Petrographic Analysis of Mafic and Ultramafic Rocks in Northern Thailand: Implications for CO₂ Mineralization and Enhanced Rock Weathering Approach

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Title - appropriate. However, there are repeated keywords used in the title of the work. It is suggested that the authors avoid repeated terms.

As for the topic addressed, it is a very current and pertinent subject, considering the entire theme involving global climate change and carbon sequestration. It is up to the authors to make the importance of the study more robust and explicit, inducing more attention and interest from the reader.

The abstract is coherent, presents the subject in a broader and more general way, following the objectives and results obtained in the research.

In section 2, the geological approach in which the studied rocks are inserted is well contextualized, exploring the characteristics of the rocks and a good amount of references to support the subject. Figure 1 presents maps that detail the location of the units that contemplate the context of the studied rocks. However, the detailed location of the units that contain the mafic and ultramafic rocks of interest (Figure 1, right) has very poor resolution and it is not up to the authors to induce the reader to access a repository of geospatial information to better understand the geological distribution of the materials researched in the work. This can even be maintained, but it is recommended that the authors modify this figure to provide more detail and clarity in visualizing the location of the study materials.

Regarding the methodology, it is very well described and detailed. This is a strong point of the manuscript. Section 3.1 includes the sampling procedures, description of the locations and geology of each lithology selected in the research. Figure 2 has a very adequate resolution and observation scale. Sections 3.2 and 3.3 are well written, allowing for a good reading flow of the text. It is suggested that the authors evaluate the possibility of including an additional figure with the equipment used.

In the results, sections 4.1 to 4.6, the authors present detailed mineralogical and textural aspects and information of the 6 rocks studied (and their varieties) in a very articulate manner, with excellent quality microphotographs. A deficiency found in the results of petrographic analyses is the way in which the modal composition of the main mineral phases that make up each rock is presented. For example, ln 170, change from "...8.71 modal%..." to "...8.71 modal%...". And so on for all other citations, up to ln 501, unless the reviewer is mistaken. It is also suggested that the authors evaluate the possibility of presenting the diffractograms of greatest interest to the study, in addition to the data presented in Table 2.

The discussion of the results and conclusions are very well presented and convergent with the objectives of the work. However, it is suggested that the authors open a comparison with results from other international research with similar rocks, removing the regional character of the manuscript presented here. For example, in Brazil, a country of continental dimensions and with several important agricultural frontiers, the subject has been intensively studied and with many interesting works available in the literature.

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "Petrographic analysis of mafic and ultramafic rocks in Northern Thailand: Implications for carbon sequestration by enhanced rock weathering" by Tadsyda Taksavasu is a well written petrographic description of some mafic and ultramafic igneous rocks from Northern Thailand. The author proposes the hypothesis that some of these rocks may be good candidates for use as carbon sequestration purposes and that "Reducing the rock size to be equal to the average size of the reactive minerals could be considered as one of practical designs in enhanced rock weathering activities.  This is an interesting hypothesis but absolutely no evidence, either experimental or theoretical, in support of the hypothesis is provided. Good petrographic descriptions of the various rocks are provided. These descriptions could, however, be summarized in table form allowing a roughly 50% reduction in manuscript length. The manuscript describes the proposed enhanced rock weathering (ERW) technique as simply "... grinding the rocks into finely grained powders and spreading them on the lands, coasts, or in the oceans". No word of caution is provided pertaining to the potentially disastrous consequences of spreading asbestos bearing rock powders across the land. And most of the described rocks contain significant quantities of asbestos. Unfortunately, most ultramafic rocks, including the Northern Thailand rocks, are at least partially altered, and the most typical alteration product is serpentine. Chrysotile serpentine is almost always present (as is the case with the described rocks) in minor to major quantities together with minor fibrous amphiboles such as tremolite/actinolite. Chrysotile is asbestos by definition and is regulated for good reasons. A few line-by-line questions and comments follow:

Line 3 – The title is misleading. The phrase "enhanced rock weathering" implies that the manuscript includes some evidence pertaining to the subject.  

Line 19 – "could be considered" but is it? Absolutely no experimental evidence is presented.

Lines 77- 125 – The introduction is well written and encouraging but the Geologic Background and the Material and Methods sections are disappointing. The rock units (DCv, Cb, Cpu, etc.) are given ambiguous lithologic descriptions in table 1 and nothing about the age of the rocks is provided. Cenozoic peridotites and basalts are typically much different than Precambrian rocks.  Instead of unmapped "Sample Sites", sample numbers should be mapped.

Line 150 – Replace "blinded" with blended.

Results section  – The petrography is well written with the exception of a few terms that some readers may not be familiar with such as "oikocrysts" (on lines 232 and 237); "bowlingite" line 236; intercumulus line 267 and nematoblastic, line 304.  Please provide brief definitions in the text.

Lines 400-401 – Why and how does the author distinguish between secondary minerals and alteration products? It does not seem to be a useful distinction.

Line 407 – Fibrous pale green amphibole is probably tremolite; actinolite is distinctly dark green. It should be noted that both minerals are commonly asbestiform.

Figure 7c and 7d  both look like there is abundant chrysotile. Figure 7c Antigorite serpentine is characterized by "mesh texture".

Line 469 – If the serpentine is fibrous it is very likely chrysotile. Lizardite displays a " ribbon texture" and is very common.  Is the author sure there is no lizardite?

Line 470 – How old it the tuff?  The glass shards of tuff devitrophy with age.

Line 493 – Are there any calcite amygdule? 

Line 541 – Again; I would expect at least some lizardite.

