Review on Chemistry of Water-Containing Calcium Carbonates and Their Transformations into Amorphous and Crystalline Carbonate Modifications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript entitled "Review on chemistry of water-containing calcium carbonates and their transformations into amorphous and crystalline carbonate modifications" provides a comprehensive overview of hydrated calcium carbonates, including both amorphous and crystalline solvatomorphs (ACC, MHC, ikaite, and CCHH). The authors present in detail their preparation methods, crystal structures, physicochemical properties, as well as the mechanisms of mutual transformations. The topic is timely and relevant given the importance of calcium carbonate in geological processes, biogenic structures, and in the development of modern functional materials.

The manuscript is very well written, clearly structured, and supported by an extensive body of literature, which significantly enhances its value as a review article. A broad range of relevant studies has been carefully analyzed, and the key contributions in the field are well highlighted.

In my opinion, the paper can be accepted for publication after minor revisions, primarily addressing technical and/or linguistic details in order to further improve clarity and readability.

1. In Section 2 (Natural occurrence, preparation, and properties of hydrated calcium carbonates), I would encourage the authors to expand the discussion for each individual solvatomorph. The subdivision into 2.1. Ikaite, 2.2. Monohydrocalcite (MHC), 2.3. Calcium carbonate hemihydrate (CCHH), and 2.4. Amorphous calcium carbonate (ACC) is well structured; however, in many places the text mainly lists previous works without sufficiently describing the experimental procedures those authors employed. Including short summaries of the synthesis methods used in the literature would add significant value and improve the review’s usefulness. A comparative table summarizing the synthetic routes for each solvatomorph would also be highly beneficial for readers.

Furthermore, the meaning of the term “Natural occurrence” is not entirely clear in this context. If the intention is to describe the natural presence of these phases in geological or biological systems, this should be explicitly stated.

At present, the four CaCO₃ solvatomorphs are not discussed with the same level of detail in this respect. For example, Section 2.3 on Calcium carbonate hemihydrate (CCHH) appears rather brief compared to the other subsections.

Finally, I would recommend reconsidering the placement of Table 1 (Crystalline parameters of the different hydrated CaCO₃ solvatomorphs). Since this table primarily addresses structural aspects, it would fit more logically in Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates), rather than in the subsection devoted to CCHH. This would help align the content more clearly with the section headings.

2. I would like to suggest that Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates) be reorganized to follow a clearer and more consistent structure. One possible approach would be to present this section in the same manner as Section 2 (Natural occurrence, preparation, and properties of hydrated calcium carbonates), with subsections dedicated to each CaCO₃ solvatomorph (3.1. Ikaite, 3.2. Monohydrocalcite (MHC), 3.3. Calcium carbonate hemihydrate (CCHH), and 3.4. Amorphous calcium carbonate (ACC)). This would make the text much easier to read and more pleasant to follow, as it would allow direct comparison between the occurrence, preparation, and spectroscopic/structural features of each phase.

Alternatively, the section could also be divided according to the spectroscopic and structural methods employed (e.g., IR, Raman, NMR, XRD, etc.), which would provide readers with a method-based comparative overview. Either of these approaches would improve readability and strengthen the logical flow of the manuscript.

3. In continuation of my previous comment regarding Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates), I would also recommend including at least one representative IR and/or Raman spectrum. Such a figure would greatly enhance the clarity of the discussion, allowing readers to directly visualize the characteristic vibrational features of the different solvatomorphs.

This addition would not only complement the detailed textual description and the existing table but also provide readers with a clearer comparative overview of the spectroscopic distinctions among ikaite, monohydrocalcite, calcium carbonate hemihydrate, and amorphous calcium carbonate. I believe this would significantly improve the accessibility and scientific value of the review.

4. Section 4 (Transformations of hydrated calcium carbonates) is rather difficult to read and follow in its current form. At present, the synthesis procedures, analytical details, and transformation pathways are all described together, which makes the text somewhat dense and less clear. It is evident that the authors have reviewed a large amount of literature and made a significant effort to synthesize this knowledge into a comprehensive review, but I believe this final section requires further refinement.

One possible way to improve clarity would be to divide the section into subsections, for example by describing the transformations of each individual CaCO₃ solvatomorph separately. Another option would be to present only the transformation processes in this section, without repeating detailed synthesis and analytical methods, which have already been covered earlier in the manuscript.

