A best-practices guide to X-ray diffraction studies of sedimentary carbonates

The accurate identification of sedimentary dolomites and related Ca-Mg carbonates is critical for understanding their formation and, by extension, the evolution of Earth's oceans and climate. However, the literature contains numerous examples of misapplication and misinterpretation of X-ray diffraction (XRD) data. For instance, experimental studies claiming successful low-temperature dolomite synthesis have frequently misidentified very high-Mg calcite (VHMC) or other phases as dolomite due to incorrect evaluation of XRD data. Rigorous analytical standards are needed to avoid misinterpretations and to advance the understanding of sedimentary dolomite formation. This paper provides a best-practices guide for powder XRD analysis of trigonal carbonate minerals, with particular emphasis on Rietveld refinement methods for evaluating cation ordering, unit-cell parameters, crystallite size, strain, and stoichiometry. Recommendations are presented for sample preparation, data collection, and analysis, aimed at improving the reliability of carbonate mineral identification and characterization, with particular emphasis on dolomite. Additionally, the nomenclature and classification challenges in distinguishing calcite, high-Mg calcite, VHMC, protodolomite, dolomite, ferroan dolomite, and ankerite are addressed. Finally, the implications of dolomite mineralogy for interpreting past environmental conditions are critically assessed, highlighting the pervasive effects of recrystallization over geological timescales.