Preliminary characterization of the petrologic, bulk, and mechanical properties of a lithophysal zone within the Topopah Spring Member of the Paintbrush Tuff

Ten large samples of lithophysal tuff were studied as part of the nuclear waste repository project at Yucca Mountain in southern Nevada. Macroscopic and microscopic examination led to division of the tuff into three components: (1) a very fine-grained, relatively nonporous, feldspar- and cristobalite-rich matrix; and (2) a coarser, more porous, feldspar- and tridymite-rich vapor-phase-altered material usually found encircling; and (3) lithophysae, which are cavities often lined with tridymite and occasionally contain carbonate-rich fillings. Results from mechanical tests provided compressive strengths which are lower, and Young`s moduli which are higher, than values predicted from effective porosity. 17 figures, 7 tables.