Extreme Alteration of Chevkinite-(Ce) by Pb-CO₂-Rich Fluids: Evidence from the White Tundra Pegmatite, Keivy Massif, Kola Peninsula

Round 1

Reviewer 1 Report

The paper cannot be published in this form. I order to be published, it must be completely rewritten and resubmitted as a new manuscript.

In the Discussion section, the authors write: “Pb values in Group 2 analyses are up to 1.1 apfu, i.e. about one-quarter of the A-site.”. Thus, the Pb-Ti-Fe-Si phase is considered as a mineral isostructural with chevkinite. However, this is not confirmed by X-ray data. Moreover, the analytical totals are very low (around 80 wt.%). For a chevkinite-type mineral they should be about 100%. Probably, the studied minerals are metamict and hydrated, but even this assumption is insufficient for the explanation of such low totals.

The REE silicate treated as chevkinite contains 0.11 – 0.58 wt.% of Ca which is unusual for chevkinite which typically contains 2 to 4 wt.% of Ca. It is to be noted that chevkinite, perrierite, dingdaohengite and mauniupingite cannot be reliably distinguished by composition, but all these minerals contain significant amounts of Ca.

Chevkinite does not have microporous structure. Its structure is dense and substitution of Ca and REE for Pb and U could be possible only after the complete decomposition of the initial chevkinite structure.

Furthermore, the authors do not provide sufficient substantiation of their assumption that the Pb-Ti-Fe-Si phase substitutes chevkinite rather that it was growing separately or around presumed chevkinite grains. Figure 5 is insufficiently convincing. More data on spAcial relationships between the REE and Pb,U phases should be provided. Alternative hypotheses on the formation and nature of the Pb-Ti-Fe-Si phase must be discussed.

The authors should check whether the formulae of presumed chevkinite-type phases are charge-balanced.

The title does not exactly correspond to the content of the paper because it provides also chemical data for bastnaesite and chevkinite.

To conclude: In order to be reviewed further, the paper must be completely rewritten and entitled differently (e.g. “Association of Pb,REE,U-minerals in  the White Tundra pegmatite, Keivy massif, Kola Peninsula”).

Author Response

First, may we thank the Academic Editor and reviewers for their comments and suggestions. We hope that we have responded to them satisfactorily.

 

Reviewer 1

(i) We feel, unfortunately, that the reviewer has either not carefully read, or has misunderstood, several sections of the text. We did not claim that the Pb-Ti-Fe-Si phase was isostructural with chevkinite; we have clearly shown by EBSD that it is amorphous. We did show that the Group 2 phase is crystalline and that Pb can be taken to occupy about one-quarter of the A-site. Some of the Group 2 analytical totals are low, which we ascribe to incorporation of hydrothermal water and/or to vacancies, typical features of hydrothermally altered chevkinite (see review by Macdonald et al., 2019). The very low totals in Groups 3 and 3 are related to the fact that they are amorphous, most probably containing significant water and CO₂.

(ii) The reviewer seems unaware of the full range of chevkinite compositions. In a compilation of 164 analyses of chevkinite, Macdonald et al. (2019) found that the range of CaO values was 0.26-5.50 wt%.

(iii) A successful, if empirical, compositional discriminant between chevkinite and perrierite has been available since 2002 (Macdonald et al., 2002; updated by Macdonald et al., 2009). The introduction of maoniupingite and dingdaohengite is irrelevant; recent research has suggested that neither phase exists and should be discredited as valid species (Belkin et. al., 2009; Macdonald et al., 2019; Nagashima et al., 2020).

(iv) Nowhere did we suggest that the chevkinite was microporous – the term was applied to the amorphous Pb-Ti-Fe-Si material.

(v) The textural evidence, such as the intergrowth of Groups 1 and 2 and the residual patches Groups 1 and 2 in Groups 3 and 4, is unequivocal that the relationship is replacive.

(vi) Charge balance. We are unaware of what the reviewer means here by “chevkinite-type phases”. Does she/he assume that the amorphous material should be charge-balanced?

(vii) We presume that here the reviewer is referring to aeschynite and not chevkinite? These analyses form only a minor part of the paper and do not merit inclusion in the title.

References

Belkin, H.E., Macdonald, R. and Grew, E.S. (2009) Chevkinite-group minerals from granulite-facies

    metamorphic rocks and associated pegmatites from East Antarctica and South India. Mineralogical

    Magazine 73, 149-164.

Macdonald, R. and Belkin, H.E (2002) Compositional variation in minerals of the chevkinite group.

    Mineralogical Magazine, 66, 1075-1098.

Macdonald, R., Belkin, H.E., Wall, F. and Bagiński, B. (2009) Compositional variation in the chevkinite

    group: new data from igneous and metamorphic rocks. Mineralogical Magazine, 73, 777-796.

Macdonald, R., Bagiński, B., Belkin, H.E. and Stachowicz, M. (2019) Composition, paragenesis, and

    alteration of the chevkinite group of minerals. American Mineralogist, 104, 348-369.

