Vibrational Analysis of Natural Rhodochrosites by Raman Spectroscopy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present some interesting Raman spectra of the well-studied mineral rhodochrosite and then use these spectra to calculate some thermodynamic parameters. The paper is well written but some improvements in the English are required. I have also added a number of comments directly in the manuscript for the authors. Some additional comments are given below.

My principal concern is that the majority of analyses have been conducted on natural specimens. One presumably pure (no analysis is given) manganese carbonate from Sigma-Aldrich is also included but is not specifically referred to in the paper. Some of the natural samples show visible colour variation which indicates compositional changes but these is no chemical analysis given. To rule out the effects of compositional changes, chemical mapping by micro-XRF or electron microprobe is needed.

The authors also claim that there are important variations in intensities of the low wavenimber librational and translational modes. However, it is difficult to judge the magnitude of this variation as no Raman spectra are shown and there is no scale accompanying the coloured maps. he reasons given for this variation are rather speculative and the authors themselves admit that the variations were not observed in the high T experiments. The natural rhodochrosites appear to have some fluorescence/luminescence associated with the Raman spectra which is said to be background corrected. However, no detials of the correction procedure are given. This needs to included. it is possible that the background correction may be causing some of the observed intensity differences. Figure 3 also shows that the intensity differences correlate with different colour zones in the crystals which suggests that the changes are related to compositional variations.

In terms of the variations of the Raman wavenumbers with temperature and pressure, the authors report better fitting results with the use of quadratic fitting procedures. However, the differences are minor compared with the relative error in frequency measurement whjch is not given in the figures. I think it would also be useful to use a peak fitting procedure to improve the accuracy of these measurements. However, having said this, it appears that the fitting procedures had little effect on the calculated thermodynamic parameters. It would be helpful to provide a Table giveing a comparison of the parameters obtained in this study with the other results in the literature. Overall, for publication, there needs to be much more detailed data provided about the chemical compositional mapping of the crystals or simply just use pure chemicals and more reference is needed to the prexisting studies of rhodochrosite and its thermodynamic parameters.

 

 

 

 

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Some sentences are not clear and minor corrections are required.

Author Response

Reviewer 1: “My principal concern is that the majority of analyses have been conducted on natural specimens. One presumably pure (no analysis is given) manganese carbonate from Sigma-Aldrich is also included but is not specifically referred to in the paper. Some of the natural samples show visible colour variation which indicates compositional changes but these is no chemical analysis given. To rule out the effects of compositional changes, chemical mapping by micro-XRF or electron microprobe is needed.”

Authors: We acknowledge the author’s comment, since the chemical analysis reinforces our arguments. As a result, we have included the MnCO₃ Sigma-Aldrich standard data and energy dispersive spectra (EDX) in the supplementary material. It's important to note that the EDX analysis only serves to indicate the possible main composition of the mineral sample and provides a semi-quantitative result. However, for Raman spectroscopy, the result is sufficient to support our discussion.

Reviewer 1: “The authors also claim that there are important variations in intensities of the low wavenumber librational and translational modes. However, it is difficult to judge the magnitude of this variation as no Raman spectra are shown and there is no scale accompanying the coloured maps.”

Authors: We have realized that some additional information needs to be included to clarify the previous discussion. The difference in relative intensity between the T and L vibrational modes is significant enough to cause the inversion of these bands. Therefore, we have included one figure to illustrate this inversion. Although the software does not provide a scale for the colored maps, the purpose of the figures in this case is to demonstrate that there are spectra where the T band is more intense than the L band, and vice versa. The intensity of the bands is represented by the vibrancy of the pixel colour. Dark pixels, appearing black, indicate that the relative intensity of the two bands is very similar, as the red and green colours cancel each other out.

Reviewer 1: “The reasons given for this variation are rather speculative and the authors themselves admit that the variations were not observed in the high T experiments.”

Authors: In this part of the discussion, we are considering two possible explanations for the observation. One of them involves the local heating of the sample due to the laser power. To test this first possibility, Raman measurements were carried out with temperature variations and lower laser power. It was observed that there was no inversion of the relative intensity in these experiments. Therefore, the changes in the relative intensity were attributed to the interaction between radiation and matter.