Additionally, I would strongly encourage the authors to include a schematic illustration or diagram summarizing the possible transformation pathways between the different hydrated and anhydrous phases. Such a figure would significantly improve readability and provide the reader with a clear visual overview of the complex relationships among the various forms.

5. In several instances, terms such as amorphous calcium carbonate (ACC), solvatomorphs, and hydrated calcium carbonates appear to be used interchangeably. I suggest that the authors standardize the terminology throughout the manuscript.

6. I suggest that the authors consider adding a dedicated section discussing the morphology of CaCO₃ solvatomorphs. Each form exhibits distinct and interesting features, and highlighting these differences in a separate section would enhance the clarity and depth of the manuscript.

Author Response

We would like to express our gratitude to the Reviewers, for their efforts to improve our manuscript. We have addressed their issues and revised the manuscript accordingly. Below, we reported all their comments and our point-to-point responses, marked in green both here and in the text).

Reviewer 1

The manuscript entitled "Review on chemistry of water-containing calcium carbonates and their transformations into amorphous and crystalline carbonate modifications" provides a comprehensive overview of hydrated calcium carbonates, including both amorphous and crystalline solvatomorphs (ACC, MHC, ikaite, and CCHH). The authors present in detail their preparation methods, crystal structures, physicochemical properties, as well as the mechanisms of mutual transformations. The topic is timely and relevant given the importance of calcium carbonate in geological processes, biogenic structures, and in the development of modern functional materials.

The manuscript is very well written, clearly structured, and supported by an extensive body of literature, which significantly enhances its value as a review article. A broad range of relevant studies has been carefully analysed, and the key contributions in the field are well highlighted.

In my opinion, the paper can be accepted for publication after minor revisions, primarily addressing technical and/or linguistic details in order to further improve clarity and readability.

1. In Section 2 (Natural occurrence, preparation, and properties of hydrated calcium carbonates), I would encourage the authors to expand the discussion for each individual solvatomorph. The subdivision into 2.1. Ikaite, 2.2. Monohydrocalcite (MHC), 2.3. Calcium carbonate hemihydrate (CCHH), and 2.4. Amorphous calcium carbonate (ACC) is well structured; however, in many places, the text mainly lists previous works without sufficiently describing the experimental procedures those authors employed. Including short summaries of the synthesis methods used in the literature would add significant value and improve the review’s usefulness. A comparative table summarizing the synthetic routes for each solvatomorph would also be highly beneficial for readers.

We appreciate the Reviewer's suggestion, which helps to improve the quality of our manuscript. To address the issue, we included possible synthetic routes for each solvatomorphs at the end of each subsection in section 2 and inserted a new table (Table 1), which summarizes the possible synthetic methods.

Furthermore, the meaning of the term “Natural occurrence” is not entirely clear in this context. If the intention is to describe the natural presence of these phases in geological or biological systems, this should be explicitly stated.

Based on the Reviewer’s suggestion, we modified the title of section 2 into the following: “Occurrence in geological environment and/or biological systems, preparation, morphology, and properties of hydrated calcium carbonates”

At present, the four CaCO₃ solvatomorphs are not discussed with the same level of detail in this respect. For example, Section 2.3 on Calcium carbonate hemihydrate (CCHH) appears rather brief compared to the other subsections.

The Reviewer is right. Section 2.3 is short compared to other sections. CCHH was only discovered in 2019, and since then, only a limited number of articles, all of which are included, have been published on this topic.

Finally, I would recommend reconsidering the placement of Table 1 (Crystalline parameters of the different hydrated CaCO₃ solvatomorphs). Since this table primarily addresses structural aspects, it would fit more logically in Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates), rather than in the subsection devoted to CCHH. This would help align the content more clearly with the section headings.

We appreciate the Reviewer’s suggestion and we moved the original Table 1 into Section 3 as the new Table 3.

