Shale Mineralogy Analysis Method: Quantitative Correction of Minerals Using QEMSCAN Based on MAPS Technology

Round 1

Reviewer 1 Report

Very interesting and useful article. Some little comments:

Line 38. What about XRF analysis? It is not included. XRF is a commun technique used in ore and mineral elemental analysis. Togheter XRD or SEM, precise mineralogical reconstruction can be made. I think it is important to include it and also discuss advantages and disadvantages.

Line 76. Any additional comments on the response times of these techniques and the level of training required, with respect to the other methods? 

Results and discusion. The precision of the results obtained is not clear. Hor do you corroborate or validate the results? What other technique was used to compare and validate the mineralogical composition obtained by QEMSCAN-MAP?

Author Response

• Line 38. What about XRF analysis? It is not included. XRF is a commun technique used in ore and mineral elemental analysis. Togheter XRD or SEM, precise mineralogical reconstruction can be made. I think it is important to include it and also discuss advantages and disadvantages.

Reply: Thanks for your suggestion, XRF should be introduced in this paper. We have added comments on XRF in the introduction. X-ray fluorescence spectrometry (XRF) is based on the principle that under X-ray excitation, the inner electrons of the atoms of the element undergo an energy level jump and emit secondary X-rays (X-fluorescence). Different elements have characteristic X-ray spectra with different wavelengths, and the fluorescence intensity of each spectral line is related to the concentration of the element. Based on the wavelength and intensity of the characteristic X-ray spectra, qualitative and quantitative analysis of elements can be performed. The detection limit of XRF is up to 1PPM, which means the trace and ultratrace elements can be detected. XRF has the advantages of fast analysis, measurement of many elements, and the ability to measure multiple elements at the same time. However, the quantitative analysis of XRF has an error of 5-10%. In addition, the conversion of elemental content to mineral content is subject to error and requires a large number of standard samples for calibration.

• Line 76. Any additional comments on the response times of these techniques and the level of training required, with respect to the other methods? 

Reply: Thanks for your suggestion. We briefly summarized the response time and training requirements of these techniques. We supplemented this information in the introduction. Rock petrographic analysis, such as microscopic observation, is the most time-consuming and requires a long period of training and practice in order to achieve eligibility. The newest XRD, FESEM, FIB-SEM, XRF, QEMSCAN and MPAS are usually separately equipped with data processing software systems, making them relatively easy to use. Regardless of the sample preparation process, the measurement response time (per unit volume or area)of XRD, FESEM and XRF is relatively shorter, while FIB-SEM, QEMSCAN and MPAS has relatively longer measurement response time because of high resolution. Due to their different use purposes, it is difficult to compare the training level requirements. But there is reason to believe that as computer-assisted technology continues to improve, these devices will become less difficult to operate.

• Results and discusion. The precision of the results obtained is not clear. Hor do you corroborate or validate the results? What other technique was used to compare and validate the mineralogical composition obtained by QEMSCAN-MAP?

Reply: We have added a subsection (5.2) that discusses how to use XRD to corroborate or validate the results. The results after correction and normalization for mineral content, were compared with XRD test data. Image correction did not result in significant changes in the major mineral contents. Overall, the clay mineral and non-clay mineral contents identified by the QEMSCAN technique were comparable to the XRD results.

Reviewer 2 Report

In order to improve the quality of the article, I would like to suggest that the following points be included:

1. BSE detector specification - it is important that the BSE detector specification is included to enable reproduction of the data by other researchers. It is known that machine configurations vary in ThermoFEI, so it would be interesting to present this specification in the methodology.

2. Influence of the FEGSEM tip life - as the tip crystal is used, there is significant current stability variation in these machine models. Regular QEMSCAN users have this current stabilization issue vs. grayscale of the BSE image. Therefore, it is important for the authors to mention how this can be mitigated or tackled during MAPS analyses.

3. Border features vs. Large samples (large stages) - How is possible stage misalignment handled during multiple sweeps for a large sample? In micrometric scale analyses, this problem is imperceptible. But on the scale of structures compatible with the sizes of clay minerals, is this misalignment serious?

Author Response

• BSE detector specification - it is important that the BSE detector specification is included to enable reproduction of the data by other researchers. It is known that machine configurations vary in ThermoFEI, so it would be interesting to present this specification in the methodology.

Reply: The BSE detector specification we used is T1 in-lens backscatter detector, which is positioned close to the sample to collect the maximum amount of signal. Unlike other backscatter detectors, this fast detector makes materials contrast available at all times, even when navigating, while tilted, or at short working distances.

• Influence of the FEGSEM tip life - as the tip crystal is used, there is significant current stability variation in these machine models. Regular QEMSCAN users have this current stabilization issue vs. grayscale of the BSE image. Therefore, it is important for the authors to mention how this can be mitigated or tackled during MAPS analyses.

Reply: The device we used for MPAS is equipped with multiple detectors. Different detectors are used to detect different signals into the image. Therefore, this current stabilization issue vs. grayscale of the BSE image is avoided to some extent.

• Border features vs. Large samples (large stages) - How is possible stage misalignment handled during multiple sweeps for a large sample? In micrometric scale analyses, this problem is imperceptible. But on the scale of structures compatible with the sizes of clay minerals, is this misalignment serious?

Reply: The device we used for MAPS has SmartAlign Technology and FLASH Technology FEI introduced on a workhorse SEM platform. SmartAlign Technology virtually removes the need for user alignments. FLASH Technology enables automatic image fine tuning, undo, user guidance, maps tiling, and stitching. On the scale of structures compatible with the sizes of clay minerals, our MPAS images show almost no misalignment.

Reviewer 3 Report

Comments/Suggestions

p.72-74. Do you mean “overcome” instead of “navigated”. Please rephrase to clarify.

p.75-77 “…to realize quantification and refined characterization of shale minerals”  Do you mean to improve quantification?

p.83-84 “…and cross-sections of QEMSCAN and MAPS scans were developed for vertical bedding/stratification.” Should this be referred here? In fact they are materials, but maybe should this be referred in 2.2.2 as input for scanning?

Author Response

• 72-74. Do you mean “overcome” instead of “navigated”. Please rephrase to clarify.

Reply: Thank you for your good questions, which really helped improve the quality of our paper. We replaced “navigated” with “overcome” to make the paragraph more accurate.

• 75-77 “…to realize quantification and refined characterization of shale minerals”. Do you mean to improve quantification?

Reply: Yes, we improved the quantification of shale minerals. This sentence has been rewritten as “…to realize an improvement in the quantitative analysis and refined characterization of shale minerals.”

• 83-84 “…and cross-sections of QEMSCAN and MAPS scans were developed for vertical bedding/stratification.” Should this be referred here? In fact they are materials, but maybe should this be referred in 2.2.2 as input for scanning?

Reply: Thanks for your suggestion. We've repositioned this sentence because it's more appropriate for 2.2.3. We add XRD method in 2.2.2.