Reviewer 1: “The natural rhodochrosites appear to have some fluorescence/luminescence associated with the Raman spectra which is said to be background corrected. However, no details of the correction procedure are given. This needs to be included. It is possible that the background correction may be causing some of the observed intensity differences.”

Authors: The OPUS software made corrections on the instrument itself to produce coloured maps. The intensity value used to create the maps was determined by measuring the height of the band after a baseline correction as a straight line within an interval containing the Raman band – 320 to 260 cm^-1 for the L band and 200 to 165 cm^-1 for T band. In the spectra displayed in the manuscript figures, only normalization by the height of the Raman band was applied. It's important to note that the observed background is due to changes in the objective focus region and is not related to fluorescence or luminescence effects. The Raman maps cover relatively large sample areas and even a flat sample shows slight focal changes when using a 50X magnification objective. To address this issue, we have included detailed explanations of how this data was obtained in the manuscript.

Reviewer 1: “Figure 3 also shows that the intensity differences correlate with different colour zones in the crystals which suggests that the changes are related to compositional variations.”

Authors: The EDX data that was included supports that part of the discussion. Also, the mapping doesn't show any spectra that are different from the standard Rhodochrosite Raman spectrum. The spectra used in the mapping in Figure 3 only consider the T and L bands of rhodochrosite. If there are other metals present besides Mn2+, it would significantly change the Raman shift and width, but only at higher concentration values. This information has also been added to the manuscript.

Reviewer 1: “In terms of the variations of the Raman wavenumbers with temperature and pressure, the authors report better fitting results with the use of quadratic fitting procedures. However, the differences are minor compared with the relative error in frequency measurement which is not given in the figures. I think it would also be useful to use a peak fitting procedure to improve the accuracy of these measurements. However, having said this, it appears that the fitting procedures had little effect on the calculated thermodynamic parameters. It would be helpful to provide a Table giving a comparison of the parameters obtained in this study with the other results in the literature.”

Authors: The supplementary material presents a table with previous data comparisons, that we believe will be useful for the discussion. Additionally, we have included more statistical analyses to strengthen our arguments.

Reviewer 1: Overall, for publication, there needs to be much more detailed data provided about the chemical compositional mapping of the crystals or simply just use pure chemicals and more reference is needed to the prexisting studies of rhodochrosite and its thermodynamic parameters.

Authors: We acknowledge all the advice and comments we have received, and we use it to enhance the quality of our work. A few corrections were made based on the comments provided in the manuscript document, the changes are indicated in pink colour.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "Vibrational analysis of natural rhodochrosites by Raman spectroscopy" offers new data on the behavior of carbonate minerals at different temperature and pressure conditions.

The text is well-written, and the references are coherent with the aims of the manuscript.

There are some little considerations to be done:

1. The resolution of the instrumentations used needs to be adequately indicated in the text.
2. Authors should eventually explain if the presence of trace elements in the rhodochrosites could influence the overall investigations.
3. Based on point n. 2, the authors could introduce a chemical analysis of the samples to understand how pure they are.
4. Authors can try to analyze synthetic pure rhodochrosite to evaluate differences with natural rhodochrosite. 

Comments on the Quality of English Language

There are no comments. 

Author Response

Reviewer 2: “The resolution of the instrumentations used needs to be adequately indicated in the text.”

Authors: To improve our methods description more details about it were added to the manuscript. The changes are indicated in pink.

 

Reviewer 2: Authors should eventually explain if the presence of trace elements in the rhodochrosites could influence the overall investigations.

Authors: This discussion was also added to the work. We added some energy dispersive spectra (EDX) of our samples to verify the presence of other metal elements besides Mn²⁺ in supplementary material. The presence of different cations in the mineral sample interferes with the Raman shift and the width of the Raman band and only changes the Raman spectrum at considerable concentrations. That was not observed, in this work, we are discussing the variation of the relative intensity from the same symmetry Raman bands.

 

Reviewer 2: “Based on point n. 2, the authors could introduce a chemical analysis of the samples to understand how pure they are.”

Authors: To endorse the discussion EDX was added in supplementary material.

