Manganese minerals in geodes from Chihuahua, Mexico

Seven manganese minerals have been observed in geodes from Chihuahua, Mexico. Ramsdellite occurs in a number of different habits, but the morphology and precession X-ray patterns suggest that it is pseudomorphous after groutite. Pyrolusite is found only in association with ramsdellite and is believed to be an alteration product of that mineral. Todorokite generally occurs as a late-stage fibrous overgrowth in many of the geodes. Rancirite, birnessite, hollandite, and cryptomelane are present in trace amounts. This assemblage of manganese minerals is typical of supergene mineralization.