Effect of pH Cycling and Zinc Ions on Calcium and Magnesium Carbonate Formation in Saline Fluids at Low Temperature

Veerle Vandeginste

doi:10.3390/min11070723

Abstract

The formation of dolomite is very challenging in the laboratory under ambient conditions due to kinetic inhibition. The goal of this study was to test the impact of pH cycling and zinc ions on the formation of magnesium-rich carbonates in saline fluids at a low temperature. Batch reactor experiments were conducted in two series of pH cycling experiments, one without and one with zinc ions, at 43 °C. The results after 36 diel pH cycles indicate a reaction product assemblage of hydromagnesite, aragonite and magnesite in the experiments without zinc ions, and of magnesite and minor aragonite in the experiments with zinc ions. The presence of zinc ions leads to a decrease in the pH in the acid phase of the cycling experiments, which likely plays a role in the reaction product assemblage. Moreover, the hydration enthalpy and other specific ion effects could be additional factors in the formation of magnesium-rich carbonate. The results show a clear evolution towards increasing incorporation of magnesium in the carbonate phase with cycle number, especially in the experiments with zinc ions, reflecting a ripening process that is enhanced by pH cycling. Hence, repeated pH cycling did not lead to more ordered dolomite (from protodolomite), but rather to the formation of magnesite with 92 mol% MgCO3 after 36 cycles, even though geochemical models indicate a higher saturation index for dolomite than for magnesite.

Highlights

The formation of dolomite has fascinated many researchers worldwide because of the so-called “dolomite problem”, referring to the abundant presence of dolomite in ancient sedimentary systems in contrast to its rare occurrence in recent systems and the difficulty in forming abiotic dolomite in the laboratory under ambient temperature and pressure [1,2]
Despite the fact that seawater is oversaturated with respect to dolomite, the latter does not precipitate due to kinetic inhibition at a low temperature [3]
This study reported low-temperature cyclic pH experiments with saline solutions with and without zinc ions to test the impact on the formation of magnesium-rich carbonate

Summary

Introduction

The formation of dolomite (calcium magnesium carbonate) has fascinated many researchers worldwide because of the so-called “dolomite problem”, referring to the abundant presence of dolomite in ancient sedimentary systems in contrast to its rare occurrence in recent systems and the difficulty in forming abiotic dolomite in the laboratory under ambient temperature and pressure [1,2]. Magnesite (magnesium carbonate) forms quite slowly in laboratory conditions at ambient pressure and temperature [4]. At low temperature, hydrated magnesium carbonates are rather formed, such as hydromagnesite [Mg5(CO3)4(OH)2·4H2O], dypingite [Mg5(CO3)4(OH)2·xH2O], lansfordite [MgCO3·5H2O] and nesquehonite [MgCO3·3H2O] [5]. It was shown that the addition of zinc ions, which have a stronger hydration enthalpy (−2046 kJ·mol−1) than magnesium ions, can accelerate dolomitization in saline fluids at a high temperature [7]. Besides the slow dehydration of magnesium ions, experiments in water-free environments indicate an additional entropic inhibition preventing the formation of longrange ordered crystallographic structures at ambient conditions [8]. A few studies that claim successful formation of dolomite or magnesite at low temperature made use of carboxylated polystyrene microspheres [10,11]

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