New Insights to the Formation of Dolomite and Magnesite Through Hydrothermal Alteration of Ca-Carbonates: An Experimental Approach

Hydrothermal precipitation experiments were performed to trace and quantify elemental (Ca, Mg and Sr) and stable isotopic δ18O signatures during the (trans)formation of intermediate aragonite and low-Mg calcite to more stable dolomite and magnesite in the presence of Mg-/Na-chloride rich brines by reacting 0.1M inorganic CaCO3 seed material (aragonite or calcite) with an artificial brine originally containing 0.2M MgCl2(aq) and 0.1 or 0.05M of NaHCO3 within Teflon-lined, steel autoclaves at temperatures of 150, 180 and 220°C over the course of 365 days. The evolution of reaction products and of the experimental solutions was monitored by ICP-OES, CRDS, FTIR, XRD, EMPA and SEM analyses as well as pH and alkalinity measurements. Based on apparent solid-phase compositions and reactive fluid chemistry the following sequence of mineral growth was established: aragonite and/or low-Mg calcite reacted with aqueous Mg2+ ions to form intermediate huntite, brucite and high-Mg calcite, subsequently altered to Ca-excess dolomite and Ca-rich magnesite and finally converted to nearly stoichiometric endmembers. A progressive evolution in the stoichiometry of dolomite (from 42 to 50 mol% MgCO3) and magnesite (from 80 to 98 mol% MgCO3) as well as the increase in the degree of cation order in dolomite (from 0.26 to 0.74) were observed during this reaction sequence, implying a kinetic drive towards the (thermodynamically stable) end members. Results from our study indicate that in the presence of Mg-rich brines metastable CaCO3 polymorphs are transformed into more stable magnesite and dolomite via the formation of intermediate Mg-Ca carbonates. The experimental results are discussed in the scope of dolomitization of limestone platforms in natural surroundings.