Idriss Mohdeb X-ray diffraction (XRD) peak intensities after van der Waals (VDW) exfoliation can be influenced by factors such as crystal structure, degree of exfoliation, sample preparation and processing, and instrumental factors. Combining XRD with techniques like Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) is recommended for a comprehensive understanding of exfoliation success and to visualize the exfoliated material's thickness and topography.

My Dear Mr. Idriss Mohdeb,

The exfoliation of Vermiculite is best described with diagram and explanation as below:

· Vermiculite Exfoliation involves a volume expansion with individual platy particles expanding perpendicular to the cleavage planes, bloating in an accordion or concertina like fashion to up to 20 - 30 times their original volume

· Mechanism of exfoliation is explained below in the Diagram schematically:

Ø Particles consisting of undergrown mosaic of Vermiculite, Hydrobiotite and Mica domains (or vermiculite and chlorite domains) shown in (a)

Ø They get heated during Fire which leads to Gas phases produced by shock thermal treatment, or breakdown of hydrogen peroxide are wrapped in dead ends at phase boundaries causing pressure build up. The result is (b) an exfoliated Product

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Natural vermiculite flakes are characterized by high values of aspect ratio. In clay science, exfoliation involves a degree of separation of the layers of a host structure where units, either individual layers or stacking of several layers, are dispersed (freely oriented and independent) in a solvent or polymer matrix. This may be achieved by intercalation, by mechanical procedures, or by other methods. Exfoliation implies that the orientation between the layers of the host structure is lost, and that interlayer cohesive forces are overcome. If delamination or exfoliation cannot be distinguished, the terms “intercalation” or elimination/exfoliation” should be used.

Delamination is a term used to describe a layer-separation process between the planar faces of adjacent layers of a particle. Delamination describes a process where intercalation occurs: guest material is introduced between the layers while the stacking of layers remains. When delamination cannot be distinguished from exfoliation, the terms “intercalation” or “delamination/exfoliation” should be used to describe the process.

Intercalation is a general term to describe the movement of atoms, ions or molecules into a layered host structure, often a swelling clay mineral. The resulting structure is an “intercalated structure” .

Exfoliated vermiculite can be produced either by a thermal or a chemical treatment. Heating a particle of vermiculite rapidly above about 200 °C results in the transformation of the interlayer water into steam. The thermal shock is the best-known procedure for dissociating the macroscopic packet of vermiculite. The pressure of the steam separates silicate layers to the several orders thicker than were the fundamental layers. Such exfoliated particles exhibit the “accordion” type morphology. The large thermal expansion of the vermiculite after explosive dehydration of interlayer water causes cracking of vermiculite flakes.

This exfoliation generates a 10 to 20 times volume expansion but the basal dimensions of the particles are unchanged. This exfoliation is associated with a sudden release of water molecules between the silicate layers and also the hydroxyl water gradually releasing on heating from about 500 to 850°C . The water rapidly vaporized causes a disruptive effect upon the particles, in industry called exfoliation.

Several authors have found that the presence of the mixed-layer vermiculite–mica or mixed-layer minerals phases containing vermiculite in different hydration states contributed to the exfoliation of vermiculites

[1-6, 7,8,9-11].

In a very interesting Article (8), Machinable Glass-ceramic was produced by sintering method from vermiculite, K2CO3 and MgF2. The sintering behavior and machinability of glass-ceramic composition was investigated.

Following are detected crystalline phases and the macro images of XRD patterns of the samples depending on sintering temperatures-

Temperature Detected Phases

a) 1100°C Forsterite, Phlogopite, Jadeite- diopside, Augite

b) 1200°C Forsterite, Phlogopite, Jadeite- diopside, Spinel

c) 1300°C Forsterite, Phlogopite, Spinel

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Figure-X R Ds and SEM images of the sintered samples at a)1100 °C, b) 1200 °C and c) 1300 °C

The main object of this study was toproduce machinable glass-ceramic materials from vermiculite by sintering method.

Phlogopite, Augite, Forsterite, Jadeite-Diopside, and MgAl2O4 Spinel phases were detected by XRD.

Diopside and Phlogopite phases are common phases for machinable glass-ceramic systems. Phlogopite, Augite and Diopside phase intensities decreased with increasing temperature. When temperature reached to 1300 °C, Spinel phase is major phase. This phase transformation was proved by SEM images. Sharp igneous Diopside crystals disappeared in SEM image of the sample sintered at 1300 °C. It has no negative effect on machinability. All samples were machined, successfully. When macro images of samples were scrutinized, firing shrinkage and also darkening of color was observed clearly depending on increase in sintering temperature. (8)

In short, the best answer could be – with increasing temperatures- peaks of Phlogopite, Augite, Forsterite, Jadeite-Diopside [ images a) and b)] reduces and at higher temperature of 1300° C Spinel is a major peak in phase is detected- (image c.)

REFERENCE LIST- 2.7 Vermiculite

Phlogopite, Augite, Forsterite, Jadeite-Diopside

[1] P.A. Ciullo, Industrial Minerals and Their Uses: A Handbook & Formulary, 1996.

[2] M. Valášková, G.S. Martynková, in: Clay Miner. Nat. - Their Charact. Modif. Appl., IntechOpen, 2012.

[3] H. J.R., in: J.E. Kogel, N.C. Trivedi, M.J. Barker, S. Krukowski (Eds.), Ind. Miner. Rocks, Commod. Mark. Uses, 7th ed, 2006, pp. 1015–1026.

[4] S. Hillier, E.M.M. Marwa, C.M. Rice, Clay Miner. 48 (2013) 563582.

[5] R.A. Schoonheydt, C.T. Johnston, EMU Notes Mineral. 11 (2011) 337–373.

[6] Y. Xue, S. Zhang, W. Yang, J. Coatings Technol. Res. 12 (2015) 357–364.

[7] G.E. Wilson, C.W. Lehnert, Fire Resistant Gypsum Composition, 1987.

[8] Umut Önen, Ediz Ercenk, Şenol Yılmaz- Cumhuriyet University, Sakarya University – Türkiye- The Effect of Sintering Temperature on Machinability -Properties of Vermuiculite Based Glass-Ceramics -UCTEA Chamber of Metallurgical & Materials Engineers . IMMC 2016 | 18th International Metallurgy & Materials Congress .

[9] THE VERMICULITE ASSOCIATION, www.vermiculite.org

[10] H D M - Hydro Design Management Co. Pvt. Ltd. - India, www.dmglobal.com

[11] Arabian Vermiculite Industries – Dammam- Kingdom Of Saudi Arabia.

www.avi-sa.com

The phenomenon of exfoliation, particularly in layered materials like graphene or clays, involves separating layers from the bulk material, resulting in a change in the material's structure and properties.

Layer separation: Exfoliation involves separating layers within a material. If the layers were strongly bound together in the original material, their separation could result in a decrease in peak intensity due to the disruption or weakening of the crystal lattice.

Size Effects: Exfoliation can lead to the formation of smaller particles or thinner layers. These size effects can impact the diffraction patterns, potentially resulting in changes in peak intensity or shifts in peak positions.