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Hydrotalcite mineral

Hydrotalcite is the magnesium-aluminium layered double hydroxide carbonate mineral Mg6Al2(OH)16(CO3)(H2O)4. The IMA-CNMNC September 2026 list gives approved status A and 2016 s.p., indicating a special-procedure action rather than a conventional sequential approval number. This species is distinct from the hydrotalcite group and from generic synthetic layered double hydroxides containing other metal ratios or interlayer anions. A Niue calcrete contains source-XRD-assigned hydrotalcite associated with optically interpreted biofilms; microbial formation is not demonstrated. Elemental magnesium or aluminium criticality is not inherited.

Microbial & Biochemical Mechanisms (1)

Field association of hydrotalcite-rich calcrete with source-interpreted biofilms

other Microbial

In a coastal calcrete from Niue Island, powder XRD was assigned predominantly to hydrotalcite with magnesian calcite and goethite. Reflected-light petrography shows hydrotalcite-filled fractures bordered by material the authors interpret as biofilms. This is a source-defined microbial association in a natural mixed rock, not a culture experiment or direct demonstration of microbial mineral synthesis. The proposed depositional model involves dolomite dissolution, transport of weathered soil and atmospheric/seawater carbon inputs; a contribution from microbial respiration is hypothesized but is not separately measured or shown necessary.

Experimental Context & Analytical Limits:
A representative hand-sized sample of the rare HT-C coastal calcrete near Vaiea, Niue, was investigated alongside a distinct calcite-rich interior calcrete PC-C. The coastal rock is described as about 1.1 m long, around 3.2 m above sea level and wave-exposed. No sampling date, formation duration, directly measured precipitation temperature, inoculum, strain or experimental mineral loading is established. Several grams of representative powdered rock were used for Cu K-alpha XRD, with integrated peak-intensity calibration; polished thin sections were examined by reflected-light microscopy and fused powders by XRF. Approximately 1 mg powdered aliquots reacted with phosphoric acid at 50°C for isotope analysis; this analytical temperature is not a mineral-formation condition. There is no sterile, killed-biomass or matched abiotic formation control, no sequence or molecular assay authenticating a microbial taxon, and no industrial recovery experiment.
Curated Evidence Points:
Sections 3.1–3.2 and Figure 4B report the hydrotalcite-bearing phase assemblage: the diffraction figure labels 65.3% hydrotalcite, 30% magnesian calcite and 4.7% goethite; the text rounds hydrotalcite to 65%. These are source-calibrated bulk phase estimates, not an isolated pure product or recovered mass. Figure 6A, explicitly labelled HT-C, shows hydrotalcite-filled fractures encased in source-interpreted biofilms. Section 3.2 incorrectly points its HT-C description to Figure 5, which is instead labelled PC-C; the accepted field association is grounded to Figure 6, not the calcite-rich specimen.
The phase assignment comes from a mixed-rock diffraction pattern and integrated peak intensities, not a reported phase-specific composition or independent single-crystal refinement. Table 1 is bulk XRF chemistry and cannot assign every Mg, Al or Fe atom to hydrotalcite. Optical films alone do not establish a living organism, metabolic pathway, precipitation chronology, or a biological rate enhancement. Proposed dolomite–seawater formation in Sections 4.2 and 5 is an interpretation supported by a separately cited synthesis study, not a new synthesis performed in this paper.
Section 3.4 and Table 2 report paired bulk HT-C carbon and oxygen isotope measurements, not separated hydrotalcite-specific isotope values. The authors state that hydrotalcite fractionation factors and the phosphoric-acid oxygen correction are unknown. Atmospheric CO2, seawater bicarbonate, dolomite dissolution and microbial respiration therefore remain non-unique possible contributors; no fractional microbial carbon contribution or net sequestration rate is curated. The stronger soil-respiration argument for the separate PC-C calcite is not transferred to HT-C.
The Figure 1 map locates the island in the southern hemisphere while its coordinate caption prints north latitude; exact coordinates are not adopted. The source calls some goethite/gibbsite-bearing material Mg-Al silicates, although those are not silicate species; that classification is not adopted. Suggested Holocene age and depositional history are interpretations rather than direct dates. These source inconsistencies do not justify treating the observed calcrete as a demonstrated microbial synthesis or an engineered extraction process.

Sources & References

  1. ima_202609: IMA-CNMNC master list, September 2026: definitions pp. 1–2 and Hydrotalcite row p. 98
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-14)
  2. handbook_hydrotalcite: Handbook of Mineralogy: Hydrotalcite; formula and mineral identity, version 1. Historical nomenclature does not replace the current IMA status.
    https://www.handbookofmineralogy.org/pdfs/hydrotalcite.pdf (accessed 2026-10-14)
  3. aharon_2024: Aharon and Whitehead (2024). Abiotic and Biotic Processes Controlling Deposition of Calcite and Hydrotalcite Calcretes on Niue Island, Southwest Pacific. Minerals 14(9):877. Complete 12-page published article, Sections 3–4, Figures 1–6 and Tables 1–2.
    https://doi.org/10.3390/min14090877 (accessed 2026-10-14)

Curation History

Timestamp Curator Action Summary
2026-10-14T19:43:00.297803+00:00 codex (GPT-6; codex) CREATE Created a distinct carbonate hydroxide mineral owner from complete primary and authority evidence, retaining field-association, phase, control and source-consistency limits.

Record source: data/records/hydrotalcite.yaml