A New Technique for Quantifying Opal Content in Colorado's Upper Eocene Castle Rock Conglomerate

Abstract

Hydrous amorphous silica (aka opal) is a common cement in the Upper Eocene Castle Rock Conglomerate (CRC) of the southwestern Denver Basin. Petrographic study of standard thin sections indicates that this opal forms from 5% to as much as 40% of any given sample. It also commonly occurs as a precursor to fibrous length-fast chalcedony, a crystalline form of quartz cement. Similar opal cement apparently derived from shards of volcanic glass is even more common in the subjacent Wall Mountain Tuff, a welded deposit of volcanic ash that was the most likely source of silica in the opal cement in the CRC. This paper provides a first-of-its-kind attempt to quantify the amount of opal cement in selected samples of the CRC based on X-ray diffraction (XRD) analysis. Because opal is amorphous, its abundance cannot be quantified using standard XRD techniques, but experimenting with heating indicates that it is possible to convert powdered amorphous opal to a crystalline form of silica in less than 48 hours at a temperature of 550°C. Comparison of pre- and post-heat treatment XRD diffractograms thus provides a potential tool for quantifying opal content in the CRC and other opal-cemented samples. This new analytical technique and its results are described in this paper.