IntroductionThe large block heater test, to be conducted at Fran Ridge &in et al., 1994), is designed to provide a database with which to test codes that simulate hydrological, geochemical, and geomechanical processes that may occur within the repository block The geochemical processes that may occur include rock-water interaction within the matrix of fi-actum bounded blocks, and with the minerals that line fractures (see, for example, Buscheck and Nitao, 1992,1993a,b, 1994Glassley, 1993). As a first step in evaluating these interactions, characterization of the fractures, and of the matrix that is adjacent to those fractures, must be completed. Characterization of the fractures and matrix before the large block test is started will allow a "baseline" set of data to be collected that will describe the properties of the large block prior to the test. After the test is completed, the block will be dismembered, and characterization of the matrix and fractures will be repeated. Changes in matrix and fracture mineralogies will allow documentation of the mineralogical consequences of rock-water interaction resulting from heating of tuff under the conditions of the test.The pre-test characterization of mineralogical properties will be conducted on samples ("small blocks") collected during excavation of the block. The post-test samples will be obtained from the block itself. The latter samples will be obtained from specific temperature and hydrothermal zones, identified through techniques described in the study plan for the large block test (Scientific Investigation Plan, 1993;Lin, 1993). This data report describes the results of small block characterization completed to date.
Qualitative Description of the Small BlocksThe small blocks were collected during the excavation of the large block test located at Fran Ridge, Yucca Mountain. The rock is near-surface, devitrified, welded Topopah Spring Member of the Paintbrush tuff, predominantly composed of tridymite, quartz, cristobalite, and alkali feldspars (Bish, et al., 1984;Knauss, 1984; Schuraytz, et 1 al., 1986;Broxton et al., 1987). The blocks exhibit complex fracture networks with associated alteration. Fracture orientations am usually either subhorizontal or steeply inclined, with notable differences in nature and thickness of fracture fillings.Textural and color differences in the tuff delimit regions of contrasting alteration (Figure 1). In the least altered portions of fracture-bound blocks, lithic fragments within the devitrified and welded tuffs are clearly evident as dark, chocolate brown areas, millimeters to a few centimeters in cross section, set in a lighter, orange-brown matrix. Alteration around fractures tends to color the rock a lighter hue, masking the color contrast between lithic fragments and the enclosing matrix, thus tending to make the rock appear more homogeneous than it really is.The surfaces of the first set of fractures are coated by a thin sheet of dark material <. 1 mm thick, which defines the fracture plane. These fractures are surroun...