Coarse dolomite crystals in the Taum Sauk Limestone (Upper Cambrian, southeast Missouri) contain both calcite and iron oxides in rhombohedral zones and along fractures and cleavage traces. The dolomite crystals are demonstrably nonstoichiometric, averaging 57 mole percent CaCO 3 , and are compositionally zoned, ranging from 1.40 to 4.56 mole percent FeCO 3 . Cathodoluminescene study and microprobe analyses of the dolomite, coupled with scanning electron microscope (SEM) examination of etched fracture surfaces reveal that: 1) Scattered dolomite within the calcite/iron oxide zones is the most iron-rich. 2) Calcite within dolomite rhombs is brightly luminescent, nonferroan and is intergrown with iron oxides. 3) The iron oxides display two crystal habits--thin, red (hematite?) circular plates, typically 0.7 to 1.0 microns across, grouped in rosettes; and prismatic bundles and prismatic radiating aggregates of brown crystals (goethite?), typically 1 to 3 microns in length. 4) Narrow veinlets of dolomite associated with calcite and iron oxides along cleavage traces form a three-dimensional network. This feature, which aided in interpreting the diagenetic history of the dolomite, was not observed during petrographic examination but was readily observed with the SEM. The occurrence of nonferroan calcite and ferric oxides in rhombohedral zones in Taum Sauk dolomite suggests that dedolomitization occurred by the oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to Recent weathering.