Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular

Round 1

Reviewer 1 Report

The authors present a study of cation ordered grossular. The study employs suitable methods and shows adequate figures and tables. A major challenge is the partial substittuion in these natural minerals.

Inaddition to the required significant improvement of the English, I have two remarks:

Why do the authors use the formula (H4O4)4- for the substituted oh groups instead of (OH)sub4super4a as there is no such structural unit, it basically are simply four (OH)- groups

The authors give the refinement results in good detail. And I understand that the selected space group is preferred as is takes into account all observed extinctions correctly and produces the lowest Rint. However, the higher symmetric model in the cubic space group Ia-3d gives the lowest R value after refinement despite less degrees of freedom (smaller number of free, refinable parameters). This finding needs to be explaned. 

In conclusion, I suggest maor improvement of the English and minor revisions of the content.

Author Response

Dear Vinny Mei,

We are very grateful to you and both reviewers for the constructive comments. We tried to revise our manuscript as best as we can.

Below please find our responses to the reviewers’ comments.

Comments by First reviewer:

Why do the authors use the formula (H₄O₄)^4- for the substituted oh groups instead of (OH)sub4super4a as there is no such structural unit, it basically are simply four (OH)- groups

Response:
The hydrogarnet substitution intensively discussed in literature, for example, Geiger and Rossman,(2018, 2020) (https://doi.org/10.2138/am-2018-6160CCBY, https://doi.org/10.2138/am-2020-7256). This works argued that OH- is located as isolated (H₄O₄)^4– groups. From crystal chemical point of view one structural unit (SiO₄)⁴⁻ exchanged by another one (H₄O₄)^4–. We just use standard terms for the hydrogarnets.

Comment:

The authors give the refinement results in good detail. And I understand that the selected space group is preferred as is considers all observed extinctions correctly and produces the lowest R_int. However, the higher symmetric model in the cubic space group Ia-3d gives the lowest R value after refinement despite less degrees of freedom (smaller number of free, refinable parameters). This finding needs to be explained.

Response:

 R-factor is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. In other words, it is a measure of how well the refined structure predicts the observed data.[1] The value is also sometimes called the discrepancy index, as it mathematically describes the difference between the experimental observations and the ideal calculated values.[2] It is defined by the following equation:

where F is the so-called structure factor and the sum extends over all the reflections of X-rays measured and their calculated counterparts respectively. The structure factor is closely related to the intensity of the reflection it describes:

The main parameter characterizing intensity of reflections is I/I(σ) ratio. For the cubic Ia-3d model the I/I(σ) ratio is 60.8 at the same ratio for the I4₁/a model is 41.2. Such difference explains lower R₁ value for the cubic structural model.

Comment:

In conclusion, I suggest maor improvement of the English and minor revisions of the content.

Response:

We improve English as best as we can.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Article entitled: Merohedral mechanism twining growth of natural cation ordered tetragonal grossular by Panikorovskii et al. has got very well described and finally concluded mineralogical problem. It fills the crystal data gap for unusual grossular from the Wiluy River area in Russia.  However, there are some parts, witch can be improved. I attached the main comments below, where some technical comments are listed in separated PDF file.

The text needs one more figure with the occurrences of the samples as well as geological background. The main aim of the script is to focus on the crystallogenic mechanism, but do not forget, that it is still geological article at all.

Table 1 presents the EMPA data. Some of them seem to be weak. I cannot agree the Cl measured  content was 0.04 (spot 3). The same with Cr₂O₃ (spot 3). What is the sense to indicate 0 (spot 4; Cr₂O₃) or 0.00 (spot 3; SO3) and e.g., nd spot 2 Cr₂O₃. I deeply sorry, but the whole sulphur content was not as in the table. I guess that “nd” means “not detected”. So, you can use also “bdt” below detection limit. Please, give the REAL limits of detection for every element.

Your article presents a lot of a new, valuable crystallographic data. Becouse of that it is too long. I recommend to move tables 3 and 4 to supplementary material (appendixes).

 Do not use short grammar forms like: that’s, doesn’t, etc.

Comments for author File: Comments.pdf

Author Response

Dear Vinny Mei,

 

We are very grateful to you and both reviewers for the constructive comments. We tried to revise our manuscript as best as we can.

 

Below please find our responses to the reviewers’ comments.

 

Comments by Second reviewer:

t fills the crystal data gap for unusual grossular from the Wiluy River area in Russia.  However, there are some parts, witch can be improved. I attached the main comments below, where some technical comments are listed in separated PDF file.

Response:

We applied all corrections proposed in the PDF-file to manuscript.

Comment:

The text needs one more figure with the occurrences of the samples as well as geological background. The main aim of the script is to focus on the crystallogenic mechanism, but do not forget, that it is still geological article at all.

Response:

Unfortunately, after the flooding of the classical deposit of achtarandite, wiluite and grossular, geological works were not carried out in this area; therefore there are no current geological maps. We can state with assurance that the large block of the Ordovician carbonate rocks was altered at the contact with a large intrusion of the Siberian Traps (we have supplemented the text). The reader can find old schematic maps of this area in the book [40], which is fully available at https://rebus.us.edu.pl.

Comment:

Table 1 presents the EMPA data. Some of them seem to be weak. I cannot agree the Cl measured  content was 0.04 (spot 3). The same with Cr₂O₃ (spot 3). What is the sense to indicate 0 (spot 4; Cr₂O₃) or 0.00 (spot 3; SO3) and e.g., nd spot 2 Cr₂O₃. I deeply sorry, but the whole sulphur content was not as in the table. I guess that “nd” means “not detected”. So, you can use also “bdt” below detection limit. Please, give the REAL limits of detection for every element.

Response:

We agree, the measured content of the S and Cl are in the border of detection limit. The same related to the Cr. For the convenience we delete Cr, S and Cl content from the table. Contents for these elements do not exceed 0.01 apfu and absent in empirical formula.

Comment:

Your article presents a lot of a new, valuable crystallographic data. Becouse of that it is too long. I recommend to move tables 3 and 4 to supplementary material (appendixes).

Response:

We transfer tables 3, 4 and 5 to the supplementary materials as tables S1, S2 and S3.

Comment:

 Do not use short grammar forms like: that’s, doesn’t, etc

Response:

We add corresponding corrections and all short forms is changed.

 

Author Response File: Author Response.pdf