COMPARISON OF VERY HIGH-MG CALCITE (PROTODOLOMITE) AND WELL-ORDERED DOLOMITE USING HIGH RESOLUTION POWDER DIFFRACTION METHODS (Invited Presentation)

High-resolution diffraction data (lab X-ray, synchrotron, and neutron) were collected for two natural Ca-Mg carbonate samples that represent endmembers in terms of stoichiometry and cation ordering. These samples include the Bonneterre Dolomite (Cambrian, Missouri, USA) used here as a dolomite standard and a very high-Mg calcite (VHMC) or “protodolomite” (Holocene, Andros Island, Bahamas). Both minerals replaced similar peritidal microbial laminate facies. The Cambrian dolomite is stoichiometric (Ca:Mg = 50:50, estimated using the 104 peak shift method) with well-developed cation ordering reflected in all diffraction patterns. The Holocene Ca-Mg carbonate is enriched in Ca relative to stoichiometric dolomite (Ca:Mg = 58:42). Initial lab X-ray patterns of the Holocene sample display no evidence of cation ordering, suggesting calcite symmetry. After treatment with EDTA to remove excess aragonite the sample displayed very weak ordering reflections visible with synchrotron and lab X-ray measurements. The neutron diffraction pattern does not display evidence of ordering for the EDTA-treated Holocene sample. Weak ordering observed in X-ray data for the Holocene sample may be due to scattered domains of cation ordering in an otherwise disordered crystal structure. An initial disordered VHMC phase is predicted by high-temperature (>100°C) synthesis of dolomite. The initial disordered product of the high-temperature experiments transitions over time to an ordered, stoichiometric dolomite through a series of recrystallization reactions (Ostwald’s step rule). Our new diffraction data have bearing on the interpretation of VHMC synthesized at ambient Earth-surface temperatures. Early X-ray data collected on inorganically synthesized VHMC relied on powder camera results that display no evidence of cation ordering. More recently published results of VHMC, synthesized in association with of microbial activity, rely on poor quality X-ray diffraction patterns that also display no evidence of cation ordering. However, diffraction evidence for ordered domains may become visible for Ca-Mg carbonates synthesized at low temperature given better sample preparation and better quality X-ray diffraction data.