Mineralogy, nucleation and growth of dolomite in the laboratory and sedimentary environment: A review

Abstract

Dolomite [CaMg(CO 3 ) 2 ] forms in numerous geological settings, usually as a diagenetic replacement of limestone, and is an important component of petroleum reservoir rocks, rocks hosting base metal deposits and fresh water aquifers. Dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of Ca 2+ and Mg 2+ cations interspersed with CO 2À 3 anion layers normal to the c-axis. Dolomite has R 3 symmetry, lower than the (CaCO 3 ) R 3c symmetry of calcite primarily due to Ca-Mg ordering. High-magnesium calcite also has R 3c symmetry and differs from dolomite in that Ca 2+ and Mg 2+ ions are not ordered. High-magnesium calcite with near-dolomite stoichiometry (%50 mol% MgCO 3 ) has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high-magnesium calcite. Many dolomites display some degree of cation disorder (Ca 2+ on Mg 2+ sites and vice versa), which is detectable using transmission electron microscopy and X-ray diffractometry. Laboratory syntheses at high temperature and pressure, as well as studies of natural dolomites show that factors affecting dolomite ordering, stoichiometry, nucleation and growth include temperature, alkalinity, pH, concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of the carbonate being replaced, and surface area available for nucleation. In spite of numerous attempts, dolomite has not been synthesized in the laboratory under near-surface conditions. Examination of published X-ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near-ambient conditions by microbial mediation are unsubstantiated. These laboratory products show no evidence of cation ordering and appear to be very high-magnesium calcite. Elevated-temperature and elevated-pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high-magnesium calcite formation. It remains to be demonstrated whether microbial-mediated growth of very high-magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high-temperature experiments.

Biogeochemistry of intertidal microbial mats from Qatar: New insights from organic matter characterisation

“…The 015* ordering peak of dolomite (2.54A at ca. 35.4 o CuKα) did appear to be present in some XRD plots, but was mostly weak or unconvincing, such that the mineral could be a high Mg (50:50) calcite rather than true dolomite (Gregg et al, 2015). Peaks of other minerals could have masked the ordering peaks in some cases.…”

What do we really know about early diagenesis of non-marine carbonates?

“…6B, C) (Table A2). Early dolomitisation processes often yield non-stoichiometric and poorly ordered Mg-rich carbonates (Bontognali et al, 2014;Gregg et al, 2015). Much has been published and discussed on a variety of dolomitisation models (Machel, 2004).…”

Experimental calibration of clumped isotope reordering in dolomite

“…On the other hand, models of precipitation rate suggest that even small temperature increases will preferentially raise dolomite crystallization rates with respect to calcite, which may contribute to the often-observed pseudomorphic replacement of calcite by dolomite during early diagenesis (Arvidson and Mackenzie, 1999). Temperature is by no means the only parameter regulating dolomite formation; fluid chemistry, rock porosity, water/rock ratio, and the interplay between dissolution, diffusion, and precipitation all have roles in determining when and where dolomite forms (Gregg et al, 2015;Jonas et al, 2015;Machel, 2004;Warren, 2000;Blättler et al, 2015). Nevertheless, constraining the temperatures of dolomitization is critical for evaluating competing models for this process (e.g., Murray and Swart (2017)).…”