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Controls on the Development of Cation Ordering in Dolomite


Cation ordering, an essential property of the mineral dolomite [Mg, Ca(CO3)2], is evidenced by the presence of well-defined ordering peaks in x-ray diffraction (XRD) patterns. The precise positions and intensities of these ordering peaks reflect the degree to which magnesium and calcium are distributed in alternating cation layers within the carbonate lattice. It has long been postulated that the development of cation ordering represents one of the biggest hurdles for dolomite formation under earth surface conditions. Cation ordering has also recently been cited in discussions about whether or not some published laboratory experiments have actually produced dolomite. When stringent cation ordering criteria are applied to these XRD data, it has been shown that low-temperature abiotic experiments have universally failed in producing ordered dolomite as have nearly all low-temperature laboratory experiments involving microbes (Gregg et al. 2015; Kaczmarek et al. in press). Instead, it has been argued, the precipitates formed in these experiments include hydromagnesite, Ca-phosphates, aragonite, and very-high magnesium calcite (VHMC), which can have the same MgCO3 composition of dolomite, but importantly lacks XRD evidence of cation ordering. In high-temperature dolomitization experiments, dolomite invariably forms through a series of metastable precursors including VHMC (“protodolomite”), weakly-ordered dolomite, and finally well-ordered dolomite. In nature, some carbonate sediments appear to contain VHMC, and older dolomites are generally better ordered than younger dolomites. Interestingly, what these various observations suggest is that cation ordering develops over time. The fundamental controls on cation ordering development in dolomite are, however, poorly understood. Here, the first comprehensive assessment of new and previously published data from high-temperature (>60 °C) dolomitization experiments and low-temperature sedimentary dolomites is conducted to evaluate potential controls on the development of cation ordering. These data clearly show that factors such as temperature, Mg/Ca and molarity of the dolomitizing fluid, reactant mineralogy, and the concentrations of certain additives, like NaCl, KCl, and organic matter, can significantly impact the rate at which cation ordering develops in dolomite.