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

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 [Mg_3.4-3.8Al_0.1-0.4)_∑3.8-3.9(Si_5.9-6.0Al_0-0.1)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.