Near-Infrared Spectroscopic Study of OH Stretching Modes in Kaolinite and Dickite

Round 1

Reviewer 1 Report

The MS presents results of a study of a number of kaolinite group mineral samples containing kaolinite and dickite polymorphs, identified by IR and NIR spectroscopy. The MS is generally well written and results given could be of interest for people working in the field.

However there a number of points which, in my opinion, should be addressed and revised. Some of them: 

The readers might not know the meaning of the hardness terms RI and SG in Table 1. Please specify.

It would be helpful to have a description of the crystal structure of the two polymorphs.

Rather than stating “collected by using transmission mode, KBr compression” it would be better to say: “collected in transmission mode in a pellet / matrix of KBr”

The Authors state “Raman spectroscopy can detect the vibrations that do not involve a dipole moment change, and it can allow an evaluation of the vibration types that are difficult to detect by infrared.” Strictly speaking, the statement is not correct. A certain vibration is not Raman active because it does not involve dipole-moment change. It is because different selection rules apply.

The Authors make many of their assignments on the basis of what they call “theoretical calculations”, I assume from literature data in the references given  (where however I could not find the calculations). I would suggest to give some more details. Also, the expression “theoretical calculations” should be avoided.

I cannot understand the sentence “Due to the non-ideality of the structure and composition, the average factor between the OH stretching vibration and its overtone is 1.95 (±0.003);, it is not twice as high, as suggested by the theoretical calculation”. Are the Authors meaning that system does not behave harmonically? Are they expecting the energy of the overtone to be twice the energy of the fundamental mode? What the theoretical calculation is, in this case? I know there has been some discussions about the anharmonicity of the OH stretching in Clays and  Kaolinites (Phys Chem Minerals (2004) 31: 585–592, and Ref. 21 of the MS), but proper expressions should be used or Figure 9 becomes meaningless.

Author Response

• The readers might not know the meaning of the hardness terms RI and SG in Table 1. Please specify.

All the abbreviations have been explained (NIR, MIR, RI, SG).

• It would be helpful to have a description of the crystal structure of the two polymorphs.

Thank you for your suggestion. At first I thought about describing the structure, but later I decided to remove it, because the focus of this manuscript is spectral difference and OH assignment. In the second paragraph of the INTRODUCTION, I mentioned that kaolinite group minerals have internal and external OH groups, and the three inter-surface OH groups in octahedral sheets are not equivalent. This is related to the theme. I think too many structural descriptions will distract readers' attention.

• Rather than stating “collected by using transmission mode, KBr compression” it would be better to say: “collected in transmission mode in a pellet / matrix of KBr”

Thank you for your more appropriate description, I have revised it.

• The Authors state “Raman spectroscopy can detect the vibrations that do not involve a dipole moment change, and it can allow an evaluation of the vibration types that are difficult to detect by infrared.” Strictly speaking, the statement is not correct. A certain vibration is not Raman active because it does not involve dipole-moment change. It is because different selection rules apply.

Changed to: “Raman spectra can detect the vibrations of nonpolar groups that cause the change of polarizability, these vibrations may be non-infrared active. So it can be a supplement of the infrared spectra.” (And an additional explanation was added to the front)

• The Authors make many of their assignments on the basis of what they call “theoretical calculations”, I assume from literature data in the references given (where however I could not find the calculations). I would suggest to give some more details. Also, the expression “theoretical calculations” should be avoided.

(This question is the same place referred in the next comment, and I will answer it below)

• I cannot understand the sentence “Due to the non-ideality of the structure and composition, the average factor between the OH stretching vibration and its overtone is 1.95 (±0.003);, it is not twice as high, as suggested by the theoretical calculation”. Are the Authors meaning that system does not behave harmonically? Are they expecting the energy of the overtone to be twice the energy of the fundamental mode? What the theoretical calculation is, in this case? I know there has been some discussions about the anharmonicity of the OH stretching in Clays and Kaolinites (Phys Chem Minerals (2004) 31: 585–592, and Ref. 21 of the MS), but proper expressions should be used or Figure 9 becomes meaningless.

My unclear description makes your understanding complicated. I just want to express that "the average factor is not exactly twice, but less" (Ref. 21 mentioned： "The vibrational energy of the overtone band is not simply twice the original band, but is somewhat less as explained by quantum mechanics."). For a more accurate description, I changed to "the average factor between the oh stressing vibration and its overtone is 1.95 (± 0.003), not simply two but is something less as explained by quantum mechanics [32]."

