Microtextural Characteristics of Ultramafic Rock-Forming Minerals and Their Effects on Carbon Sequestration
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript under review is dedicated to the crucial issue of utilizing greenhouse gases (in particular, CO2) by binding them in stable mineral forms. The authors study the possibility of using ultramafic rocks (wehrlites) from the Ma-Hin Creek region in northern Thailand to capture CO2. The paper describes experiments on interaction of crushed wehrlites with CO2 in an aquatic environment. In result, the authors assessed the prospects for using the studied rocks to capture carbon dioxide as positive.
The manuscript may be of interest to specialists in the field of greenhouse gas utilization, but requires significant revision. Provided below are major comments on the manuscript.
Geological background
Line 109
Figure 1 looks oversimplified. It should be detailed. It would be good to indicate some administrative units in the insets, i.e. countries, cities, regions, etc. The geological map shows no structural elements (faults, rock strike and dip) and intrusive bodies.
It is unclear whether the studied ultramafic rocks are bedrock or found in the form of fragments or pebbles? The fact is that the map does not show the geological bodies of these rocks. What formation do they belong to? Ophiolites? Layered intrusions? Or something else? Next, the authors speak about a creek, which should be also be marked on the map. Its valley is probably composed of Quaternary sediments, they should be indicated on the map either. The authors state that ultramafic rocks appeared in the area in result of the “closure of the Tethys paleo-ocean.” So these are ophiolites? Then why are they absent on the geological map? Further, the authors discuss reverse faults and shifts, but they are also not indicated on the geological map.
Materials and methods
Line 135
“intrusive igneous origins” – interpretation. It should be removed to Discussion
Line 136
Figure 2 is not informative. The image of the samples should be made more readable and it’s better to be placed on a drawing with petrographic images or removed to Supplementary Materials
Line 139
I strongly recommend making polished sections, uncoated, for the SEM-EDS study and determining the composition of minerals.
Line 193
It should be noted that the EDS analysis of unpolished materials provides no quantitative results. In my opinion, it is more correct to study polished preparations before and after the experiment.
Results
Line 198-200
Figure 4 does not correspond to the description given in the text: 1) olivine is intensively replaced by serpentine and it is not possible to assess its belonging to cumulus, 2) pyroxene occurs both in the form of xenomorphic grains (intercumulus) and elongated prisms (cumulus?), 3) images of plagioclase do not show “multiple twinning.” It is necessary to supplement Fig. 4 with more representative photographs that would correspond to the description in the text. For example, Fig. S5 can be used to illustrate cumulus olivine.
Line 225-226
“A cumulate texture that comprises primary and secondary minerals”.
The cumulative texture can only apply to primary igneous mineral assemblages. Please, correct the text.
Lines 207-217
I strongly recommend you to provide more accurate information on minerals, including composition. It can be easily done by using the SEM-EDS method in samples of the original rocks. Mineral composition can have a critical impact on the carbonation process.
Line 244 (Table 1)
How was the volume estimated to determine the density?
Lines 266-280
The discussion of qualitative EDS-spectra for geochemical characterization of products is insufficient. First, polished specimen should be prepared from the products, and then it is possible to obtain quantitative data on the composition of the resulting phases and visualize their structural relationships.
Line 308
This conclusion is doubtful and requires more rigorous proof. An increase in mass is not the evidence of an increase in density. How was the density of the starting materials and products determined? How was the volume of the material determined?
Discussion
The text extensively discusses the absorption of CO2 by minerals, but no single reaction that determines the absorption is provided. It is necessary to give them. It should also be explained how the minerals that maximally absorb CO2, i.e. olivine and serpentine, containing no calcium, produce only calcite and not magnesite.
Line 397-404
It is necessary to make references to the original data source, i.e. those who obtained these results.
The question arises: What reactions determine the “absorption of CO2” by olivine and serpentine? Why didn’t the authors observe the formation of magnesite than? These reactions should be provided and explained.
Line 413
Is olivine replaced by pyroxene? This must be a mistake. Remove pyroxene.
Line 408-411
It is unclear from the text, whether this means a comparison of two definitions for the starting material or for the starting material and the one obtained after interaction with CO2. Please, clarify.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe carbonation tests were performed with only CO2, slightly heated sample. (line 165). The reaction between the materials and CO2 is not very fast, and the reaction time of 60 hours might have been too short to obtain more relevant results.
From literature it is known that either NaHCO3, higher pressure, and/or longer times will be required for obtaining a higher conversion degree. The mass increase of 0.36% o 0.04% are modest. Especially given the high olivine and other reactive components. The article would greatly gain influence if the experiments would be performed both for 60 hours and for > 200 hours.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsDear editor!
The authors responded to most of the comments and made the necessary corrections and additions to the text. In my opinion, this manuscript can be published.