2. I would like to suggest that Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates) be reorganized to follow a clearer and more consistent structure. One possible approach would be to present this section in the same manner as Section 2 (Natural occurrence, preparation, and properties of hydrated calcium carbonates), with subsections dedicated to each CaCO₃ solvatomorph (3.1. Ikaite, 3.2. Monohydrocalcite (MHC), 3.3. Calcium carbonate hemihydrate (CCHH), and 3.4. Amorphous calcium carbonate (ACC)). This would make the text much easier to read and more pleasant to follow, as it would allow direct comparison between the occurrence, preparation, and spectroscopic/structural features of each phase.Alternatively, the section could also be divided according to the spectroscopic and structural methods employed (e.g., IR, Raman, NMR, XRD, etc.), which would provide readers with a method-based comparative overview. Either of these approaches would improve readability and strengthen the logical flow of the manuscript.

We appreciate the Reviewer’s suggestion and based on it we separated Section 3 into 5 subsections.

3. In continuation of my previous comment regarding Section 3 (Spectroscopic and structural studies on hydrated calcium carbonates), I would also recommend including at least one representative IR and/or Raman spectrum. Such a figure would greatly enhance the clarity of the discussion, allowing readers to directly visualize the characteristic vibrational features of the different solvatomorphs.

This addition would not only complement the detailed textual description and the existing table but also provide readers with a clearer comparative overview of the spectroscopic distinctions among ikaite, monohydrocalcite, calcium carbonate hemihydrate, and amorphous calcium carbonate. I believe this would significantly improve the accessibility and scientific value of the review.

Following the Reviewer's suggestion, we inserted a new figure (as new Figure 2), which shows the IR and Raman spectra of the various hydrated calcium carbonates.

4. Section 4 (Transformations of hydrated calcium carbonates) is rather difficult to read and follow in its current form. At present, the synthesis procedures, analytical details, and transformation pathways are all described together, which makes the text somewhat dense and less clear. It is evident that the authors have reviewed a large amount of literature and made a significant effort to synthesize this knowledge into a comprehensive review, but I believe this final section requires further refinement.

One possible way to improve clarity would be to divide the section into subsections, for example by describing the transformations of each individual CaCO₃ solvatomorph separately. Another option would be to present only the transformation processes in this section, without repeating detailed synthesis and analytical methods, which have already been covered earlier in the manuscript.

We appreciate the reviewer’s suggestion and, based on it, we have divided Section 3 into five subsections and relocated the synthesis parts to new paragraphs in Section 2. As a result, Section 4 became clearer and more focused on transformation.

Additionally, I would strongly encourage the authors to include a schematic illustration or diagram summarizing the possible transformation pathways between the different hydrated and anhydrous phases. Such a figure would significantly improve readability and provide the reader with a clear visual overview of the complex relationships among the various forms.

Following the reviewer’s suggestion, we have inserted a new figure (Figure 7) that summarizes the various transformation pathways.  

5. In several instances, terms such as amorphous calcium carbonate (ACC), solvatomorphs, and hydrated calcium carbonates appear to be used interchangeably. I suggest that the authors standardize the terminology throughout the manuscript.

We have tried to standardize the terminology; however, hydrated calcium carbonates (including amorphous calcium carbonate) are solvatomorphs of each other. However, we inserted in the following sentences: In spite of its simple composition, CaCO₃ has a variety of polymorphs and solvatomorphs. Polymorphs are anhydrous forms, whereas solvatomorphs are hydrated calcium carbonates and these terms are interchangeable used in the text.”

6. I suggest that the authors consider adding a dedicated section discussing the morphology of CaCO₃ solvatomorphs. Each form exhibits distinct and interesting features, and highlighting these differences in a separate section would enhance the clarity and depth of the manuscript.

In order to address this issue, we included a short discussion about the typical morphology of CaCO₃ solvatomorphs and inserted a new figure (Figure 1) in Section 2.

Reviewer 2 Report

Comments and Suggestions for Authors

The article by Béres and coauthors is devoted to a review of data on aqueous calcium carbonates. The authors summarize a large set of information on natural finds, synthesis, transformation pathways, and identification methods.

In my opinion, the article could be improved due to two main points.
1) The text needs to be reformatted for a more understandable division of parts. Thus, information on synthesis and diagnostics is scattered throughout the text, making it difficult to perceive the data.
I propose to highlight 4 large sections (by phases) - ikaite, monohydrocalcite, calcium carbonate hemihydrate, and amorphous calcium carbonate. And then in each section, one could write about: crystal structure, finds in nature, synthesis. Then, two general sections could be made - one on mutual transformation, and the second on identification. This work does not imply significant changes to the text, but it would help the reader navigate it. In addition, in the last two sections, comparative tables and figures could be made.

