Synchysite-(Ce) from Cinquevalli (Trento, Italy): Stacking Disorder and the Polytypism of (Ca,REE)-Fluorcarbonates
Round 1
Reviewer 1 Report
Nice presentation of polysomatism and polytypism in the fluorocarbonate synchysite completing the work started with the description of the same aspects in parasite.
I corrected some spelling errors and made some wording suggestion concerning, the latter are to take or to leave. Both are contained in the annotated and commented version of the pdf-file of the paper Below are some comments concerning some scientific issues.
Introduction: The author starts with presenting the polysomatic (Ca,Ree)-fluorocarbonate polysomatic series using the most popular BS notation e.g. based on bastnäsite and synchisite layers. He indicates, however, that he will for obvious reasons use notation based on vaterite and bastnäsite layers e.g. the VB notation. I would add a sentence showing the relationship between both notations e.g B in (BS) remains B in (VB) notation, S in (BS) notation becomes VB in (VB) notation. V alone has no equivalent in (BS) notation. Example: the sequence BSS becomes BVBVB.
line 101: Did you calibrate the K-values of your system or did you take the values provided by Oxford?
line 116 and following: Rwp ad chi2 of the refinements? What about preferred orientation of the (Ca,REE)-fluorocarbonates?
line 164: The BSE contrast of the darker fine rim could also be due to the average atomic of the anion in this zone which may be OH instead of oxygen, since BSE contrast is mainly depending on average atomic number of a compound and not density
line 216: I do not understand the last part of the sentence "...and retain metastable."
line 249: "ii any polytypic..." you mean that any polytypic disorder, if present, can only have a component along the observation direction?
line 254: If you take the simulated contrast, not only carbonate groups are resolved, but also the the Ce and Ca atoms are resolved as dark spots. In parts of the filtered image 7c the resolution is good enough to see these spots,but I agree with you that in unfiltered images the Ca and Ce atoms are not resolved an show up as continuous fringes.
line 255: you describe the contrast of the CeF and Ca layers as "dark bands". Further down you, however, indicate the same layers in the HRTEM image as "fringes". I would call them either or, I would prefer "fringe" over "band"
line 256: "laterally offset": this is ambiguous, better "the average positions of carbon atoms is offset along a* across the Ca-layers whereas no shift is seen across a CeF-layers"
line 275: "Lattice fringe distance": usually such phrasing indicates next nearest fringes, in the present case, if you address the contrast of CeF and Ca layers as (dark) fringes, the distance you mention e.g. 14.1A is the distance between four fringes, the normal 18.3A periodicity is the distance between every fifth fringe. As shown in your simulation, the fringes corresponding to CeF layers are slightly wider . In the slightly misaligned Figure 8, specially in the thicker area at the bottom, each fifth fringe is darker can be picked up . In this part of the image the c axis periodicity corresponds to the distance between the darker fringes .
line 292: What means "dense compositional...."?
line 294: I would better explain the how the contrast in the misaligned figure 9 relates to the synchysite structure and that the faults despite the different appearances are all of the same naturee.g."The contrast in the HRTEM image shown in figure 9 is characterised by rows of bright spots 9.1 Ang apart e,g, corresponding to the width of a synchysite half cell. In several places, the distances between these rows are different from the 9.1 periodicity, indicating the presence of compositional faults. The fault in the center, closer to the boundary with bastnäsite is characterised by a larger distance between neighbouring bright fringes e.g. 14.1 Ang corresponding to a s additional B layer introduced in the regular VBVB sequence of synchysite e.g.corresponding to a parasite half cell The bright fringes characterising the faults in the upper part of the image are only 5Ang apart, which corresponds to the width of a B layer. Despite the difference in contrast, also these faults correspond to additional B layer in the regular VBVB sequence of synchysite. The difference in appearance is probably due to differences in thickness and/or orientation.
line 301: I would indicate by a subscript that [100] direction refers to synchysite.
line 315: I would explain shortly the cause of ploytypic disorder and how it is seen in HRTEM images. e.g. "Polytypism and polytypic disorder in florocarbonates arises through the different possibilities to link the vaterite layers with the adjacent bastnäsite layers (s. below), which are revealed [110] HREM images by the relative positions of the bright spots in adjacent synchysite half cells. Taking the bright spots as corners of a half cell, the different stacking possibilities can lead to three different geometries of the half cells: a rectangular cell (labeled 0 in Fig. 10) , and two oblique cell, one with a left (+), the other with a right slant (-)
line 346: Did you observe pinite, or id it the most likely explanation for the presence of muscovite peaks in the XPRD pattern?
line 358:For such low concentrations, considering that you have also preferred orientation problems with mica and most likely also with the fibrous fluorcarbonates, the uncertainty is larger than the observed difference between the two concentration. I would keep the phase determination, the lattice refinements of the fluorocarbonates, the concentrations of the phases in the result chapter, but not comment the difference in the fluorocarbonate concentrations e.g. I would skip the paragraph.
line 437: I would write in the first row: Mica polytype symbols to make it clear that although the stacking sequence in micas and fluorocarbonates can be compared, the layer periodicity, which is taken into account the entire mica polytype structure, is only valid for the carbonate layers in the fluorocarbonates.
