Manganese oxide minerals: Crystal structures and economic and environmental significance

Manganese oxide minerals have been used for thousands of years-by the ancie nts for pigments and to clarify glass, and today as ores of Mn metal, catal ysts, and battery material. More than 30 Mn oxide minerals occur in a wide variety of geological settings. They are major components of Mn nodules tha t pave huge areas of the ocean floor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chemical reactions that affect groundwater and bulk soil comp osition. Their typical occurrence as fine-grained mixtures makes it difficu lt to study their atomic structures and crystal chemistries. In recent year s, however, investigations using transmission electron microscopy and powde r x-ray and neutron diffraction methods have provided important new insight s into the structures and properties of these materials. The crystal struct ures for todorokite and birnessite, two of the more common Mn oxide mineral s in terrestrial deposits and ocean nodules, were determined by using powde r x-ray diffraction data and the Rietveld refinement method. Because of the large tunnels in todorokite and related structures there is considerable i nterest in the use of these materials and synthetic analogues as catalysts and cation exchange agents. Birnessite-group minerals have layer structures and readily undergo oxidation reduction and cation-exchange reactions and play a major role in controlling groundwater chemistry.