Manganese oxide minerals are common in soils, but seldom studied because of their small crystal size and lack of crystallinity. The minerals are identified from a wide range of environments-acid to alkaline and from many sedimentary deposits. The Southern High Plains of New Mexico and Texas is noted for the Ogallala caprock and is usually cited as an unconformity formation between the Pliocene and Pleistocene. The environmental conditions during the transition period has been described as semi-arid to arid with the formation of saline lakes, which persisted for what is believed to be several thousands years. Lacustrine sediments are common and have a unique composition. The carbonate mineral assemblage is typically dominated by calcite with dolomite and the dominant silicate clay mineral is sepiolite. A black, fine grained mineral with a dendritic pattern is characteristic within many of these deposits. A study was initiated to determine the distribution of these lacustrine sediments. At one study site, a meter thick layer had the appearance of a coal seam. The black mineral was identified by X-ray diffraction as birnessite with opal CT, calcite and sepiolite. The birnessite is a naturally occurring Mg-birnessite with fibrous morphology that has crystallized within the opal during a desiccating environment. Scanning transmission electron microcopy shows a mixture of birnessite and sepiolite. It is possible that the fibrous birnessite is a pseudomorph of sepiolite. The formation processes of these mineral assemblages and the source of the both the manganese and magnesium is being investigated.