Occurrence of primary magnesium silicates (palygorskite-sepiolite) in karst terranes

摘要

Palygorskite-sepiolite clay minerals occur in a greater number of geochemical environments than any other clays. These range from hydrothermal deposits, as pedogenic constituents, as co-crystallization phases in evaporite sequences, as minerals reported in halmyrolic environments, and as the products of neoformational processes. Clays formed by the latter are especially well represented in both the Florida-Georgia Tertiary strata of southeastern United States and in Cretaceous to recent sediments on the Yucatan Peninsula of Mexico. Processes resulting in the formation of palygorskite and sepiolite in karst sediments of southeastern U.S. and the Yucatan are directly related and can be attributed to both neoformation in marginal schizohyaline marine waters and reactions attendant with the release of magnesium during transformation of original high magnesium calcite to a more stable low magnesium form during diagenesis. It appears that palygorskite-sepiolite formation requires "aragonitic" seawater that contains adequate dissolved silica and alumina in the absence of detrital Al-beating clays; hence, the association with carbonate platforms.