Abstract
The crystallochemical models of clay mineral structures are described in this chapter because clay properties and inherent reactivity depend entirely on the specificities of those structures. Clay reactivity is strongly conditioned by the surface properties of clay mineral particles that are basically dependent on the particle global electric charge and its spatial distribution pattern, different from basal surfaces to edge surfaces. Particle electric charge results mainly from atomic substitutions that occur in both the octahedral and tetrahedral layers. In clay minerals bearing interlayer spaces chemical reactivity also possibly occurs on internal surfaces.
The reactivity of the clay–water system is quite common in natural clays. It is often used in mud therapy and peloid therapy in the form of paste for topical therapeutic and cosmetic applications, and it is also studied as well as the clay–water system that in the form of dispersion (“argillic water”) is used for internal therapeutic purposes. Clay’s ionic exchange capacity and color, two important properties whenever clay is used in the field of health, are also discussed.
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Notes
- 1.
Mineralogists qualify this layer as the “brucite layer” by analogy with brucite, magnesium hydroxide. Various atomic substitutions can exist in the layer, in particular Mg substituted by Al. The classification table and general formulas are given in the appendix.
- 2.
For example, the diameter of the ion Fe2+ is 0.148 nm, whereas that of Mg2+ is 0.132 nm.
- 3.
They are substitutions between ions bearing different electric charges.
- 4.
As with some other ions such as Br+, Cl−, and so on.
- 5.
The sepiolite is made of pseudo-layers that give it a truly three-dimensional structure. A well-known variety of sepiolite is “sea foam,” often carved to manufacture bowls of pipes. Still a use close to the field of health!
- 6.
Let us recall, avoiding any confusion, that the clay particle in general has several layers separated by an interlayer space; these groups of named layer particles are themselves separated by a broader space, known as interparticle space, easily accessible to a fluid that remains free of all restraints compared to the mineral.
- 7.
Electronic microscopy in transmission and the diffraction of X-rays make it possible to check that after an extremely strong crushing the clay microparticles lose their crystalline character to become almost amorphous. Dispersion by very powerful ultrasound has the same effect (Gouami 1996).
- 8.
“Specific surface” is defined by the ratio of the surface of a particle to its mass. When a body has increasingly smaller dimensions, its specific surface becomes increasingly larger, conferring on the material a prevalence of its surface properties relative to its volume properties.
- 9.
Classically, the exchange capacity is expressed in milli equivalent grams for 100 g of clay. In the case of cations it is named CEC (cation exchange capacity); currently, it is expressed in Cmol/kg (centimole(+)/kg).
These measurements are carried out on “dried” clay previously heated up to 100°C (up to 125°C if it is clay-Ca). Thus, all the water that does not belong to the layers is eliminated.
- 10.
The notations Clay-Ca or Clay-Na mean that the internal charges of the layer are compensated by calcium or sodium ions in interlayer positions; they are called, respectively, calcic or sodium clay.
- 11.
Two very distinct situations lead to coloring. In the case of “kaolin rock,” very fine particles of oxides or metal hydroxides intervene in mixture with the other components. For kaolinite, internal substitutions for the crystalline structure can exist simultaneously with the small particles external to the structure.
- 12.
Certain functional groups of atoms are colored; the color can also come from local electronic defects.
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Rautureau, M., Figueiredo Gomes, C.d.S., Liewig, N., Katouzian-Safadi, M. (2017). Clay Reactivity Depending Upon the Crystallochemical Properties of Clay Minerals. In: Clays and Health. Springer, Cham. https://doi.org/10.1007/978-3-319-42884-0_5
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