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

TLDR: In this paper, it was shown that the existence of very high-magnesium calcite is not a sign of cation ordering in natural dolomite, but rather of a reaction path in high-temperature experiments.

Abstract: Dolomite [CaMg(CO3)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 Ca2+ and Mg2+ cations interspersed with CO32− anion layers normal to the c-axis. Dolomite has R3¯ symmetry, lower than the (CaCO3) R3¯c symmetry of calcite primarily due to Ca–Mg ordering. High-magnesium calcite also has R3¯c symmetry and differs from dolomite in that Ca2+ and Mg2+ ions are not ordered. High-magnesium calcite with near-dolomite stoichiometry (≈50 mol% MgCO3) 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 (Ca2+ on Mg2+ 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.