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Nontronite mineral

Nontronite is an iron(III)-rich smectite mineral, distinct from the broader smectite group, montmorillonite and clay commodities. The September 2026 IMA list gives Na0.3Fe3+2(Si,Al)4O10(OH)2·nH2O and approval by a 1962 special procedure; natural interlayer cations and composition vary. The curated material is the source-characterized CMS NAu-2 specimen and its separated size fraction, not every nontronite or a chemically pure ideal endmember. No criticality designation is inherited from constituent iron or other elements.

Microbial & Biochemical Mechanisms (1)

Humic-acid-assisted microbial iron reduction and partial clay transformation

redox transformation Microbial

Associated Organisms: Shewanella oneidensis MR-1 (NCBITaxon:211586)

MR-1 cultures reduced iron associated with nontronite NAu-2. Humic acids increased the measured iron(II) production relative to live-cell cultures without humic acid, and sterile comparisons showed much less iron(II). Altered diffraction and electron-microscopy observations support partial conversion to source-assigned illite-like domains within mixed products. The observations do not establish complete conversion, a pure illite product, phase-specific recovery of iron, or an industrial process.

Experimental Context & Analytical Limits:
Liu2017 purchased CMS NAu-2, soaked ground material in 0.01 M NaCl for 24 hours, isolated a 0.5–2 micrometre fraction by centrifugation, washed it until chloride was undetectable by the stated silver-nitrate test, and freeze-dried it. MR-1 ATCC 700550 was grown overnight in tryptic soy broth at 30 degrees C, harvested and washed with 10 mM PIPES before anaerobic use. The main reduction assay used 50 mL serum bottles with 30 mL deoxygenated sterile 10 mM PIPES, 0.03 g NAu-2, 3 mg KCl, approximately 10^8 cells per mL and 10–200 mg/L Aldrich, Suwannee River or Elliott soil humic acid; no-humic-acid cultures were compared. Sealed bottles were shaken in darkness at 30 degrees C. Other arms compared 1, 5 and 10 g/L NAu-2 with 50 mg/L Elliott soil humic acid (ESHA), and whole, sorbed and unbound ESHA fractions. The inspected main text and supplement do not explicitly supply the reduction-assay electron donor or PIPES pH; the pH 7 humic-stock/model conditions are not substituted for these missing recipe details. The separate SWy-2 montmorillonite experiments are not evidence for this species.
Curated Evidence Points:
Methods pp.7–8 and Figures 1a–c, 2a and 3 show increased total iron(II) in NAu-2 cultures with humic acid, with ESHA generally producing the strongest stimulation among the tested acids. Main comparisons use living cells without humic acid, rather than sterile controls. Means and standard deviations derive from at least three experiments as reported. The source's initial iron content/speciation is attributed to earlier characterization; no new phase-specific inventory or calculated extraction percentage is inferred here.
Figure S5b includes separate sterile arms with and without humic acid, both with low iron(II) relative to cell-containing arms. This supports microbial involvement in the iron-reduction signal. Figure S5a/b also separates dissolved from solid-associated/adsorbed iron(II); increased total reduced iron is not equivalent to dissolved iron recovery. The supplement does not provide a corresponding complete sterile XRD/TEM transformation series.
Figure 6 compares starting NAu-2 with reduced solids using ethylene-glycolated XRD, TEM/HRTEM and SEM-EDX. Broadened/shifted nontronite diffraction and source-assigned illite reflections accompany approximately 1.0 nm domains next to remaining approximately 1.3 nm nontronite layers. EDX supports aluminium/silicon/potassium-containing material but is not alone a pure-phase identification. Added KCl supplies potassium externally; the observation is partial mixed-solid transformation, not complete production of an ideal illite endmember.
Figures S7–S10 show time-dependent domains and discrete source-assigned illite after ESHA-assisted reduction. The few approximately 0.364 nm lattice fringes described as siderite are a tentative secondary-phase interpretation, not an accepted pure siderite recovery claim. Table S4 reports aqueous aluminium as undetectable without humic acid and detected with it, alongside greater silicon; the undefined aluminium detection threshold is not zero. The table lacks a fully matched sterile elemental-release series, and its potassium addition/elemental basis is not used for a potassium mass balance. No concentration cells are imported into the mg/kg observation model.
Figures 3–5, electrochemical measurements and separate sorption experiments support redox-active humic fractions and altered iron(II) partitioning as explanations for stimulation. Electron shuttling, complexation and reduced surface inhibition are mechanistic interpretations rather than uniquely isolated molecular pathways. Cross-study AQDS comparisons in the Discussion use different mineral loadings, organisms or conditions; they are not matched evidence that natural humic acid is universally faster or more effective.
Context_only Source: liu_2017_si
Text S1's iron-complexation estimate is a pH-dependent model, not a measured culture yield. Table S5 combines this study with distinct previously published experiments; those entries are not independent replications of the present recipe or additional accepted mechanisms. No field deployment or isolated-product recovery is demonstrated.
Context_only Source: ncbi_mr1
The verified strain taxon grounds MR-1 nomenclature. It does not supply missing assay recipe details or establish a strain-specific molecular pathway.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026; definitions pp.1–2 and Nontronite row PDF p.157, A, 1962 s.p. Retained official original re-inspected 14 October 2026.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-14)
  2. nontronite_handbook: Handbook of Mineralogy: Nontronite, 2001 version 1.2; complete one-page entry, monoclinic smectite identity and variable interlayer cations.
    https://www.handbookofmineralogy.org/pdfs/nontronite.pdf (accessed 2026-10-14)
  3. liu_2017: Liu, G.; Qiu, S.; Liu, B.; Pu, Y.; Gao, Z.; Wang, J.; Jin, R.; Zhou, J. (2017). Microbial reduction of Fe(III)-bearing clay minerals in the presence of humic acids. Scientific Reports 7:45354. DOI 10.1038/srep45354. Complete nine-page published original, full JATS and Figures 1–6 inspected.
    https://doi.org/10.1038/srep45354 (accessed 2026-10-14)
  4. liu_2017_si: Liu et al. (2017) Supplementary Information, srep45354-s1.pdf; complete nineteen-page original acquired from the Europe PMC supplementary archive for PMC5371790. Text S1, Figures S1–S10 and Tables S1–S5 inspected.
    https://www.ebi.ac.uk/europepmc/webservices/rest/PMC5371790/supplementaryFiles (accessed 2026-10-14)
  5. ncbi_mr1: NCBI Taxonomy: Shewanella oneidensis MR-1, taxon 211586, strain rank; authoritative XML checked 14 October 2026. The study reports ATCC 700550.
    https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=211586 (accessed 2026-10-14)

Curation History

Timestamp Curator Action Summary
2026-10-14T14:50:14.105882+00:00 codex (GPT-6; codex) CREATE Created nontronite species record from complete Liu2017 primary and nineteen-page supplement, retaining sterile controls, partial mixed transformation, unresolved assay recipe and no recovery-yield inference.

Record source: data/records/nontronite.yaml