WHOLE-ROCK CLAY MINERAL ANALYSIS USING INFRARED REFLECTANCE SPECTRA: A QUALITATIVE ASSESSMENT OF SAMPLE PREPARATION TECHNIQUES

Clay minerals from shallow marine cores provide a means of assessing deposition conditions, regional weathering environments, and paleoclimate. Infrared (IR) reflectance spectroscopy allows rapid identification of clays, taking advantage of characteristic absorption and scattering of photons resulting from OH^- and H₂O vibrational processes, combined with metal-OH^- bending plus OH^- stretching near 2.2-2.3 µm. This nondestructive technique is often applied to whole-rock samples with no sample preparation.

We analyzed (core) samples from Pleistocene-Paleocene, South Carolina Coastal Plain units. To better assess the quality of undisturbed, whole-rock analysis using this technique, we compared IR spectra for ten samples after grinding to < 60 mesh and removal of carbonate minerals using HCl (~ 5N). Separate whole-rock x-ray diffraction analysis of a subset of four samples indicates abundant calcite in some samples (20- 84%) and quartz (58-84%) in others; however, clay mineral identification is non-conclusive.

Results indicate that grinding samples had minimal, if any impact on characteristic spectral features of clay minerals; however, reduced grain size did improve signal continuity between spectrometers at 0.97 µm. Calcite dissolution improved clay mineral characteristic spectra in the 2.2 µm range, particularly for samples with abundant calcite. The symmetry of the 2.2 µm absorption feature is critical for distinguishing kaolinite-smectites interstratified clays from montmorillonite in these samples.