A smectite (from Ancient Greekσμηκτός (smēktós) 'lubricated'; from σμηκτρίς (smēktrís) 'walker's earth, fuller's earth'; lit.'rubbing earth; earth that has the property of cleaning')[1] is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.[2]
Scanning electron microscope (SEM) photograph of smectite clay – magnification 23,500 – U.S. Geological Survey – Tuckup CanyonTypical cracks pattern of a smectite-rich bentonite after its desiccation and shrinkage
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 structureedit
2:1 clay minerals crystallographic structure made of three superimposed sheets of tetrahedra-octahedra-tetrahedra (TOT layer unit), respectively
The 2:1 layer (TOT) structure consists of two silica (SiO2) tetrahedral (T) layers which are electrostatically cross-linked via an Al2O3 (gibbsite), or Fe2O3, octahedral (O) central layer. The TOT elementary layers are not rigidly connected to each other but are separated by a free space: the interlayer hosting hydrated cations and water molecules. Smectite can swell because of the reversible incorporation of water and cations in the interlayer space.
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 processedit
Detailed molecular structure of pure montmorillonite, the best known end-member of the smectite group. The interlayer space between two successive TOT layers is filled with hydrated cations (mainly Na+ and Ca2+ ions) compensating the negative electrical charges of the TOT layers and with water molecules causing the interlayer expansion.
The main cations in the smectite interlayers are Na+ and Ca2+. The sodium cations are responsible for the highest swelling of smectite while calcium ions have lower swelling properties. Calcium smectite has significantly less swelling capacity than sodium smectite but is also less prone to shrinking when desiccated.[3]
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.
Smectites are formed from the weathering of basalt, gabbro, and silica-rich volcanic glass (e.g., pumice, obsidian, rhyolite, dacite). Many smectites are formed in volcanichydrothermal system (such as geyser system) where hot water percolating through the porous matrix or the cracks of the volcanic ash deposit (pumice, pozzolan) dissolves most of amorphoussilica (up to 50 wt.% of SiO2 can be dissolved), leaving smectite in place. This mechanism is responsible for the formation of the bentonite deposit (Serrata de Nijar) of Cabo de Gata in the south-east region of Almeria in Andalusia (Spain). Wyoming MX-80 bentonite was formed in a similar way during the Cretaceous Period when volcanic ashes were falling in an inner sea on the American continent. The highly porous (with a large and easily accessible specific surface) and very reactive volcanic ashes rapidly reacted with seawater. Because of silica hydrolysis, most of silica was dissolved in seawater and removed from the ashes giving rise to the formation of smectites. Smectites found in many marine clay deposits are often formed in this way as it is the case for the Ypresian Clays found in Belgium and very rich in smectites.
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.
^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
^Barast, Gilles; Razakamanantsoa, Andry-Rico; Djeran-Maigre, Irini; Nicholson, Timothy; Williams, David (June 2017). "Swelling properties of natural and modified bentonites by rheological description". Applied Clay Science. 142: 60–68. Bibcode:2017ApCS..142...60B. doi:10.1016/j.clay.2016.01.008.
Further readingedit
Meunier, Alain (2005). Clays. Springer Science & Business Media. pp. 108–. ISBN978-3-540-21667-4.
