Simple, Reproducible Synthesis of Pure Monohydrocalcite with Low Mg Content

Takuma Kitajima,Yasuo Takeichi,Yoshio Takahashi,Masahiro Yoda,Keisuke Fukushi

doi:10.3390/min10040346

Abstract

Monohydrocalcite (MHC) is a metastable hydrous calcium carbonate that requires Mg in the mother solution during formation in the laboratory. MHC prepared by previously reported methods always contains a large amount of Mg (Mg/Ca ratio up to 0.4) because of the simultaneous formation of amorphous Mg carbonate during synthesis, which has hindered detailed elucidation of the mineralogical characteristics of MHC. Here, we synthesized MHC at low temperature (5 °C) and found that it contained little Mg (Mg/Ca ratio < 0.01). X-ray absorption near-edge structure analysis of synthesized MHC revealed that the Mg present was structurally incorporated within the MHC, and that the chemical speciation of this Mg was similar to that of Mg in aragonite. Thus, low-temperature synthesis is an effective means of producing MHC without also producing amorphous Mg carbonate.

Highlights

Monohydrocalcite (MHC, CaCO3 ·H2 O) is a hydrous calcium carbonate, and it is metastable with respect to anhydrous phases (CaCO3 ), such as calcite and aragonite [1,2]
When the initial Na2 CO3 concentration was 0.07 mol/kg, small peaks attributed to aragonite were observed in the X-ray diffraction pattern, but when the initial Na2 CO3 concentration was 0.08 or greater, the solid contained only MHC. These findings are consistent with those of Nishiyama et al [11] who examined the formation of MHC using the same concentrations of CaCl2 and MgCl2 but a higher temperature (25 ◦ C) than that used in the present study (5 ◦ C) and found that the solids obtained when the initial Na2 CO3 concentration was greater than 0.07 mol/kg contained only crystalline MHC (Table 1)
◦ C5with near near-edge structure spectrum ofsolid the solid obtained °C with an initial

Summary

Introduction

Monohydrocalcite (MHC, CaCO3 ·H2 O) is a hydrous calcium carbonate, and it is metastable with respect to anhydrous phases (CaCO3 ), such as calcite and aragonite [1,2]. MHC most frequently occurs as an authigenic mineral in alkaline saline lakes [3,4]. Recent research has suggested that the water chemistries of alkaline saline lakes, such as pH and the concentrations of Ca and Mg, are controlled by the formation of MHC in association with amorphous Mg carbonate (AMC, MgCO3 ·nH2 O) [5]. The metastable nature of MHC makes it a promising material for environmental remediation [1]. A simple, reproducible method for the synthesis of MHC containing low levels of impurities would be useful

Methods

Results

Conclusion