Table 2 – Five of the ten listed rocks contain major to moderate amounts chrysotile.  They should not even be considered as good candidates for ERW carbon sequestration projects. It is likely that some of the remaining five rocks contain at least trace amounts of either chrysotile or tremolite and should be carefully examined for asbestos content.

Line 561 – Explain "that could be resulted in opposite direction."

Line 623 – Serpentine group minerals are a poor choice.

Line 639 – The author seems to ignore the distribution of chromite, magnetite, and ilmenite which clearly must occur in most of the described rocks.  But chromite can also be a pollution problem.

Line 725 – The author seems to ignore the importance of permeability on weathering rate. 

Lines 737 -743 This is a plausible hypothesis but needs experimental verification.  Grinding coarse grain olivine will also increase the reactive surface area. The proposed concept is not clearly stated.

Lines 742-742. The crushability of the rock is also a factor. Crushing typical mafic rock is easier than crushing typical meta-quartzite or fresh, unweathered granitic rock.  I have crushed hundreds of samples of all rock types for chemical analysis and find ultramafic rocks are among the least difficult; typically because of their soft chlorite or serpentine content; but even fresh dunite is not difficult. Grain size does not seem to be a major factor; but this is just speculation; I have not seen any pertinent scientific data.         

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Interesting research that leads to the practical application of CO₂ uptake for carbon sequestration by CO₂ mineralization. In further work, the practical use of 0/2 fractions, partly found as by-products in quarries, should also be considered.

line 200	Please check the dimensions of the Cpx. Figure 3a shows that they are up to 0.4 mm, not 0.1mm.
line 571	Will the resulting carbonate mineral precipitation be a waste material? How will it be treated from an environmental perspective?
line 726	Do you think the required grain size of minerals is drawn from the 0/2 mm fraction, as a common fraction of igneous rocks in quarries?
line 734	Can by-products from ultramafic rock quarries be used for this purpose in practical applications?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is ready for publication.

Author Response

Thank you so much, I highly appreciate Reviewer 1 for the recommendation and all previous comments and suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

The revised version of the manuscript originally titled "Petrographic analysis of mafic and ultramafic rocks in Northern Thailand: Implications for CO2 Mineralization and Enhanced Rock Weathering Approach" by Tadsyda Taksavasu is little changed from the original draft. Some of the question and suggestions that I made were complied with but others were ignored.  The length of the manuscript is still much longer than necessary.  Some of the mineral content data is summarized in Table 2 that is  based on X-ray diffraction determinations, but the column labeled "Trace <2%" is empty because powder XRD technology is not designed to pick out trace mineral peaks from all the ambiguous minor peaks of the major minerals. This is confirmed by reviewing the hundreds of overlapping secondary peaks submitted in the supplemental file.  Important minerals like chromite, magnetite, ilmenite, and apatite are not listed but are probably present in trace amounts. Instead a data table needs to be compiled that is based on the thin-section observations made by the author. There are only 10 samples, so a table presenting thin section observations would help summarize the project. It would also be useful to assess the asbestos content of each of the ten samples to determine if any are free of asbestos. Otherwise it would be potentially disastrous to grind the asbestos bearing rocks "...  into finely grained powders and spreading them on the lands, coasts, or in the oceans".

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

200	Please check the dimensions of the Cpx. Figure 3a shows that they are up to 0.4 mm, not 0.1mm.	OK
571	Will the resulting carbonate mineral precipitation be a waste material? How will it be treated from an environmental perspective?	OK (720-730)
726	Do you think the required grain size of minerals is drawn from the 0/2 mm fraction, as a common fraction of igneous rocks in quarries?	OK (752-757)
734	Can by-products from ultramafic rock quarries be used for this purpose in practical applications?	OK 609

Comments on the Quality of English Language

You answered the questions in detail. These are frontier studies concerning geology, technology, and the environment. The proposed products' applications can help solve the global problem of increasing CO2. I enjoyed reading the paper. As a geologist in the construction industry, I am familiar with manufacturers' difficulties with by-products. This applied research should be further funded to bring some products to market. Good luck with your future work. 

If it's not too late, you can include some answers to these questions in your paper as part of your discussion. I'm interested in hearing your thoughts. You could also add the time rate of conversion to carbonates.

Author Response

Thank you so much for your support and encouragement for this work! I really enjoyed answering your insightful questions. All of your questions are crucial to be considered for the next moving steps and definitely lead to many more interesting topics we need to study. I would take this opportunity to express my gratitude to Reviewer 3 for the insightful review of this manuscript and for providing helpful suggestions.

In addition, I agree to include a brief detail regarding the initial idea of using byproducts of mafic-ultramafic quarries for mineral CO₂ sequestration since they are already in fine-grained sizes. The detailed descriptions have accordingly been added in Lines 781-788 on page 22 as presented in blue highlighted. However, the prediction and details of carbonate (and silica) byproducts and the time rates (or in terms of reaction rates) to convert the CO₂-reactive materials to the carbonates (and silica) might not be able to be included and specific in this manuscript due to their variations depending on various factors, including environmental conditions, treatments, and CO₂ concentration. It would be practical to be consistent in the mineralogical and textural characteristics of the rocks for the very first step to predicting rock behaviors with CO₂ and assessing their mineral CO₂ sequestration potential. Further experimental studies to prove these concepts and to test the possible rates of carbonate precipitation, or even the initial mineral dissolution, are strongly considered for the following steps as suggested by the reviewer.

Again, Thank you very much for your support of this work! I definitely highly appreciate all of your suggestions and comments for the manuscript improvement.