Lowering R3m Symmetry in Mg-Fe-Tourmalines: The Crystal Structures of Triclinic Schorl and Oxy-Dravite, and the Mineral luinaite-(OH) Discredited

Round 1

Reviewer 1 Report

This paper will bury all the concerns about the position of luinaite-(OH) in the tourmaline-supergroup classification scheme. I agree with the conclusions of authors about possible monoclinic and triclinic members.

I have only a few recommendations, which, in my opinion, would improve the paper for the general audience.

Firstly, I checked the structure from the original proposal and here and found out that Kolitsch et al. chose completely different unit cel but the unit cell of authors is more appropriate for the tourmaline-structure topology. This could be mentioned to avoid any discussion about vast differences in structural parameters.

Figure 4: I think it would be better for reader to compare structure of "luinaite" to trigonal schorl visually. I mean to compose images of both structures to illustrate their topological similarity. There is a space for it.

There is a typo in Table 4 - populsation -> population

Author Response

Reviewer 1

This paper will bury all the concerns about the position of luinaite-(OH) in the tourmaline-supergroup classification scheme. I agree with the conclusions of authors about possible monoclinic and triclinic members.

I have only a few recommendations, which, in my opinion, would improve the paper for the general audience.

Firstly, I checked the structure from the original proposal and here and found out that Kolitsch et al. chose completely different unit cell, but the unit cell of authors is more appropriate for the tourmaline-structure topology. This could be mentioned to avoid any discussion about vast differences in structural parameters.

--- OUR ANSWER. We fully agree with this comment. However, we prefer not to comment further on the choice on the cell made by Kolitsch et al.... We are sure that Kolitsch et al. will be able to motivate such a choice in an upcoming manuscript.

 

Figure 4: I think it would be better for reader to compare structure of "luinaite" to trigonal schorl visually. I mean to compose images of both structures to illustrate their topological similarity. There is a space for it.

--- OUR ANSWER. Triclinic and trigonal structures are very similar: they are related by a very small crystal structure distortion. If you compare the two structures visually, it is hard to see the difference between them. Figure 4 is triclinic (a = 15.95, b = 15.94, c = 7.19 Å, a = 90.03°, b = 89.94° and g = 119.85°), yet it looks like the classic trigonal tourmaline structure (a = b = 15.96, c = 7.19 Å, a = 90°, b = 90° and g = 120°). So, adding a trigonal structure would not help to show the differences in terms of structural distortion. This comparison, however, can easily be done using a CIF and any crystal structure visualization program.  

 

There is a typo in Table 4 - populsation -> population

--- OUR ANSWER. Done. Thank you!

Reviewer 2 Report

Dear authors,

good work.  

Please check the attachment with a few comments.

Comments for author File: Comments.pdf

Author Response

Reviewer 2

good work. 

Please check the attachment with a few comments.

--- OUR ANSWER. Thank you for the suggestions. We followed several of them, except for: “Several localities where tourmalines with lower symmetry are reported by”, we prefer to maintain the original sentence.

--- OUR ANSWER. Regarding the question: “Are all fibrous?”, Yes!

Reviewer 3 Report

To comfirm the structrual transformation from normal uniaxial trigonal (R3m) to biaxial monclinic or triclinic systems, I suggest the authors to do convergent polarized light interference patterns. That is very simple and straightforward. 

Author Response

Reviewer 3

To comfirm the structrual transformation from normal uniaxial trigonal (R3m) to biaxial monclinic or triclinic systems, I suggest the authors to do convergent polarized light interference patterns. That is very simple and straightforward.

--- OUR ANSWER. Yes. We did it.

Reviewer 4 Report

Very nice paper indeed. Only two comments. Provide information on the source of the study material - private collection or institutional one? Further, I found that ranges of k and l indices in table 3 are quoted twice, please correct. 

Author Response

Reviewer 4

Very nice paper indeed. Only two comments. Provide information on the source of the study material - private collection or institutional one? Further, I found that ranges of k and l indices in table 3 are quoted twice, please correct.

--- OUR ANSWER. Private collection.

--- OUR ANSWER. We corrected Table 3. Thank you!