Electron microprobe analyses are presented for plagioclase, biotite, hornblende, magnetite, microcline, and chlorite in core samples from Granite Test Hole One at the Los Alamos Geothermal Test Site. The analyses and accompanying petrographic descriptions characterize material that is being exposed to artificial hydrothermal systems in laboratory experiments. Plagioclase in granitoid rocks exhibits significant compositional variation (albite rims on calcic oligoclase) and alteration to sericite and epidote.