We report here the high-pressure synthesis of well-sintered millimeter-sized bulks of superhard BC 2 N and BC 4 N materials in the form of a nanocrystalline composite with diamond-like amorphous carbon grain boundaries. The nanostructured superhard B-C-N material bulks were synthesized under high P-T conditions from amorphous phases of the ball-milled molar mixtures. The synthetic B-C-N samples were characterized by synchrotron x-ray diffraction, high-resolution transmission electron microscope, electron energy-loss spectra, and indentation hardness measurements. These new high-pressure phases of B-C-N compound have extreme hardnesses, second only to diamond. Comparative studies of the high P-T synthetic products of BC 2 N, BC 4 N, and segregated phases of diamond + cBN composite confirm the existence of the single B-C-N ternary phases.

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Site Characterization Project. An Affirmative Action/Equal Opportunity Employer This report was prepared as an account of work sponsored by an a,_eHcy of the United States Govermnent. Neither The Regents of the University Of California, the United States Govermnent nor any agency thereof, nor any of their employees, makes ally warranty, express or implied, or assumes any h'Ral liability or responsibility for the accuracy, compleh,ness, or tlsefllhtess of any information, apparatus, product, or process disclosed, or represents that its use wouht not infi'inge privately owned r_hts. Reference herein to any specific commercial product, process, or service by trude name, trMemark, tllllllll]ilCllll'et*, or" otherwise, does not necessarily constitute or imply its endorsement, reconunend.ticm, or firooring by 771e Regents of the University of California, the United States Government, or any agency there@ The views and opinions of authors expressed herein do not .,lecessarily shire or reflect those oI The Regents of the University # California. the United States Govcrnmenl, or any agency lhereof.

The Topopah Spring Member of the Paintbrush Tuff and the Lithic-rich tuff are two Tertiary volcanic units that occur in cores from drill holes UE25a-l and USW-Gl at Yucca Moun¬ tain, Nevada. Recently they have been suggested as possibly suitable for the permanent storage of high-level radioactive waste. This report augments earlier petrologic characteriza¬ tion of these units.The Topopah Spring Member (approximately 350 m thick) has two compound cooling units.The upper, thinner unit is densely welded to vitrophyric.The lower unit ranges from norwelded to vitrophyric, and its nonwelded base is exten¬ sively zeolitized to clinoptilolite and mordenite. Heulandite occurs as fracture fill in the overlying vitrophyric part, but zeolites are absent above that vitrophyre.Here primary devitrification plus vapor-phase crystallization dominate the mineralogy. Vapor-phase effects are especially prominent between the two vitrophyres in both cores and in¬ clude numerous large lithophysal cavities throughout most of this moderately to densely welded tuff.The Lithic-rich tuff extends from 1203 to 1506 m in the USW-Gl drill core.It is nonwelded to partly welded but is well indurated due to pervasive intergrowths of authigenic minerals.Thesr phases are analcime, albite, alkali feld¬ spar, sericite, chlorite and quartz.The transition from analcime to secondary albite corresponds to Iijima's zeolite Zone IV boundary, and this boundary appears in USW-Gl at 1326 m.However, analcime remains as a prominent phase through most of the Lithic-rich tuff.Further work is necessary to assess the suitability of either of these horizons for a waste repository.In the Topopah Spring Member, both mechanical and hydrologic proper¬ ties of the thick lithophysal zone must be studied, as well as the complete sequence of fracture fill.For both units, zeolite and clay mineral stabilities need to be investigated.