2) In my opinion, the figures need to be redone (and their number reduced). At present, all the figures presented are simply reproductions. The review involves generalizing the data, including illustrations.
2.1. There is no point in providing pictures of structures from articles when you can draw them yourself and show the necessary details (location of carbonate groups and hydrogen bonds). You can make 1 or 2 generalizing pictures with crystal structures.
2.2. There is no generalizing picture that would show the differences in the morphology of various carbonates. Instead, many scattered photographs from different articles are provided, mainly on anhydrous calcium carbonates. In addition, I did not find a single good picture of ikaite.
2.3. Pictures with small captions (for example, Fig. 8 and 15) and detailed electron structures (Fig. 2) look redundant.

Minor comments:
1. the issue of ikaite synthesis should be discussed in the context of temperature (e.g. https://doi.org/10.1038/s41598-020-64751-5) and additives (phosphorus, sulfur, magnesium; e.g. https://doi.org/10.1007/s10498-023-09418-z)
2. the same is true for monohydrocalcite. There are data on its synthesis at low temperatures and in the absence of magnesium (but in the presence of copper or cobalt; https://doi.org/10.3390/min13121497)
3. Table 1. The title (top line) should contain ikaite (as well as monohydrocalcite).
4. The phases in different figures are signed differently (since these are reproductions). This needs to be unified. Thus, in Figure 4, the amorphous carbonate is labeled ACCH2O instead of ACC. In Figure 7, there are some BC and GS, and so on.

Author Response

We would like to express our gratitude to the Reviewers, for their efforts to improve our manuscript. We have addressed their issues and revised the manuscript accordingly. Below, we reported all their comments and our point-to-point responses, marked in green both here and in the text).

Reviewer 2

The article by Béres and coauthors is devoted to a review of data on aqueous calcium carbonates. The authors summarize a large set of information on natural finds, synthesis, transformation pathways, and identification methods.

In my opinion, the article could be improved due to two main points.

1) The text needs to be reformatted for a more understandable division of parts. Thus, information on synthesis and diagnostics is scattered throughout the text, making it difficult to perceive the data.

I propose to highlight 4 large sections (by phases) - ikaite, monohydrocalcite, calcium carbonate hemihydrate, and amorphous calcium carbonate. And then in each section, one could write about: crystal structure, finds in nature, synthesis. Then, two general sections could be made - one on mutual transformation, and the second on identification. This work does not imply significant changes to the text, but it would help the reader navigate it. In addition, in the last two sections, comparative tables and figures could be made.

We greatly appreciate the Reviewer’s suggestions. We have inserted new subsections, prepared new tables and figures to clarify the discussion and the text. However, we feel the current form of the sections facilitates easier comparison of the occurrence, synthesis, crystal structure, spectroscopic data, and mutual transformation among the various solvatomorphs. Therefore, we would like to keep the order of the sections.

2) In my opinion, the figures need to be redone (and their number reduced). At present, all the figures presented are simply reproductions. The review involves generalizing the data, including illustrations.

The figures included in this review were chosen to highlight important pieces of information that we would like to emphasize in our paper. To avoid losing valuable information, we combined some of these figures to reduce the overall number of figures, according to the Reviewer’s suggestion. Furthermore, we also prepared a new figure that summarizes the possible transformation pathways between the different hydrated and anhydrous phases.

2.1. There is no point in providing pictures of structures from articles when you can draw them yourself and show the necessary details (location of carbonate groups and hydrogen bonds). You can make 1 or 2 generalizing pictures with crystal structures.

The reviewer is right, drawings of the structure models could be generated based on the database. However, these drawings would not add new information compared to the published ones. Therefore, we decided to use published figures. However, we revised the previous figures and merged to one (Figures 1, 3, and 6 into one single figure (new Figure 3)) to show the key elements of the structures in our paper and help the readers to extract the main information and find the structures of the different solvatomorphs (and to reduce the number of the figures), we merged

2.2. There is no generalizing picture that would show the differences in the morphology of various carbonates. Instead, many scattered photographs from different articles are provided, mainly on anhydrous calcium carbonates. In addition, I did not find a single good picture of ikaite.