Comments for author File: 
Comments.pdf
Author Response
Nice presentation of polysomatism and polytypism in the fluorocarbonate synchysite completing the work started with the description of the same aspects in parasite.
I corrected some spelling errors and made some wording suggestion concerning, the latter are to take or to leave. Both are contained in the annotated and commented version of the pdf-file of the paper Below are some comments concerning some scientific issues.
Introduction: The author starts with presenting the polysomatic (Ca,Ree)-fluorocarbonate polysomatic series using the most popular BS notation e.g. based on bastnäsite and synchisite layers. He indicates, however, that he will for obvious reasons use notation based on vaterite and bastnäsite layers e.g. the VB notation. I would add a sentence showing the relationship between both notations e.g B in (BS) remains B in (VB) notation, S in (BS) notation becomes VB in (VB) notation. V alone has no equivalent in (BS) notation. Example: the sequence BSS becomes BVBVB.
R: That sentence has been added to the introduction.
line 101: Did you calibrate the K-values of your system or did you take the values provided by Oxford?
R: As stated, we used a standardless method, therefore theoretical k-factors encoded in the analytical system.
line 116 and following: Rwp ad chi2 of the refinements? What about preferred orientation of the (Ca,REE)-fluorocarbonates?
R: wRp = 0.1122 and Chi^2 = 6.703. These values, along with Rp (0.0818) has been added to the text. As observed in TEM studies, where sample are prepared by grinding and wet deposition, there is not a marked preferential orientation, since all the principal orientations could be found in different grains with similar chances, with minor tilt adjustment.
line 164: The BSE contrast of the darker fine rim could also be due to the average atomic of the anion in this zone which may be OH instead of oxygen, since BSE contrast is mainly depending on average atomic number of a compound and not density
R: Average atomic number and density are, of course, correlated. Hydroxyl bearing compounds are usually less dense than their dry counterparts are. The sentence has been modified according to the referee suggestion.
line 216: I do not understand the last part of the sentence "...and retain metastable."
R: The sentence has been modified as it follows: ”… and considered improbable because metastable.”
line 249: "ii any polytypic..." you mean that any polytypic disorder, if present, can only have a component along the observation direction?
R: Yes, text modified accordingly.
line 254: If you take the simulated contrast, not only carbonate groups are resolved, but also the the Ce and Ca atoms are resolved as dark spots. In parts of the filtered image 7c the resolution is good enough to see these spots,but I agree with you that in unfiltered images the Ca and Ce atoms are not resolved an show up as continuous fringes.
R: Yes, figure 7 is OK, Ca and Ce can be distinguished by the different alignment of the adjoining white dots, but in other images such distinction was not so easy because of sample misalignment or beam damage…
line 255: you describe the contrast of the CeF and Ca layers as "dark bands". Further down you, however, indicate the same layers in the HRTEM image as "fringes". I would call them either or, I would prefer "fringe" over "band"
R: Text modified accordingly.
line 256: "laterally offset": this is ambiguous, better "the average positions of carbon atoms is offset along a* across the Ca-layers whereas no shift is seen across a CeF-layers"
R: Text modified accordingly
line 275: "Lattice fringe distance": usually such phrasing indicates next nearest fringes, in the present case, if you address the contrast of CeF and Ca layers as (dark) fringes, the distance you mention e.g. 14.1A is the distance between four fringes, the normal 18.3A periodicity is the distance between every fifth fringe. As shown in your simulation, the fringes corresponding to CeF layers are slightly wider . In the slightly misaligned Figure 8, specially in the thicker area at the bottom, each fifth fringe is darker can be picked up . In this part of the image the c axis periodicity corresponds to the distance between the darker fringes .
R: Ok, text revised according to the referee suggestion.
line 292: What means "dense compositional...."?
R: A higher frequency… I rephrased the text to make it clearer.
line 294: I would better explain the how the contrast in the misaligned figure 9 relates to the synchysite structure and that the faults despite the different appearances are all of the same naturee.g."The contrast in the HRTEM image shown in figure 9 is characterised by rows of bright spots 9.1 Ang apart e,g, corresponding to the width of a synchysite half cell. In several places, the distances between these rows are different from the 9.1 periodicity, indicating the presence of compositional faults. The fault in the center, closer to the boundary with bastnäsite is characterised by a larger distance between neighbouring bright fringes e.g. 14.1 Ang corresponding to a s additional B layer introduced in the regular VBVB sequence of synchysite e.g.corresponding to a parasite half cell The bright fringes characterising the faults in the upper part of the image are only 5Ang apart, which corresponds to the width of a B layer. Despite the difference in contrast, also these faults correspond to additional B layer in the regular VBVB sequence of synchysite. The difference in appearance is probably due to differences in thickness and/or orientation.
