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Sepiolite mineral

Sepiolite is the hydrated magnesium silicate mineral Mg4Si6O15(OH)2(H2O)2·4H2O, an orthorhombic ribbon-like clay mineral. It is distinct from elemental magnesium, magnesium commodities, the biotite mineral group and phlogopite. The September 2026 IMA list gives grandfathered status and original-description year 1847, not a unique approval accession. The accepted experiments concern laboratory abiotic dissolution of source-characterized, Mg-deficient sepiolite. No constituent criticality, microbial mechanism, recovered magnesium product or industrial deployment is inferred.

Microbial & Biochemical Mechanisms (1)

Abiotic preferential magnesium release during acidic sepiolite dissolution

other

Chemical dissolution at 25 °C released Mg and Si nonstoichiometrically: Mg was preferentially released under acidic conditions and Si under alkaline conditions. In acidic citrate-bearing batch experiments, solution mass balance, EDS and XRD supported strong Mg depletion and loss of crystallinity of the remaining Si-rich solid. This is dissolution and solid alteration, not microbial activity or isolated-product recovery. Channel-mediated proton/Mg exchange and formation of a Si-rich altered layer are mechanistic interpretations; interface-coupled dissolution and reprecipitation is discussed as an alternative.

Experimental Context & Analytical Limits:
Two sepiolite samples described as from Valdemore, Spain, were obtained from the Clay Mineral Society; no catalog accession is supplied. The samples were ground, sieved to below 63 µm, washed five times in ethanol with sonication and settling, and dried at 50 °C for 24 h. Section 3.1 reports XRD-assigned sepiolite with other crystalline phases below an approximately 2 volume-percent detection limit, rather than absolute purity. XRF gives Mg:Si = 3.34:6, ±5%, with trace Al and Fe, compared with ideal 4:6. Flow experiments used 250 mL reactors with a reported 1 g mineral/3 g inlet-solution slurry enclosed in 12–14 kDa dialysis tubing, citrate-free or citrate-bearing solutions and nominal inlet pH 1.7–10. Three batch series used 0.1 g mineral and 50 mL citrate/phosphate solution in sealed 60 mL bottles shaken at 25 °C. Initial pH is tabulated as 2.56, 3.40 and 4.31, with 2.57 used elsewhere for the first series. Seven sacrificial bottles per series represent sampling times, not seven independent endpoint replicates. Filtered solutions were analyzed for Mg and Si, and recovered solids by XRD, SEM and EDS. The chemical-reagent citrate was not produced by a tested microbe; no organisms or biomass were used. Separate no-mineral blanks and independent replication of each condition are not specified.
Curated Evidence Points:
Sections 3.1–4.1, Table 3 and Figures 2–4 support the initial mineral assignment and pH-dependent, nonstoichiometric release. The citrate-bearing acidic flow experiments showed less unequal Mg/Si release than citrate-free experiments. Rates were calculated from inlet/outlet concentrations, flow, stoichiometric factors and initial BET area; they are normalized dissolution rates, not recovered-metal yields. Buffer composition changes with pH, and the small citrate-bearing subset is not a fully crossed design isolating every chemical variable.
Section 4.2, Tables 2 and 4, Figures 5–7 and Appendix 2 support preferential Mg removal in the acidic batch series. The authors infer approximately 98 percent of the original solid Mg removed in the lowest-pH series and approximately 43 percent at initial pH 4.31, while Si removal did not exceed about 20 percent. These are source-derived element mass-balance estimates relative to the starting mineral, not measured isolated-product recovery, product purity or whole-feed conversion. Figure 5 concentrations are normalized by the source stoichiometric coefficients; they should not be read as unnormalized solution concentrations.
The abstract, prose, Tables 2/4 and Figure 7 caption use a final duration of 688 h, whereas Appendix 2 and the final labels in Figures 6/7 show 699 h. The exact final duration is unresolved. The text describes buffered batch pH as stable, but Appendix 2 records drift, including 2.56 initially to 2.82 at 699 h in the lowest-pH series and 4.31 to 4.58 in the highest-pH series. The endpoint estimate is therefore retained with both time labels and initial, rather than assumed constant, pH.
Figures 6–7 show retained needle-like morphology but progressive loss of diffraction intensity, including the source-highlighted peak near 8.46 degrees 2theta, and an amorphous final pattern in the lowest-pH series. EDS shows strong Mg depletion. Retained shape does not establish surviving crystalline sepiolite, and a Si-rich amorphous residue is not a newly identified crystalline silica species or a quantified pure product. Initial EDS Si/Mg in Table 4 differs from the XRF bulk ratio in Section 3.1; these measurements are not treated as interchangeable bulk compositions.
Sections 5.1–5.4 propose proton exchange for structural Mg, transport through channels and subsequent Si-network alteration, but explicitly discuss interface-coupled dissolution/reprecipitation as another explanation. Molecular causality is not uniquely resolved by these dissolution and ex situ solid measurements. PHREEQC saturation calculations and the hypothetical upper-bound brucite mass estimate are not direct observations of brucite, talc or chrysotile precipitation. No separate new-mineral product is assigned from those calculations.
Section 5.5 estimates about 15 percent uncertainty in calculated release rates from analytical, pH, flow and initial-area contributions, with additional unquantified effects from changing reactive area. Comparisons between batch and flow systems do not independently isolate dialysis effects. The source discusses potential CO2 sequestration relevance but does not perform a carbon-capture demonstration, microbial experiment, magnesium-product isolation or industrial test. The accepted evidence is one study; related earlier same-group sepiolite work is not counted here as independent replication.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026; definitions pages 1–2 and Sepiolite row PDF page 197: Mg4Si6O15(OH)2(H2O)2·4H2O, G, 1847. Official original acquired again and row visually inspected on 14 October 2026.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-14)
  2. handbook: Handbook of Mineralogy. Sepiolite, version 1.2, 2001; complete one-page identity entry, orthorhombic mineral and composition Mg4Si6O15(OH)2·6H2O.
    https://www.handbookofmineralogy.org/pdfs/sepiolite.pdf (accessed 2026-10-14)
  3. mulders2020: Mulders, Josephina J.P.A. and Oelkers, Eric H. (2020). An experimental study of sepiolite dissolution rates and mechanisms at 25 °C. Geochimica et Cosmochimica Acta 270:296–312. DOI 10.1016/j.gca.2019.11.026. Complete official UCL accepted manuscript, 48 PDF pages: Sections 3–5, Tables 1–4, Figures 1–8 and Appendices 1–2. Manuscript pagination differs from the journal; source endpoint and pH inconsistencies are retained.
    https://doi.org/10.1016/j.gca.2019.11.026 (accessed 2026-10-14)

Curation History

Timestamp Curator Action Summary
2026-10-14T18:02:34.098303+00:00 codex (GPT-6; codex) CREATE Created source-grounded sepiolite mineral identity and one abiotic dissolution mechanism from the complete UCL accepted manuscript, preserving phase, stoichiometry, endpoint/pH, causal and recovery limitations; microbial sepiolite and phlogopite leads remain deferred.

Record source: data/records/sepiolite.yaml