LA-9021-MS, "Detailed Mineralogical Characterization of the Bullfrog and Tram Members in USW-G1, with Emphasis on Clay Mineralogy."

The detailed mineralogy of the Bullfrog and Tram Members of the Crater Flat Tuff from drill hole USW-G1 has been examined, primarily to characterize fully the amounts and types of clay minerals in the tuffs and the possible effects clay minerals have on rock properties. Results of bulk sample x-ray diffraction analyses agree closely with previous determinations, although slightly higher clay mineral contents were found in this study. X-ray diffraction analysis of fine fractions revealed that the clay minerals in the tuffs are sodium-saturated montmorillonite-beidellites with typical layer charges and no high-charge layers. These smectites are found in virtually all samples of the Bullfrog and Tram, and there is no correlation between the amounts of smectites and the amounts of zeolite, quartz, and feldspar. Smectites are present in both welded and nonwelded horizons and are scarce in some zones with slight-to-absent welding. The montmorillonite-beidellites are all randomly interstratified with a small amount (about 10%) of illite, and there is no consistent trend in the degree of interstratification with depth, contrary to what is commonly found in pelitic sediments. This observation, together with the lack of smectites in some zeolitized zones, suggests that clay mineral formation occurred after zeolite crystallization and under conditions similar to those in the rocks today (35-451C). It is also likely that the ground water in the tuffs has inhibited the smectite-to-illite reaction. The montmorillonite-beidellites exist in a 12.6 A form under room conditions, and their basal spacings can range from 10 at 0% relative humidity or slightly elevated temperatures to approximately 20 A in a 100% relative humidity atmosphere. Further increases in water:clay ratio cause additional expansion. In view of the possible significant variations in montmorillonite-beidellite molar volume and water content with small changes in temperature and relative humidity, it is important that we understand the present conditions existing in the rocks and appreciate the changes