R: Thank you for the suggestion. Text modified accordingly.
line 301: I would indicate by a subscript that [100] direction refers to synchysite.
R: I reversed the text to avoid ambiguity.
line 315: I would explain shortly the cause of ploytypic disorder and how it is seen in HRTEM images. e.g. "Polytypism and polytypic disorder in florocarbonates arises through the different possibilities to link the vaterite layers with the adjacent bastnäsite layers (s. below), which are revealed [110] HREM images by the relative positions of the bright spots in adjacent synchysite half cells. Taking the bright spots as corners of a half cell, the different stacking possibilities can lead to three different geometries of the half cells: a rectangular cell (labeled 0 in Fig. 10) , and two oblique cell, one with a left (+), the other with a right slant (-)
R: Thank you for the suggestion. Text modified accordingly.
line 346: Did you observe pinite, or id it the most likely explanation for the presence of muscovite peaks in the XPRD pattern?
R: Pinite alteration was observed affecting K-feldspar at the optical microscope. The inhomogeneous birefringence in figure 2d paralleled by turbid appearance with single nicol (not shown) was attributed to pinite alteration.
line 358:For such low concentrations, considering that you have also preferred orientation problems with mica and most likely also with the fibrous fluorcarbonates, the uncertainty is larger than the observed difference between the two concentration. I would keep the phase determination, the lattice refinements of the fluorocarbonates, the concentrations of the phases in the result chapter, but not comment the difference in the fluorocarbonate concentrations e.g. I would skip the paragraph.
R: Paragraph eliminated.
line 437: I would write in the first row: Mica polytype symbols to make it clear that although the stacking sequence in micas and fluorocarbonates can be compared, the layer periodicity, which is taken into account the entire mica polytype structure, is only valid for the carbonate layers in the fluorocarbonates.
R: The referee is right, there is a difference between micas and fluorcarbonates polytypism that should be captured by the reader, but I don’t not think that modifying the column heading could be of any help… Moreover, the table and the entire paragraph aim to emphasize similarities, not to further stress on the differences. For these reasons, I would prefer to leave the header as it is.
Reviewer 2 Report
The work con be of interest to Minerals readers. It presents a crystallo-chemical study on two fluorcarbonates that are important ore minerals for Rare Earth Minerals. The ideas in the manuscript are well organized and the interest of the topic, the background and the methods are well explained. In general, the results are presented clearly and the conclusions are consistent with the provided results.
Please consider the following suggestions so that, at the authors' discretion, they can improve the paper:
* The title of the paper does not clearly reflect its contents. Stacking Disorder and the Polytypism is probably the core, but it includes other mineralogical and mineralogenetic characterizations of the materials that are not included in the title.
* Given the importance of the structural characterization in this paper, the inclusion of images that support the description of the structures in the introduction section would facilitate the understanding of the work.
* It would be desirable to include some of the diffractograms from which the identification and refinement of cell parameters have been made. This information is more useful than the polarization microscope hand sample images that are provided in the manuscript.
* Figure 3b needs better explanation. Does the analysis correspond to a specific area of the 3a image? In that case, to which one? In the case it is a global analysis, it offers poor information. In order to confirm the identification and the compositional particularities, if any, spectra of each phase separately would be needed.
* Please, check the format of Table 2.
Author Response
The work con be of interest to Minerals readers. It presents a crystallo-chemical study on two fluorcarbonates that are important ore minerals for Rare Earth Minerals. The ideas in the manuscript are well organized and the interest of the topic, the background and the methods are well explained. In general, the results are presented clearly and the conclusions are consistent with the provided results.
Please consider the following suggestions so that, at the authors' discretion, they can improve the paper:
* The title of the paper does not clearly reflect its contents. Stacking Disorder and the Polytypism is probably the core, but it includes other mineralogical and mineralogenetic characterizations of the materials that are not included in the title.
R: I acknowledge the referee for the suggestion, but rightly because the core of the article and the more robust data relate to polytypism, I would like to emphasize this subject on the title.
* Given the importance of the structural characterization in this paper, the inclusion of images that support the description of the structures in the introduction section would facilitate the understanding of the work.
R: I agree with the referee, a figure has been added to the paper as Figure 1.
* It would be desirable to include some of the diffractograms from which the identification and refinement of cell parameters have been made. This information is more useful than the polarization microscope hand sample images that are provided in the manuscript.
R: Actually, since the low concentration of the fluorcarbonates, the related peaks are dominated by those of the main phases, such as quartz, K-feldspar and chalcopyrite, therefore not very illuminating…
* Figure 3b needs better explanation. Does the analysis correspond to a specific area of the 3a image? In that case, to which one? In the case it is a global analysis, it offers poor information. In order to confirm the identification and the compositional particularities, if any, spectra of each phase separately would be needed.
R: the spectrum refers to the bismuth grain imaged in Fig. 3a. The caption has been reworded in order to make it clearer.
* Please, check the format of Table 2.
Format revised, thank you.
