The Fe-Mg-saponite solid solution series – a hydrothermal synthesis study

Abstract

The boundary conditions of saponite formation are generally considered to be well known, but significant gaps in our knowledge persist in respect to the influence of solution chemistry, temperature, and reaction time on the mineralogy, structure, stability, and chemical composition of laboratory-grown ferrous saponite. In the present study, ferrous saponite and Mgsaponite were synthesized in Teflon-lined, stainless steel autoclaves at 60, 120 and 180°C, alkaline pH, reducing conditions, and initial solutions with molar Si:Fe:Mg ratios of 4:0:2, 4:1:1, 4:1.5:0.5, 4:1.75:0.25, and 4:1.82:0.18. The experimental solutions were prepared by dissolution of sodium orthosilicate (Na4SiO4), iron(II)sulfate (FeSO4·6H2O) and magnesium chloride salts (MgCl2·6H2O with ≤ 0.005 mass% of K and Ca) in 50 mL ultrapure water that contained 0.05% sodium dithionite as the reducing agent. The precipitates obtained at two, five and seven days of reaction time were investigated by X-ray diffraction techniques, transmission electron microscopy analysis, infra-red spectroscopy, and thermo-analytical methods.The precipitates were composed mainly of trioctahedral ferrous saponite, with small admixtures of co-precipitated brucite, opal-CT, and 2-line ferrihydrite, and nontronite as the probable alteration product of ferrous saponite. The compositions of the obtained ferrous saponites were highly variable, (Na0.44−0.59 K0.00−0.05 Ca0.00−0.02) (Fe2+0.37−2.41 Mg0.24−2.44 Fe3+0.00−0.28)Σ2.65−2.85 [(Fe3+0.00−0.37 Si3.63−4.00)O10](OH)2, but show similarities with naturally occurring trioctahedral Fe and Mg end members, except for the Al content. This suggests that a complete solid solution may exist in the Fe-Mg-saponite series.A conceptual reaction sequence for the formation of ferrous saponite is developed based on the experimental solution and solid compositions. Initially, at pH ≥ 10.4, brucite-type octahedral template sheets are formed, where dissolved Si-O tetrahedra are condensed. Subsequent reorganization of the octahedra and tetrahedra via multiple dissolution-precipitation processes finally results in the formation of saponite structures, together with brucite and partly amorphous silica. The extent of Fe2+ incorporation in the octahedral template sheets via isomorphic substitution is suggested to stabilize the saponite structure, explaining (i) the abundance of saponite enriched in VIFe2+ at elevated Fe supply and (ii) the effect of structural Fe on controlling the net formation rates of ferrous saponite.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…The TEM-EDX spectra were acquired using a counting time of 30 s to reduce element migration and element loss. The accuracy of the EDX results was verified by replicate measurements performed on a suite of phyllosilicate standards [26][27][28]. The analytical error was determined to be ± 2 at.% for Si and Al analyses.…”

Synthesis and structural characterization of ferrous trioctahedral smectites: Implications for clay mineral genesis and detectability on Mars

“…Each of the smectites with high octahedral Al (E and F) has low octahedral site occupancies (2.57 and 2.53, respectively). Calculated octahedral site occupancies below three are common in synthetic trioctahedral smectites [ Baldermann et al ., ; Grauby et al ., ]. These low occupancies indicate a ditrioctahedral structure characterized by vacancy defects associated with trivalent Al 3+ cations in the octahedral sheet.…”

“…Previous work related to Fe-rich ferrous smectites includes both synthetic and natural samples, with characterization largely confined to major element and X-ray diffraction (XRD) analyses with limited Mössbauer and spectral reflectance analyses [Badaut et al, 1985;Baldermann et al, 2014;Chukanov et al, 2003;Harder, 1978;Lajarige et al, 1998;Parthasarathy et al, 2003]. There are Mars-relevant gaps in chemistry (particularly high-MgO and low-Al 2 O 3 ferrous smectite), and, for spectral reflectance studies, the reported wavelength ranges do not overlap those covered by the hyperspectral imaging instruments in Martian orbit (effectively~0.40 μm to 2.6 μm [e.g., Murchie et al, 2007]).…”

Geochemistry and mineralogy of the Oligo-Miocene sediments of the Valley of Lakes, Mongolia

“…About 50 mg of the sample was mixed with 5 mL of deionised water, following dispersion for 10 min in an ultrasonic bath. The clay-in-suspension was then sucked through a ceramic tile of about 4 cm 2 in order to produce oriented specimens (Baldermann et al 2014). These clay films were X-rayed from 3 to 30° 2θ with a step size of 0.02° 2θ and a count time of 2 s per step, each at air-dried states and after the solvation of the clay matter with ethylene glycol (EG) and heating of the preparations to 550 °C for 1 h. The percentage of illite layers (%I) in illite–smectite (I-S) mixed layered clay minerals was determined based on EG-solvated patterns with an analytical precision of ±5% (Baldermann et al 2013).…”