Effect of aqueous Si/Mg ratio and pH on the nucleation and growth of sepiolite at 25 °C

Publikation: Beitrag in einer FachzeitschriftArtikelForschungBegutachtung

Abstract

Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61. The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of Si–OH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS04 0 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.

Originalspracheenglisch
Seiten (von - bis)211-226
Seitenumfang16
FachzeitschriftGeochimica et Cosmochimica Acta
Jahrgang227
DOIs
PublikationsstatusVeröffentlicht - 15 Apr 2018

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sepiolite
nucleation
Nucleation
Silicates
effect
magnesium trisilicate
polymerization
Silicon Dioxide
condensation
Precipitates
Condensation
stevensite
silicate
Magnesium Hydroxide
silica
Polymerization
crystal
Talc
saponite
brucite

Schlagwörter

    ASJC Scopus subject areas

    • !!Geochemistry and Petrology

    Dies zitieren

    Effect of aqueous Si/Mg ratio and pH on the nucleation and growth of sepiolite at 25 °C. / Baldermann, Andre; Mavromatis, Vasileios; Frick, Paula M.; Dietzel, Martin.

    in: Geochimica et Cosmochimica Acta, Jahrgang 227, 15.04.2018, S. 211-226.

    Publikation: Beitrag in einer FachzeitschriftArtikelForschungBegutachtung

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    title = "Effect of aqueous Si/Mg ratio and pH on the nucleation and growth of sepiolite at 25 °C",
    abstract = "Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61. The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of Si–OH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS04 0 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.",
    keywords = "Depositional environment, Marine diagenesis, Mg-silicates, Sepiolite, Solution composition",
    author = "Andre Baldermann and Vasileios Mavromatis and Frick, {Paula M.} and Martin Dietzel",
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    journal = "Geochimica et Cosmochimica Acta",
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    T1 - Effect of aqueous Si/Mg ratio and pH on the nucleation and growth of sepiolite at 25 °C

    AU - Baldermann, Andre

    AU - Mavromatis, Vasileios

    AU - Frick, Paula M.

    AU - Dietzel, Martin

    PY - 2018/4/15

    Y1 - 2018/4/15

    N2 - Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61. The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of Si–OH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS04 0 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.

    AB - Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61. The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of Si–OH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS04 0 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.

    KW - Depositional environment

    KW - Marine diagenesis

    KW - Mg-silicates

    KW - Sepiolite

    KW - Solution composition

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    U2 - 10.1016/j.gca.2018.02.027

    DO - 10.1016/j.gca.2018.02.027

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    JO - Geochimica et Cosmochimica Acta

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