Simple, Reproducible Synthesis of Pure Monohydrocalcite with Low Mg Content

Round 1

Reviewer 1 Report

This manuscript can be accepted only if the authors response the comments and make the corrections indicated.

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

This manuscript by Kitajima et al. reports a protocol to make monohydrocalcite (MHC) with low content of Mg at a low temperature of 5 ^oC. I would not recommend its publication in the present version because it is too superficial in scientific depth and more data are needed. For example, further characterization of the MHC via (cryo)-TEM (plus elemental mapping) or SEM is highly desirable to analyze the structure and morphology.

In the Introduction part, the authors mentioned that “Previous studies have shown that MHC effectively sorbs contaminants such as arsenic, lead, and phosphorous [6-8]. Thus, a simple, reproducible method for the synthesis of MHC containing low levels of impurities would be useful.” I would suggest the authors do further experiments to test the absorption ability of the MHC with various Mg contents. That would significantly impove the scientific importance of this manuscript.

Author Response

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Reviewer 3 Report

The enclosed manuscript describes the results of a series of free drift solid carbonate synthesis experiments focusing on the Mg content as function of temperature and solid carbonate mineralogy.  In particular, the authors demonstrate that lower-Mg monohydrocalcite can be synthesized at low temperature. Prior experiments performed at 25C yielded higher bulk solid Mg/Ca values than the 5C experiments presented in this study. The manuscript is clear and well written, but could be improved significantly by some additional calculations and discussion.

What is notably absent is a discussion and comparison of the partitioning behavior of Mg - distribution coefficients are not presented or discussed at all. I suggest that the authors provide calculated distribution coefficients in Table 1, along with uncertainty estimates for all measured data. It appears that Mg has a very low distribution coefficient in MHC (3e-5 to 1e-4) compared to calcite, but it is not clear whether the difference between the Nishiyama et al. results and the results in this study can be attributed to temperature dependent partition coefficients or to the absence of AMC. For example, if Kd for Mg increases with temperature, such a temperature effect could explain the higher Mg distribution coefficient in Nishiyama.

In addition to discussion of distribution coefficients, I believe the manuscript could be improved by some additional discussion of the structural characteristics of MHC, and the relationship between structure and partitioning behavior. It is interesting that the Mg site in MHC is aragonite-like when Mg partitioning is very weak in aragonite compared to calcite. If spectroscopic data are available for Ca in these phases, it would be useful to know whether the Ca site in MHC is more similar to aragonite or calcite as well. This information can help interpret bulk Mg mass balance in systems such as the alkaline lakes mentioned in the introduction.

A few other items that would improve the manuscript:

1. Final estimated aqueous supersaturations with respect to the solid phase formed should be added to table 1. If final measurements of dissolved inorganic carbon concentration/alkalinity/pH were not made, this can be estimated based on mass balance, atmospheric CO2 equilibration, etc. This information will inform the reader how close to (metastable) equilibrium the experiments came.
2. Uncertainties on measured elemental analyses and quantities calculated based on elemental analysis (e.g. Mg/Ca ratios).

Author Response

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Round 2

Reviewer 1 Report

This manuscript can be considered for publication in the journal  

Reviewer 2 Report

Given the additional data provided in the new version, I am happy to recommend its publication in minerals.