Geochemical compositions of the rocks, ash-flow tuffs, and secondary mineral coatings in the rocks (mostly calcite and opal) were analyzed in rock samples from drill core and outcrops at Yucca Mountain. After the 7-kilometer-long Exploratory Studies Facility (ESF) was excavated under Yucca Mountain by the DOE's 25-meter-diameter tunnel boring machine (TBM), samples collected underground could be analyzed.

The calcite and opal coatings were less disturbed and could be seen at a larger scale in the tunnel at Yucca Mountain. The extent and thickness of mineral coatings, the fragility of some of the calcite crystal forms, and the extensive distribution of opal were not anticipated in ESF samples based on earlier investigations using drill core.

Image of non-welded bedded tuff
(Photo by USGS)

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