Dolomite, very high-magnesium calcite, and microbes— implications for the microbial model of dolomitization

Numerous field and laboratory studies over the past two decades claim that microbes catalyze nucleation and growth of dolomite at temperatures common in low-temperature geologic environments (25–608 C). However, a critical reexamination of the X-ray diffraction (XRD) data presented by these studies indicates that the laboratory products are not dolomite but rather a mixture of minerals, including very high-magnesium calcite (VHMC). Because VHMC can be ‘‘compositionally’’ indistinguishable from dolomite (i.e., 50 mol% MgCO₃ ), the positions of the principal (104) XRD reflection for VHMC and dolomite can be identical. Nevertheless, published XRD patterns of products derived from microbial experiments lack convincing evidence of cation ordering, which is a unique characteristic of carbonate minerals exhibiting R3 (dolomite) symmetry. The lack of cation ordering in laboratory precipitates instead indicates that the products are VHMC, which possesses R3c (calcite) symmetry. Hence, previous laboratory studies have misidentified VHMC for dolomite. Despite the failure to synthesize dolomite in microbial experiments, the low-temperature laboratory results remain interesting. High-temperature (60– 3008 C) dolomitization experiments have long shown that ordered dolomite is invariably preceded by disordered VHMC precursors that recrystallize to dolomite over time. Although recrystallization from VHMC to ordered dolomite has not been documented in the low-temperature microbial experiments, it may be common in natural settings where higher surface temperatures and longer time periods overcome kinetic barriers to dolomite formation. Mineralogical arguments aside, petrological observations show that VHMC products from microbial laboratory experiments are dissimilar to both natural dolomites and high-temperature synthetic dolomites. First, the published microbial experiments produced VHMC or other carbonates as cements via direct precipitation from solution rather than by replacement of a CaCO₃ precursor, whereas the latter is demonstrated in high-temperature synthetic dolomites and inferred for most natural dolomites. Second, these precipitates tend to be spheroidal and/or dumbbell shaped, and as such they are fundamentally different from both the dominant rhombohedral form and the mimetic replacement textures observed in natural and high-temperature synthetic dolomites. Thus, the microbial products are not only mineralogically unlike natural dolomites, they also differ with respect to their mode of formation and their morphological characteristics.