Smectite
notability.(May 2022) ) |
A smectite (from
Terminology
In clay mineralogy, smectite is synonym of montmorillonite (also the name of a pure clay mineral phase) to indicate a class of swelling clays. The term smectite is commonly used in Europe and in the UK while the term montmorillonite is preferred in North America, but both terms are equivalent and can be used interchangeably. For industrial and commercial applications, the term bentonite is mostly used in place of smectite or montmorillonite.
Mineralogical structure
The 2:1 layer (TOT) structure consists of two
The TOT layers are negatively charged because of the isomorphic substitution of Si(IV) atoms by Al(III) atoms in the two external silica tetrahedral layers and because of the replacement of Al(III) or Fe(III) atoms by Mg2+ or Fe2+ cations in the inner gibbsite octahedral layer. As the +4 charges born by Si(IV), and normally compensated by −4 charges from the surrounding oxygen atoms, become +3 due to the substitution of Si(IV) by Al(III), an electrical imbalance occurs: +3 −4 = −1. The excess of negative charges in the TOT layer has to be compensated by the presence of positive cations in the interlayer. The same reasoning also applies to the gibbsite central layer of the TOT elementary unit when an Al3+ ion is replaced by a Mg2+ ion in a gibbsite octahedra. The electrical imbalance is: +2 −3 = −1.
Role of interlayer cations in the swelling process
The main
The degree of hydration of the cations and their corresponding hydrated radii explain the swelling or the shrinking behaviour of phyllosilicates. Other cations such as Mg2+ and K+ ions exhibit even a more contrasted effect: highly hydrated magnesium ions are "swellers" as in vermiculite (totally expanded interlayer) while poorly hydrated potassium ions are "collapsers" like in illite (totally collapsed interlayer).
As the interlayer space of smectites is more open and so more easily accessible to water and cations, smectites exhibit the highest cation-exchange capacity (CEC) of clay minerals commonly found in the soils. Only more expandable vermiculite and some rarer alumino-silicate minerals (zeolites) with inner channel structure can exhibit a higher CEC than smectite.
Formation process
Smectites are formed from the
Industrial applications
Smectites are commonly used in very diverse industrial applications. In civil engineering works, it is routinely used as a thick bentonite slurry when excavating deep and narrow trenches in the ground to support the lateral walls and to avoid their collapse. It is also used as mud for drilling fluids. Smectites, more commonly called bentonite, are candidate as buffer and backfill materials to fill the space around high-level radioactive waste in deep geological repositories. Smectites also serve as additive in paints or as thickening agent for various preparations.
See also
- Argillaceous minerals
- Bentonite
- Clay
- Clay chemistry
- Clay mineral
- Clay–water interaction
- Expansive clay
- Hectorite
- Montmorillonite
- Nontronite
- Saponite
References
- ^ CNRLT (2012). "Smectite : Définition de smectite" [Smectite: Definition of smectite]. cnrtl.fr (in French). Retrieved 28 July 2022.
Terre qui a la propriété de nettoyer. Earth that has the property of cleaning
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ISBN 3-432-82986-8
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Further reading
- Meunier, Alain (2005). Clays. Springer Science & Business Media. pp. 108–. ISBN 978-3-540-21667-4.
- Mitchell, J. K. (2001). Physicochemistry of soils for geoenvironmental engineering. In Geotechnical and geoenvironmental engineering handbook (pp. 691-710). Springer, Boston, MA.
- Mitchell, J. K., & Soga, K. (2005). Fundamentals of soil behavior (Vol. 3). New York: John Wiley & Sons.
- Mackenzie, R. C., & Mitchell, B. D. (1966). Clay mineralogy. Earth-Science Reviews, 2, 47–91.
- Jeans, C. V., Merriman, R. J., Mitchell, J. G., & Bland, D. J. (1982). Volcanic clays in the Cretaceous of southern England and Northern Ireland. Clay Minerals, 17(1), 105–156. https://doi.org/10.1180/claymin.1982.017.1.10
- Wagner, J. F. (2013). Chapter 9: Mechanical properties of clays and clay minerals. In: Developments in Clay Science, 5, 347–381. Elsevier. https://doi.org/10.1016/B978-0-08-098258-8.00011-0