 

Reviewer 2: “Authors can try to analyze synthetic pure rhodochrosite to evaluate differences with natural rhodochrosite.”

Authors: We agree and data from a Sigma Aldrich MnCO₃ sample was added in supplementary material and in the manuscript results discussion. We acknowledge all the advice we have received, and we use it to enhance the quality of our work. The corrections were made and are highlighted in pink colour.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have carried out a Raman spectroscopic study of natural rhodochrosite at ambient and high T and P. A large part of the paper is devoted to the mapping of variations in the intensities of the translational and librational modes of natural crystals but the speculative conclusions are later dismissed in the high P and T studies. A more sound approach would have been to first determine if these variations were present in a pure synthetic crystal. However, this was not done in the present study as the reference material was impure as indicated by additional Raman bands shown in the supplementary material.

The high P and T studies are a repeat of earlier work by other researchers. The authors show that quadratic fitting of the data gives better results but overall this has an insignificant effect on the calculated thermodynamic parameters. So while this may be worht noting it does not change what is already known about rhodochrosite.

Some other comments are given in the attached copy of the manuscript.

Comments for author File: Comments.pdf

Author Response

Authors reply to the review report – Reviewer 1

Reviewer: The authors have carried out a Raman spectroscopic study of natural rhodochrosite at ambient and high T and P. A large part of the paper is devoted to the mapping of variations in the intensities of the translational and librational modes of natural crystals but the speculative conclusions are later dismissed in the high P and T studies. A more sound approach would have been to first determine if these variations were present in a pure synthetic crystal. However, this was not done in the present study as the reference material was impure as indicated by additional Raman bands shown in the supplementary material.

Authors: It is important to consider the following points: Our goal was to investigate an unusual observation and its potential cause. Initially, it was thought that changes in relative intensities in the crystal orientation may be reasonable. However, when taking into account the symmetry of the vibrational modes in question, it became clear that this was not the case. In a single crystal spectrum, rotating the crystal in different orientations allows for the identification of the symmetries of the vibrational modes. Since the symmetry was the same, it was unexpected to observe a change in relative intensity. We proposed that the interaction with radiation was producing stress in the crystal lattice, affecting the polarizability factor and consequently the intensity of these two bands. The effect of temperature was also considered, as the excitation line could cause local heating in the laser spot region. However, we did not observe variations in relative intensity in spectra obtained under temperature variation and low laser power, as observed in the mappings. Some variation was noted in the pressure spectra. Our conclusion is based on the idea that the stress caused is not linked to physical processes of heating or compression, but rather is caused by an excessive power of the excitation line. Additionally, the size of the crystal seems to influence this process, with larger crystals offering more resistance to higher laser powers. For the synthetic sample, the resistance is minimal, and the extra bands observed indicate the beginning of a degradation process. Notably, the indicative band for the formation of manganese oxide in the spectrum of the synthetic sample differs from that obtained in the spectrum of natural rhodochrosite degraded in the heating experiment. These results also suggest that the degradation process caused by the excitation line differs from the degradation process caused by heating.

Review: The high P and T studies are a repeat of earlier work by other researchers. The authors show that quadratic fitting of the data gives better results but overall this has an insignificant effect on the calculated thermodynamic parameters. So while this may be worth noting it does not change what is already known about rhodochrosite.

Authors: Reviewer is correct, but it is straightforward to understand that this is only a part of the present work, since we are trying to show what are the min features related to the changes in intensity for L and T Raman bands when laser excitation is changed. About the calculated thermodynamic parameters, we do not agree with the reviewer opinion, since we have done a comparative study between the linear and polynomial fitting for rhodochrosite on both temperature and pressure dependences, and clearly our approach is better than we can see in the present literature. We have done an insertion on the last paragraph of the manuscript, trying to clarify all the issues about the understanding of pressure and temperature parameters being important to the discussion. Anyway, we thank to the reviewer for all comments, since they were very important in the attempt to improve the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been improved according to the suggestions made. The text and the conclusion are very clear. According to the new text, I believe the manuscript can be published in Minerals. 

Comments on the Quality of English Language

No comments in this section. 

Author Response

Many thanks for reviewer 2.