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Two hundred ninety carbonate rock samples from nearly all pre-Cenozoic periods, from widely scattered North American sites, representing all of Dunham's carbonate rock types, and containing 5%–100% ordered dolomite in the carbonate fraction were analyzed for a possible relationship between dolomite nonstoichiometry (expressed as mole percent CaCO3) and pétrographie parameters and stratigraphie position. Dolomite composition ranged from 49.3 to 57.3 mole percent CaCO3, with modes from 50.0% to 52.0% and 54.0% to 56.0%. Data indicate that dolomite nonstoichiometry is not related to insoluble residue, porosity, rock type, percentage of crinoid fragments, recrystallization (dolomite crystal size), and percentage of dolomite in the carbonate fraction in dolomitized limestones. A general trend toward more nearly stoichiometric composition with age is obscured by local effects. Crystalline dolomites tend to be more nearly stoichiometric and less variable in composition (mode 50.0%–51.0% CaCO3) relative to dolomicrites (modes 51.0%–52.0% evaporitic; 54.0%–55.0% nonevaporitic) and dolomitized lime mudstones (modes 51.0%–52.0% evaporitic; 55.0%–56.0% nonevaporitic). Evaporite-related dolomite is almost invariable near stoichiometric or calcium depleted. Open-marine and stoichiometric, evaporite-related dolomites are products of syndepositional dolomitization; nonstoichiometric nonevaporite dolomite is syndepositional to early diagenetic; and crystalline dolomite is middle to late diagenetic.

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