Hydrothermal alteration of a femic igneous intrusive near Ely, Minnesota, has resulted in formation of an unusual assemblage of three iron-rich minerals. These minerals have been studied by means of Debye-Scherrer powder photographs, flat film fiber photographs, and chemical tests and have been determined to consist of clinochrysotile, antigorite, and a one-layered orthogonal mineral similar to lizardite and chamosite in x-ray patterns. This mineral is considered to be chamosite in this paper. Partial chemical analyses show that all three minerals are iron-rich. The considerable differences in d spacings and unit cell dimensions between these minerals and those previously published are attributed to high iron content.
Field and microscopic relationships show that antigorite formed during two stages, before and after formation of clinochrysotile. The relative time of chamosite formation could not be determined because no cross-cutting relationships were found.