New Mexico Geological Society Annual Spring Meeting — Abstracts

Phyllosilicate clays and associated minerals present unique analytical challenges to the low temperature geochemist. Bulk analytical techniques (wet chemistry, x-ray fluorescence, other) yield average data of dubious value for elucidation of diagenetic or pedogenic processes. Electron microprobe analysis lacks the required resolution to provide reIiable analyses of clay minerals. Many obstacles to analysis of clay minerals are overcome by the analytical electron microscope (AEM). AEM does not, however, provide a panacea to the student of clay minerals.

Samples of Fithian illite were analyzed at -150°C and ambient temperature on a JEOL 2000 FX AEM. Analytical results were converted to "k-factors", unitless constants element of proportionality between x-ray intensity ratios and weight fraction ratios for an element of interest and a reference element in a given material. Variation of k-factors can be used to chart element loss by beam-induced volatilization during AEM analysis. The data indicate that Mg and K are lost during analysis of the interlayers in the phyllosilicate structure. Mg loss involves disruption of the octahedral layer may reflect destruction of the layer structure during analysis. Results from analyses at -150°C, however, indicate that illites may be reliably analyzed by AEM at low temperature.

Low temperature AEM analysis is being used to provide compositional data for authigenic and detrital clays from the Westwater Canyon Member of the Jurassic Morrison Formation (USGS borehole, S6, San Juan Basin, northwestern New Mexico). Analytical results will be presented for both random and ordered mixed-layer clays from coarse- and fine-grained units in the Westward Canyon Member.