Abstract
Plagioclase is completely altered to montmorillonite in some portions of a hydrothermally altered porphyritic rock located in the Liberty Pit porphyry copper mine near Ely, Nevada. In these places the altered rock is easily crumbled in one's hand and perfect pseudomorphs of montmorillonite after plagioclase can readily be picked out of the debris. In view of the high purity of this montmorillonite as compared to most montmorillonite, it was decided to investigate this material to see where it lay in the montmorillonite-beidellite group and to examine various determinative techniques in the light of the composition. The material described in this paper was collected from near the bottom of the northwest corner of the Liberty Pit at about 103,300N, 101,500E and 6500 feet elevation.
