Postcompactional late-stage ferroan dolomite commonly occurs as 1) finely disseminated rhombic microspar in argillaceous lime mudstone, 2) sparry cement within voids in argillaceous carbonates, and 3) sparry cement within carbonates adjacent to marine shales. The distribution and abundance of these dolomites correspond with that of clay deposited in the marine environment and cannot easily be explained by alternative models of dolomite formation. Conversion of smectite to illite during burial is a common diagenetic process which is capable of releasing large amounts of Fe, Mg, Ca, Na, and Si. Sandstone petrologists have emphasized the significance of this process in generating quartz cements in sandstone-shale sequences. However, the potential of this conversion for producing postcompactional dolomite cement in limestone-shale sequences or in argillaceous limestones has been neglected. Smectite-illite conversion is here invoked as the source of ions that formed late-stage (postcompactional) dolomites in examples of the Pennsylvanian of Oklahoma, Kansas, Missouri, and Iowa; the Devonian of Iowa; and the Ordovician of Oklahoma. The smectite-illite conversion model for generating dolomite may be widely applicable to many other examples of late-stage dolomitization. It should prove useful in predicting the distribution of late-stage dolomite cement that reduces reservoir quality, and may prove useful for estimating the timing of hydrocarbon migration relative to carbonate cementation.