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Saponite mineral

Saponite is an IMA-listed smectite-group mineral with variable Mg-Fe-Al composition, written (Ca,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2.4H2O in the September 2026 list. It has grandfathered status G and original-description year 1840, not a unique approval accession. The laboratory material below is the authors' saponite assignment for poorly ordered Mg-Al silicate associated with mixed microbial cultures. Its diffuse electron diffraction and lack of resolved bulk XRD peaks do not independently establish a well-crystallized, phase-pure saponite product. This owner is distinct from the smectite group and from co-precipitated carbonates; no constituent criticality designation is inherited.

Microbial & Biochemical Mechanisms (1)

Community-associated precipitation of source-assigned saponitic material

biomineralization Microbial

Associated Organisms: Bacillariophyta (NCBITaxon:2836)

Liquid cultures derived from Caballo Alba microbial mats developed Mg-Al-Si-rich layers and pseudomorphs on degraded diatom frustules, assigned by the authors to saponite; some cyanobacteria-like filaments were also coated. Dissolution of biogenic silica followed by silicate precipitation is the proposed route. Spatial association and culture controls support a community effect but do not isolate a particular strain, metabolism or living-cell requirement.

Experimental Context & Analytical Limits:
Mats collected in March 2019 from Caballo Alba lake, Segovia, Spain, inoculated modified ASN-III liquid and 20 g/L agar semi-solid media. Methods report 1 mL inoculum, triplicate experiments, 30 degrees C, seven months and dark/light conditions without an explicit photoperiod or separate light/dark mineral outcomes. No additional water was added. Initial pH was adjusted to 7.4; liquid culture endpoints were pH 8.0-8.3 versus 7.6 in the uninoculated liquid control. The medium supplied Mg, Al and dissolved silicate as well as Ca and bicarbonate, so the diatom frustule is not the only available silicon source. The inoculum was a mixed community containing at least two unnamed pennate-diatom morphotypes, cyanobacteria and other bacteria, not a pure isolate. SEM preparations were fixed, ethanol-dehydrated and dried; TEM used powdered material suspended in butanol. No in-situ phase time series, phase-specific mass yield or recovery process was measured.
Curated Evidence Points:
Methods 2.1-2.4 and Figures 3-6 show partly corroded frustules, Mg enrichment, Mg-Al-Si-bearing layers, and diatom-shaped pseudomorphs in liquid cultures. Figure 7 shows comparatively preserved frustules embedded in EPS in the semi-solid cultures. These observations support the source's culture-dependent association and dissolution/precipitation interpretation, not direct tracking of individual atoms or proof that EPS alone caused the difference.
The assignment is based on morphology and TEM/SEM EDS composition, including a reported saponitic structural-formula range. Figure 6C/E shows diffuse rings and its caption calls the aggregates amorphous; bulk XRD did not resolve clay peaks. No treated-clay diffraction series or refined phase fraction is supplied. Accordingly, the material is retained as source-assigned saponitic, poorly ordered Mg-Al silicate rather than independently confirmed phase-pure crystalline saponite.
Original Figures S2 and S4 corroborate the coated frustules/filaments and flaky Mg-Si-Al material. Figure S5 resolves halite and source-assigned coalingite in the uninoculated liquid control. The main text reports no clay mineral detected in controls, but control XRD alone cannot exclude amorphous Mg-silicate when the biotic material is itself XRD-undetected. No killed-inoculum, isolated-frustule, pH-matched or specific-metabolism control is described; the study does not establish a living-cell requirement or exclusive biological necessity.
Table 1 records substantial Si and Al depletion even in the no-inoculum control, as well as lower final Si in cultures. The printed 99.2-99.6% Si-consumption range does not match all three culture cells; the Al culture value spans rows and its replication is unclear. These solution measurements are not a saponite mass balance. The main text also cross-refers to Figure S3 for a control observation although S3 is a community chart; S5 is the actual control diffractogram. No exact uptake efficiency or phase yield is adopted.
The interpretation invokes photosynthesis, decaying biomass, EPS differences and local Mg binding, but these contributions are not separately measured. Light/dark methods are not linked to separate outcome arms. Carbonate diffraction, isotopic data and saturation modeling cannot establish the saponite phase or a saponite-specific metabolism. Reported community percentages include chloroplast assignments within 16S profiles; they are not a quantitative census of diatom species. This is laboratory evidence, with no industrial extraction or demonstrated field-scale manufacture.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026, Saponite row, PDF p192 and list definitions. Retained original re-inspected on the access date.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-14)
  2. handbook_saponite: Handbook of Mineralogy. Saponite, version 1.2 (2001). Complete one-page identity; identifies smectite-group membership and additional analytical requirements for positive smectite identification.
    https://www.handbookofmineralogy.org/pdfs/saponite.pdf (accessed 2026-10-14)
  3. del_buey_2023: del Buey, Sanz-Montero and Sanchez-Roman (2023). Bioinduced precipitation of smectites and carbonates in photosynthetic diatom-rich microbial mats: Effect of culture medium. Applied Clay Science 238:106932. Complete 12-page original, Figures 1-8 and Table 1; official supplementary DOCX, Figures S1-S5 and Tables S1-S3. Main PDF acquired from the University of Granada repository, handle 10481/112264.
    https://doi.org/10.1016/j.clay.2023.106932 (accessed 2026-10-14)
  4. del_buey_2023_si: Official supplementary material to del Buey et al. (2023), article S0169131723001199, mmc1.docx. Complete original package; original microscopy and control diffractogram inspected, chart data and table cells read from native XML.
    https://ars.els-cdn.com/content/image/1-s2.0-S0169131723001199-mmc1.docx (accessed 2026-10-14)
  5. ncbi_diatoms: NCBI Taxonomy 2836, Bacillariophyta (diatoms), phylum. Broad mapping for unidentified pennate diatoms in a mixed community, not a strain identification or causal assignment to every member.
    https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=2836&mode=Info (accessed 2026-10-14)

Curation History

Timestamp Curator Action Summary
2026-10-14T18:02:49.045937Z codex (GPT-6; codex) CREATE Created saponite identity owner with a qualified mixed-community precipitation experiment, complete original and supplementary evidence, explicit amorphous/phase-assignment uncertainty and non-causal control boundaries.

Record source: data/records/saponite.yaml