Image: Rock anhydrite (St. Louis Limestone, Middle Mississippian; subsurface gypsum mine in Martin County, Indiana, USA) 2 (29999233781)

Description: Rock anhydrite from the Mississippian of Indiana, USA. (Indiana State Museum collection) Rock anhydrite is an uncommon, crystalline-textured, ~monomineralic, chemical sedimentary rock. It is composed of the mineral anhydrite (CaSO4 - calcium sulfate), which is gypsum withouth the water (gypsum is hydrous calcium sulfate - CaSO4·2H2O). Rock anhydrite has the same origin as rock salt (halitite) and rock gypsum (gyprock) - they are all evaporites, formed by the evaporation of water (usually seawater) and the precipitation of dissolved minerals. Rock anhydrite is prone to conversion to rock gypsum, and vice versa. Rock anhdyrite to rock gypsum transformations occur commonly in nature - the accompanying volume changes often result in significant deformation of surrounding rocks. Stratigraphy: St. Louis Limestone, Meramecian Stage, Middle Mississippian Locality: subsurface gypsum mine in Martin County, southwestern Indiana, USA
Title: Rock anhydrite (St. Louis Limestone, Middle Mississippian; subsurface gypsum mine in Martin County, Indiana, USA) 2 (29999233781)
Credit: Rock anhydrite (St. Louis Limestone, Middle Mississippian; subsurface gypsum mine in Martin County, Indiana, USA) 2
Author: James St. John
Usage Terms: Creative Commons Attribution 2.0
License: CC BY 2.0
License Link: https://creativecommons.org/licenses/by/2.0
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