HYDROTHERMAL REPLACEMENT OF ARAGONITE BY MG-CARBONATES

Hydrothermal alteration pathways were studied using mm-sized fragments of aragonite (CaCO₃) of different geometry and microstructure (i.e., single crystals, bivalve shell and coral) reacting with 1 ml of a 1 M MgCl₂solution at 200 °C for up to 20 days. Reaction products were analyzed using SEM, EMPA, µ-CT and ICP-AES.

Experimental results reveal the replacement of aragonite by Mg-carbonate in all experiments. Kinetics of the replacement and resulting microstructures differ for each material. In case of the single crystal and the shell fragment, we observe the formation of a porous reaction rim built of small rhombohedral crystals, progressively replacing the aragonite. Layered rims consists of magnesite (MgCO₃) in the outer part and dolomite [CaMg(CO₃)₂] next to the unreacted core. Individual layers are separated by a sharp, irregular boundary and exhibit a concentration gradient of decreasing Mg-content with increasing proximity to the reaction interface. The highly porous coral fragment is replaced by chemically homogenous Ca-rich dolomite at higher reaction rates.

We interpret the replacement to proceed by a dissolution-precipitation mechanism. The negative volume change of the reaction causes the formation of additional fluid pathways in form of cavities. Reaction progress requires an effective transport in the pore fluid, i.e., the transport of Mg towards the reaction front countered by the removal of Ca. Chemical zoning within the rim suggest that the reaction is transport-controlled. The resulting zoning pattern mirrors the concentration gradient within the pore fluid which determines the precipitating product. This interpretation is supported by higher reaction rates for the replacement of the coral. Here, the microstructure inherited from the parent material provides additional fluid pathways that facilitate element transport and preserve a homogeneous pore fluid composition, resulting in dolomite as the only product phase.

Our study highlights the importance of sample geometry and local fluid chemistry for carbonate reaction pathways. In contrast to previous experimental studies based on experiments with powdered materials, we are able to extract quantitative information on element fluxes and reaction rates allowing us to predict the kinetics of natural carbonate replacement reactions.