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
smectite, editor, performed, search, found, that, sufficient, sources, exist, establish, subject, notability, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, newspapers,. An editor has performed a search and found that sufficient sources exist to establish the subject s notability Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Smectite news newspapers books scholar JSTOR May 2022 Learn how and when to remove this template message A smectite from Ancient Greek smhktos smektos lubricated from smhktris smektris walker s earth fuller s earth lit rubbing earth earth that has the property of cleaning 1 is a mineral mixture of various swelling sheet silicates phyllosilicates which have a three layer 2 1 TOT structure and belong to the clay minerals Smectites mainly consist of montmorillonite but can often contain secondary minerals such as quartz and calcite 2 Scanning electron microscope SEM photograph of smectite clay magnification 23 500 U S Geological Survey Tuckup CanyonTypical cracks pattern of a smectite rich bentonite after its desiccation and shrinkage Contents 1 Terminology 2 Mineralogical structure 3 Role of interlayer cations in the swelling process 4 Formation process 5 Industrial applications 6 See also 7 References 8 Further reading 9 External linksTerminology editIn 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 edit nbsp 2 1 clay minerals crystallographic structure made of three superimposed sheets of tetrahedra octahedra tetrahedra TOT layer unit respectivelyThe 2 1 layer TOT structure consists of two silica SiO2 tetrahedral T layers which are electrostatically cross linked via an Al2O3 gibbsite or Fe2O3 octahedral O central layer The TOT elementary layers are not rigidly connected to each other but are separated by a free space the interlayer hosting hydrated cations and water molecules Smectite can swell because of the reversible incorporation of water and cations in the interlayer space 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 edit nbsp Detailed molecular structure of pure montmorillonite the best known end member of the smectite group The interlayer space between two successive TOT layers is filled with hydrated cations mainly Na and Ca2 ions compensating the negative electrical charges of the TOT layers and with water molecules causing the interlayer expansion The main cations in the smectite interlayers are Na and Ca2 The sodium cations are responsible for the highest swelling of smectite while calcium ions have lower swelling properties Calcium smectite has significantly less swelling capacity than sodium smectite but is also less prone to shrinking when desiccated 3 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 edit nbsp Typical volcanic eruption plume whose ashes weathering after contact with seawater is the main source of smectite Leaching of most of amorphous silica leads to partial dissolution of obsidian the main constituent of volcanic glass Smectites are formed from the weathering of basalt gabbro and silica rich volcanic glass e g pumice obsidian rhyolite dacite Many smectites are formed in volcanic hydrothermal system such as geyser system where hot water percolating through the porous matrix or the cracks of the volcanic ash deposit pumice pozzolan dissolves most of amorphous silica up to 50 wt of SiO2 can be dissolved leaving smectite in place This mechanism is responsible for the formation of the bentonite deposit Serrata de Nijar of Cabo de Gata in the south east region of Almeria in Andalusia Spain Wyoming MX 80 bentonite was formed in a similar way during the Cretaceous Period when volcanic ashes were falling in an inner sea on the American continent The highly porous with a large and easily accessible specific surface and very reactive volcanic ashes rapidly reacted with seawater Because of silica hydrolysis most of silica was dissolved in seawater and removed from the ashes giving rise to the formation of smectites Smectites found in many marine clay deposits are often formed in this way as it is the case for the Ypresian Clays found in Belgium and very rich in smectites Industrial applications editMain article Bentonite 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 editArgillaceous minerals Bentonite Clay Clay chemistry Clay mineral Clay water interaction Expansive clay Hectorite Montmorillonite Nontronite SaponiteReferences edit CNRLT 2012 Smectite Definition de smectite Smectite Definition of smectite cnrtl fr in French Retrieved 28 July 2022 Terre qui a la propriete de nettoyer Earth that has the property of cleaning Friedrich Klockmann 1978 1891 Paul Ramdohr Hugo Strunz ed Klockmanns Lehrbuch der Mineralogie in German 16 ed Stuttgart Enke p 753 ISBN 3 432 82986 8 Barast Gilles Razakamanantsoa Andry Rico Djeran Maigre Irini Nicholson Timothy Williams David June 2017 Swelling properties of natural and modified bentonites by rheological description Applied Clay Science 142 60 68 Bibcode 2017ApCS 142 60B doi 10 1016 j clay 2016 01 008 Further reading editMeunier Alain 2005 Clays Springer Science amp 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 amp Soga K 2005 Fundamentals of soil behavior Vol 3 New York John Wiley amp Sons Mackenzie R C amp Mitchell B D 1966 Clay mineralogy Earth Science Reviews 2 47 91 Jeans C V Merriman R J Mitchell J G amp 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 0External links edit nbsp Look up smectite in Wiktionary the free dictionary nbsp Wikimedia Commons has media related to Smectite nbsp Earth sciences portal Retrieved from https en wikipedia org w index php title Smectite amp oldid 1198685935, wikipedia, wiki, book, books, library,
