Syeda Faiza Mahmuda If the clay’s 001 basal peak disappears or becomes a weak, broad hump that does not shift upon glycolation or heating, it signifies that the layered structure lacks coherent stacking and is therefore exfoliated. XRD cannot detect a well-defined 001 in such cases because the separated nanosheets no longer diffract as ordered stacks. This disappearance of a distinct 001 reflection under standard treatments is the clearest evidence that exfoliation has occurred.

Hello

If you are sure that your products are indeed the international reference clays (Swy-3, SAu-1, STX-1B), are they raw products (RP) from the Clay Mineralogical Society (in their original containers) or products that may have undergone exfoliation treatment (EP) ([1])? In the first case, it is certain that the raw products are not exfoliated. In the second case, it is possible to compare the state of the RP and EP using diffractograms acquired on the same diffractometer and under the same analytical conditions (and not just from diffractograms found in the literature).

Exfoliation aims to break down the stacks of layers to achieve a mono-layer state. This has two consequences: an increase in specific surface area (SSA) and a reduction in the number of layers in the stack, i.e., from a crystallographic point of view, a reduction in the scattering domain. For the materials studied (smectites), the increase in SSA can be monitored directly using conventional sorption methods of nitrogen (BET) or organic molecule sorption (EGME) and, indirectly, using methods of cation exchange capacity analysis (NH4 sorption, for example). This allows you to obtain real physical quantities such as the charge per m2, which will enable you to truly compare the efficiency of your products during chemical reactions (such adsorption, catalysis, etc.). By processing the diffractograms of oriented preparations (from stable suspensions), it is also possible to get an idea of the state of the scattering domain (or ‘exfoliation’ state) by using the full width at mid-height (FWHM) of the (001) reflection either after air drying or after saturation with ethylene glycol (EG). The latter option is the best and fastest because it allows you to obtain a homogeneous interplanar distance of 17 Å. In the other case, strictly speaking, homoionisation (e.g. by Ca saturation) would first have to be carried out to obtain a homogeneous interplanar distance of 15 Å. From the measurement of the FWHM of the (001)EG and applying Scherrer's law ([2], or better [3] p. 131), we can calculate the size of the coherent domain and, by dividing this by the interplanar distance value (17 Å), determine the number of layers in the stack, i.e. the ‘exfoliation’ state.

I am enclosing an Excel file that will allow you to perform this type of calculation, unfortunately in French.

(1) Das S., Prateek, Sharma P., Kumar M., Gupta R.K., Sharma H. (2023) - A review on clay exfoliation methods and modifications for CO2. Materials Today Sustainability, 23, 100427. doi.org/10.1016/j.mtsust.2023.100427.

(2) https://en.wikipedia.org/wiki/Scherrer_equation

(3) Brindley G.W., Brown G. (1980) – Crystal structures of clay minerals and their x-ray identification. Mineralogical Society Monograph N°5. Mineralogical Society Publisher, 495 p.

Alvena Shahid , thank you for the answer. However, the sample from which I collected XRD data has never been tried to exfoliate. I have emailed CMS and asked if they can clarify what exact treatments were done to this sample. And, I want to add that I used a Rigaku Miniflex 600, and match4! for the data collection and processing.

Claude Fontaine Thank you for the answer. I have received the three clays in plastic bags, no rigid container (and they were sent by a collaborator, who did not mention any treatment). And CMS have never specified the exact nature of the treatment they have performed. I emailed CMS just a while ago.

And, I want to add that I used a Rigaku Miniflex 600, and match4! for the data collection and processing.

Hello

The reference clays marketed by the Clay Mineralogical Society are normally shipped raw, without particle size fractionation or cation saturation. Whereas the diffractograms given in the literature (e.g. [1]) were generally obtained after separation of the < 2 µm fraction (to eliminate as much as possible impurities such as quartz, opal-CT, small Fe and/or Ti oxides, etc.) and homo-ionisation of the interlayer space (to 1 or 2 water layers). As a result, the (001) reflections obtained on oriented preparations are very close to 15.0 Å (Ca saturation) or 12.5 Å (Na saturation).

Having already worked with Swy-1, Swy-2 (Wyoming montmorillonite) and STX-1 (Texas montmorillonite), their behaviour under AD, EG and T treatments is very typical, with relatively intense (001) reflections. These peak profiles can therefore be used, in terms of half-width, to track changes in the size of the scattering domain in the stacking of layers before and after experimental treatments.

I have never worked with Sau-1. A priori, since it is a Mg-rich montmorillonite derived from bentonite (Arumpo, Australia), i.e. from the alteration of volcanic ash ([2]), we generally have well-crystallized smectites, even in raw powder diffraction. The results will therefore be even better after particle size fractionation and saturation ([3]).

[1] Chipera S.J., Bish D.L. (2001) – Baseline Studies of the Clay Minerals Society Source Clays : Powder X-ray Diffraction Analyses. Clays and Clay Minerals, 49, 5, 398-409.

[2] Gardam M., Mason A.J., Reid A.F., Churchman G.J., Raven M. (2008) – Arumpo bentonite deposits : distinctive indicators of past volcanic events in the Murray Basin, southeastern Autralia. Australian Journal of Earth Sciences, 55, 183-194.

[3] Churchman G.J., Askary M., Peter P., Wright M., Raven M.D., Self P.G. (2002) – Geotechnical properties indicating environmental uses for an unsual Australian bentonite. Applied Clay Science, 20, 199-209.