XRF analyses.

The following protocol is recommended for outcrop XRF analysis. (1) Prepare a flat smooth surface using an angle grinder. The prepared surface should cut below the weathering rind. Surfaces should be normal to bedding to minimize heterogeneity problems, especially in laminated shales. (2) Analyze each spot twice to test operator error. Analysis periods of 30 seconds are adequate to characterize elements of interest (depending on the instrument). (3) Elemental trends are evaluated from the data to develop qualitative shaliness indicators and estimate the number of component minerals affecting elemental concentrations. (4) Calibrate elemental ratios to mineralogy using three or more samples analyzed by both x-ray diffraction and XRF.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013