Petit (2004) is very valuable for my conclusion. I have compared this article with my discussion and added it to the references. Thank you for your suggestions.

*Petit, S.; Decarreau, A.; Martin, F. Refined relationship between the position of the fundamental OH stretching and the first overtones for clays. Physics and Chemistry of Minerals, 2005, 31(9): 585-592.

Reviewer 2 Report

The article presents the results of a comprehensive study of kaolinite and dickite using modern equipment (X-ray powder diffractometer, the Bruker Tensor II spectrometer and the Horiba LabRAM HR-Evolution laser Raman spectrometer). In addition, the authors planned to develop a methodology for assessing the differences in the near-infrared spectra of kaolinite and dickite, which can thus realize the rapid identification of the two minerals.  The interpretations of data obtained from the tests seem to have been done well; also the article is well compiled and illustrated. So, the paper adds valuable new information and can recommend for publication in Crystals but with some corrections (see comments below).

Comments. The abbreviations (e.g., NIR= near-infrared region (NIR)) must be deciphered at the first mention in the main text. For phyllosilicates it is more correct to use the term polytypes instead of polymorphs.

Author Response

• Comments. The abbreviations (e.g., NIR= near-infrared region (NIR)) must be deciphered at the first mention in the main text. For phyllosilicates it is more correct to use the term polytypes instead of polymorphs.

Thank you for your valuable advice, I’ve made clear the difference between polytypes and polymorphs.

All the abbreviations have been explained (NIR, MIR, RI, SG). Three polymorphs have been replaced by polytypes.

Reviewer 3 Report

In my opinion, the manuscript is essentially ready for publications.

However some points should be fixed:

1.  Section 2.1 - provide source of samples. Are they natural or obtained artificially? If natural, then the locality of origin should be given [like Langban (Vermland, Sweden)].

2. line 117 "PDF cards of XX minerals from ICDD(?) database" instead of "standard XRD patterns".

3. You should provide possible assignment of all bands in Table 2 (currently there are 6 bands assigned as unknown). You can provide different variants (XX and/or XX) or this can be a combination. However you have to show your suggestion regarding this "unknown" bands. Otherwise such an assignment devalues your work.

4, line 200 - accept edit.

5. line 212 what does "7000 +" means? Paraphrase.

6. Caption to figure 9 ".... vibrations of OH: (a) ...., (b)"

see pdf for easier navigation.

Comments for author File: Comments.pdf

Author Response

• Section 2.1 - provide source of samples. Are they natural or obtained artificially? If natural, then the locality of origin should be given [like Langban (Vermland, Sweden)].

Thank you for your reminding, I have added the missing parts.

• line 117 "PDF cards of XX minerals from ICDD(?) database" instead of "standard XRD patterns".

I have corrected it, thanks for your correction.

• You should provide possible assignment of all bands in Table 2 (currently there are 6 bands assigned as unknown). You can provide different variants (XX and/or XX) or this can be a combination. However you have to show your suggestion regarding this "unknown" bands. Otherwise such an assignment devalues your work.

I looked up some papers and pointed out the assignment of unknown bands. The useful papers have been added to references.

*Johansson, U.; Frost, R, L.; Forsling, W.; Kloprogge, J, T. Raman Spectroscopy of the Kaolinite Hydroxyls at 77 K. Applied Spectroscopy, 1998, 52(10): 1277-1282.

*Johnston, C.T.; Helsen, J.; Schoonheydt, R.A.; Bish, D.L.; Agnew, S.F. Single-crystal Raman spectroscopic study of dickite. American Mineralogist, 1998, 83(1-2): 75-84.

*Frost, R.L.; Tran, T.H.; Rintoul, L.; Kristof, J. Raman microscopy of dickite, kaolinite and their intercalates. The Analyst, 1998, 123(4), 611–616.

• line 200 - accept edit.

I have accepted, and supplemented this part according to the opinions of other reviewers.

• line 212 what does "7000 +" means? Paraphrase.

Sorry, this is the content when drafting, I have modified it to: above 7000 cm^-1.

• Caption to figure 9 ".... vibrations of OH: (a) ...., (b)"

thanks for your correction. Modification accepted.