Nagashima, M., Armbruster, T., Akasaka, M., Sano-Furukawa, A., Nishio-Hamane, D., Malsy, A., Imaoka,

1. and Nakashima, K. (2020) Multi-methodical study of the Ti, Fe²⁺ and Fe³⁺ distribution in chevkinite-

    subgroup minerals: X-ray diffraction, neutron diffraction, ⁵⁷Fe Mössbauer spectroscopy and electron-

    microprobe analyses. Physics and Chemistry of Minerals, 47:29.

Author Response File: Author Response.pdf

Reviewer 2 Report

In the caption and references to Figure 2 upper case letters should be used to correspond with the upper case letters on the Figures themselves.  In the discussion of REE elements analyses the notation (X/X*) does not appear to be defined?  The following typos are also noted:

 

LINE                     PROBLEM

3                           evidencefrom

70                        (Y)and

373                      mineralization is repeated

385                      rheir

432                      amazonitepegmatite

450-468              Figure captions are repeated

Author Response

First, may we thank the Academic Editor and reviewers for their comments and suggestions. We hope that we have responded to them satisfactorily.

The omissions and amendments have all been tidied up.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work White Tundra pegmatite from Keivy massif, Kola Peninsula was studied. The object is interesting with a rare hydrothermal manifestation that consists of an alteration scheme of chevkinite-(Ce) altered by Pb-CO2-rich fluids initially removed REE and Y from the chevknite-(Ce), with enrichment in Pb and U. The accurate study of rare natural processes is a contribution to fundamental scientific research. The work is very well presented, the methods are precisely described and this results in a wonderful discussion. I would ask the authors to note the markers in fig. 4 (squares, rhombus, black, gray).

Author Response

First, may we thank the Academic Editor and reviewers for their comments and suggestions. We hope that we have responded to them satisfactorily.

The point about Fig. 4 has been addressed- the symbols are explained. And we are particularly grateful for the reference to a “wonderful discussion”.

Reviewer 4 Report

This MS shows a very good level use of the modern techniques to be employed for a sensible descriptive mineralogy. People working on minerals having non-conventional composition are strongly concerned.

Fine-mineralogy, petrography and geology are well interconnected.

English language should not be improved, but only controlled.

Author Response

First, may we thank the Academic Editor and reviewers for their comments and suggestions. We hope that we have responded to them satisfactorily.

We are uncertain as to which cited references are not relevant to the text. References are there because they have an application to the developing theme of the paper.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The paper cannot be published in this form because it contains a lot of mistakes and shortcomings. My previous comments are ignored by the authors.

 Table 2: The sites A, B, C, and D are indicated. Moreover, in the Discussion section the authors write: “Pb values in Group 2 analyses are up to 1.1 apfu, i.e. about one‐quarter of the A‐site.”. Thus, the authors still consider the Pb-bearing phase as a mineral isostructural with chevkinite. Obviously, this is a gross mistake. In particular, the Si content in this phase is 2.1-2.3 atoms per formula unit (apfu) with 22 O atoms instead of 4 apfu required for chevkinite. The sites A, B, C, and D do not exist in the amorphous Pb-bearing phase! Moreover, this phase has a stoichiometry very different from that of chevkinite and the total sums of analyses are too low. I wrote this in the previous review, but these comments have been ignored by the authors.

 The authors write that the low sums of analyses may be due to the presennce of water, but this is not confirmed by analytical data. The statement “the low analytical totals may be a result of vacancies in the structure” is ignorant: vacancies do not have an influence on the sums of electron microprobe analyses.

 In Table 2 double values of formula units should be explained in a comment below the table.

 The manuscript contains a lot of incorrect terms and mistakes. It is impossible to list all of them. In particular (only a small example related to Table 2): formulae --> formula coefficients; below detection --> below detection limit; oxygens --> oxygen atoms; of Pb‐Ti‐Fe‐Si phase --> of the Pb‐Ti‐Fe‐Si phase; on the basis --> based; “Fe*” at the bottom of the table is unclear; in “Fe2+” at the bottom of the table “2+” should be a supersctipt; in “Fe3+” in another place “3+” should be a supersctipt; “;” after “detection” must be deleted; sum of cations at the bottom of the table is wrong (compare Table 1); sum of cations is given twice; Fe3+ is given twice without any comment in the footnote. This is only a small example related to Table 2. A similar concentration of incorrect terms and mistakes is overall the whole manuscript.

 The authors state that the Pb-rich phase was formed as a result of cation exchanges and losses involving mainly A-site cations. Howeever, chevkinite does not have microporous structure. Its structure is dense and substitution of Ca and REE for Pb and U could be possible only after the complete decomposition of the initial chevkinite structure. Ion exchange is impossible in chevkinite. I wrote this in the previous review, but these comments have been ignored by the authors.

 Furthermore, the authors do not provide sufficient substantiation of their assumption that the Pb-Ti-Fe-Si phase substitutes chevkinite rather that it was growing separately or around presumed chevkinite grains. Figure 5 is insufficiently convincing. More data on spAcial relationships between the REE and Pb,U phases should be provided. Alternative hypotheses on the formation and nature of the Pb-Ti-Fe-Si phase must be discussed.

 The authors should check whether the formulae of presumed chevkinite-type phases are charge-balanced.

 The title does not exactly correspond to the content of the paper because it provides also chemical data for bastnaesite and aeschynite.

 The paper cannot be published in this form. I order to be published, it must be completely rewritten and resubmitted as a new manuscript.