| Wavenumber (cm−1) | Band assignment | Reference |
| 3683, 3684, and 3693 | OH stretch for the Si-O-Si bonds on the lower octahedral surfaces of kaolinite | [23, 24] |
| 3618–3620 | Inner hydroxyl groups between tetrahedral and octahedral kaolinite sheets | [24–26] |
| 3650 | In-phase symmetrical stretch due to kaolinite sheets | [25, 26] |
| 3426 | N-H out-phase stretch due to urea | [27, 28] |
| 3340 | In-phase N-H stretch due to urea | [28, 29] |
| 3256 | OH stretching for H bonding associated with urea-oligomer substitution within kaolinite interlayer spaces | [30, 31] |
| 2970 | CH stretch/dimeric OH stretch associated with DMSO decomposition during calcination to form paraformaldehyde complexes |
[32, 33] |
| 1738 | C=O stretch due to intercalated urea |
[32, 33] |
| 1669 | Free C=O stretch and free NH2 bending | [34, 35] |
| 1587 | C-N stretch due to urea | [34, 35] |
| 1461 | In-phase CH3 bending + scissoring C-N vibrations | [34, 35] |
| 1438 | S=O sulfate ester stretching |
[32, 33] |
| 1365 | S=O sulfonyl stretching |
[32, 33] |
| 1202–1216 | Amorphous SiO2 associated with calcination | [36, 37] |
| 1113–1149 | Apical Si-O stretching | [23, 24, 26, 38] |
| 1047 | Si-O & Si-O-Al stretching due to the presence of DMSO in the kaolinite | [37] |
| 1023–1027 | In-phase Si-O planar stretching | [23, 24, 26, 39] |
| 1001–997 | Out-phase Si-O planar stretching and alternating Si-O & Al-O bonds | [23, 24, 26, 39, 40] |
| 910–900 | OH deformation of inner hydroxyl group due to Al-OH bending vibrations | [23, 24, 39, 41] |
| 788–749 | Al-O-Si asymmetric bending | [23, 24, 26, 39, 41] |
| 777 and 434 | OH stretch associated with a change from octahedral Al3+ in kaolinite to tetrahedral coordination in metakaolinite | [23, 24, 26, 39, 41] |
| 525 |
Si-O-Al bending vibration for the Al3+ in the octahedral configuration | [42] |
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