Following the reviewer’s suggestion, we have inserted a figure (as Figure 1) that shows the morphologies of the different solvatomorphs.

2.3. Pictures with small captions (for example, Fig. 8 and 15) and detailed electron structures (Fig. 2) look redundant.

We have reorganized and increased the size of the above-mentioned figures.

Minor comments:

1. the issue of ikaite synthesis should be discussed in the context of temperature (e.g. https://doi.org/10.1038/s41598-020-64751-5) and additives (phosphorus, sulfur, magnesium; e.g. https://doi.org/10.1007/s10498-023-09418-z)
2. the same is true for monohydrocalcite. There are data on its synthesis at low temperatures and in the absence of magnesium (but in the presence of copper or cobalt; https://doi.org/10.3390/min13121497)

We appreciate the Reviewer’s suggestions and we inserted these informations  into the text of the manuscript.

3. Table 1. The title (top line) should contain ikaite (as well as monohydrocalcite).

We changed the title of the Table (now it is Table 3).

4. The phases in different figures are signed differently (since these are reproductions). This needs to be unified. Thus, in Figure 4, the amorphous carbonate is labeled ACCH2O instead of ACC. In Figure 7, there are some BC and GS, and so on.

We appreciate the Reviewer’s comment and modified Figure 5 accordingly. ACC and ACC2H2O were changed to ACC-Dehyd and ACC-Hyd corresponding to dehydrated ACC and hydrated ACC, respectively. Figure 7 was also modified.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

Thank you for the edits and changes. I suggest making a few more minor edits, after which the article can be accepted for publication:

1) Table 1. It is necessary to unify the description of various syntheses, highlighting the main
1.1. the role of temperatures in the formation of various hydrated forms. This is of great importance for both ikaite and monohydrocalcite. Provide this data based on natural observations and syntheses.
1.2. The presence of magnesium, transition metals (in nature and synthesis)
1.3. The presence of phosphate ion (in nature and synthesis)
2) Figure 1. Crystals of ikaite and monohydrocalcite can be seen with the naked eye. These are not just objects accessible to SEM. Please add macroscopic images of minerals. 
3) Figure 3. I understand your reluctance to redraw the structures, but this must be done, since the captions in the figures provided are very small, the same atoms have different colors, and the carbonate groups are not visible. Please draw the crystal structures of the compounds.

Author Response

We would like to express our gratitude to the Reviewer for her/his effort to improve our manuscript. We have addressed her/his issues and revised the manuscript accordingly. Below, we reported all their comments and our point-to-point responses (marked in green both here and in the text).

Reviewer 2, Round 2

1) Table 1. It is necessary to unify the description of various syntheses, highlighting the main
1.1. the role of temperatures in the formation of various hydrated forms. This is of great importance for both ikaite and monohydrocalcite. Provide this data based on natural observations and syntheses.

1.2. The presence of magnesium, transition metals (in nature and synthesis)
1.3. The presence of phosphate ion (in nature and synthesis)

Thank you very much for your suggestion. We have modified Table 1 accordingly to highlight the presence of other ions, emphasizing their importance. This change is also reflected in the title of Table 1.

2) Figure 1. Crystals of ikaite and monohydrocalcite can be seen with the naked eye. These are not just objects accessible to SEM. Please add macroscopic images of minerals. 

Thank you very much for the suggestion; however, we chose these figures since they represent the given crystals in the most detailed manner. Furthermore, we use only SEM figures, as this approach allows the figures to be more harmonized, and additional explanations (such as differences in the scale bar) are not necessary in the title of Figure 1. 

3) Figure 3. I understand your reluctance to redraw the structures, but this must be done, since the captions in the figures provided are very small, the same atoms have different colors, and the carbonate groups are not visible. Please draw the crystal structures of the compounds.

We drew the crystalline structure of ikaite and MHC, as these data are available in the ICSD depository under numbers 98370 and 160811, respectively. In the case of CCHH, the structural data is not available in the ICSD depository, and we obtained permission only to reuse the current Figure 3(c). Currently, all colours represent the same atoms; however, to enhance understanding, a brief description of the annotations is added at the end of the title for Figure