DEHYDRATION AND REHYDRATION OF A TUFF VITROPHYRE

The basal vitrophyre of the Topopah Spring Member of the Paintbrush Tu ff at Yucca Mountain, Nevada, is a perlitic glass with 2.8 to 4.6% wat er. The dehydration of this vitrophyre was studied by thermogravimetri c analysis (TGA) arid by isothermal heating at 50-degrees, 100-degrees , 200-degrees, and 400-degrees-C for 3.4 years followed by 1.1 years o f rehydration at high controlled humidity (approximately 79% relative humidity). No crystallization of the glass was observed in long-term d ehydration or rehydration; the only observed chemical alteration was l oss of up to 60% of original fluorine. TGA studies show a characterist ic two-stage dehydration of the vitrophyre, with two-thirds to three-f ourths weight loss occurring most rapidly at temperatures ranging from 278-degrees to 346-degrees-C in 10-degrees-C/min beating experiments. The remaining water, about 1 % in all of the vitrophyre samples studi ed regardless of total water content, is lost only on second-stage hea ting to temperatures above 650-degrees-C. Long-term isothermal heating at less-than-or-equal-to 400-degrees-C releases only the first-stage water. Loss of essentially all first-stage water occurred in less than 1 hour at 400-degrees-C; proportionately lower losses were obtained a t 200-degrees and 100-degrees-C. Small (0.2%) water loss occurred in t he 50-degrees-C experiment. A time-temperature-dehydration diagram gen erated from the isothermal heating data shows a clustering of dehydrat ion contours that are the equilibrium equivalent of the rapid first-st age water loss in dynamic TGA experiments. These dry-heating experimen ts provide an end-member characterization of glass transformations for comparison with water-saturated heating experiments in which glass al teration is prominent.