Skutterudite mineral
Skutterudite is the cobalt arsenide mineral species listed as CoAs3 with grandfathered status G and original-description year 1845 in the September 2026 IMA compilation. The year is not an approval identifier. The Handbook describes cubic Im-3 symmetry and natural nickel and iron substitution. This species is distinct from nickelskutterudite, cobaltite, elemental cobalt and engineered skutterudite-type materials. Accepted microbial evidence concerns a phase-characterized, mixed Bou Azzer concentrate dominated by skutterudite; it does not establish a pure-endmember extraction rate or transfer constituent-element criticality to this mineral.
Microbial & Biochemical Mechanisms (1)
Acidophile-associated leaching of mixed skutterudite-bearing concentrate
bioleaching
Microbial
Associated Organisms:
Acidithiobacillus thiooxidans (NCBITaxon:930), Sulfobacillus thermosulfidooxidans (NCBITaxon:28034)
A living mixed acidophile culture was used to solubilize cobalt from a skutterudite-dominated natural concentrate in an acidic laboratory reactor. The reported dissolved fraction is relative to bulk concentrate cobalt, not mineral-specific dissolution or recovered cobalt product. A separate pyrite-conditioned flask produced secondary scorodite during processing. These observations support a microbial processing context; absent matched abiotic controls and phase-specific balances, they do not establish microbial necessity, enhancement over abiotic leaching or an exclusive direct-attack mechanism.
Experimental Context & Analytical Limits:
Bou Azzer, Morocco concentrate was blended from several ore bodies, supplied in late 2017 and milled through a less-than-100-micrometre mesh. Section 2.5 experiment 4 used a 2-L reactor with 1-L working volume, 1% w/v concentrate, 0.5% w/v zero-valent sulfur, 5 mM ferrous sulfate and acidophile basal salts/trace elements. Initial pH 1.8 was allowed to fall to 1.5 and then controlled with 1 M NaOH; operation was at 35 degrees C, 180 rpm and approximately 1 L/min sterile air. Inoculum combined the original iron/sulfur-oxidizing consortium with leachate from a preceding 30-degree-C flask screen. Inoculum volume and cell density are unspecified, and the concentrate was not sterilized. The abstract says 13 initial species whereas section 2.6 lists 12 named members; this discrepancy is not silently resolved. The two organisms below were significant culturable endpoint members, not the complete initial community or individually demonstrated causal agents.
Bou Azzer, Morocco concentrate was blended from several ore bodies, supplied in late 2017 and milled through a less-than-100-micrometre mesh. Section 2.5 experiment 4 used a 2-L reactor with 1-L working volume, 1% w/v concentrate, 0.5% w/v zero-valent sulfur, 5 mM ferrous sulfate and acidophile basal salts/trace elements. Initial pH 1.8 was allowed to fall to 1.5 and then controlled with 1 M NaOH; operation was at 35 degrees C, 180 rpm and approximately 1 L/min sterile air. Inoculum combined the original iron/sulfur-oxidizing consortium with leachate from a preceding 30-degree-C flask screen. Inoculum volume and cell density are unspecified, and the concentrate was not sterilized. The abstract says 13 initial species whereas section 2.6 lists 12 named members; this discrepancy is not silently resolved. The two organisms below were significant culturable endpoint members, not the complete initial community or individually demonstrated causal agents.
Curated Evidence Points:
Support
Source:
johnson2020
Sections 2.2-2.3 and 3.1, Table 1 and supplementary Figures S1-S2 identify skutterudite as the most abundant concentrate phase by XRD and SEM-EDX, with loellingite, minor silicates, trace cobaltite, bismuth arsenide, calcite and mixed-metal arsenate. Sparse nickelskutterudite grains were also identified. Table 1 reports 91.9 g/kg cobalt in the whole concentrate. These phase and elemental measurements support skutterudite-bearing mixed-feed identity, not an absolutely pure mineral or an initial refined phase mass balance.
Support
Source:
johnson2020
Section 3.3, printed pages 4-5 and Figure 3, reports 49% of the concentrate cobalt solubilized after 16 days in experiment 4. This is a bulk-feed solution fraction; no isolated cobalt product or phase-specific cobalt balance is presented. Section 3.5 and section 2.6 describe culture-based enumeration with colony 16S identification: Acidithiobacillus thiooxidans and Sulfobacillus thermosulfidooxidans were significant viable members after 16 days. This differs from the genus-level sequencing of the separate Iron Mask ore and does not identify every organism or a strain-specific contribution in the Bou Azzer system.
Partial
Source:
johnson2020
Section 2.5 describes one Bou Azzer bioreactor trial, while Figure 3's caption refers ambiguously to replicate bioreactors and shows one series without replicate uncertainty. Biological replication is therefore unresolved; section 2.8's three ICP measurements per sample are analytical repeats. No matched uninoculated, killed-cell or abiotic leach control is described. The nonsterile concentrate and transferred screening leachate also limit attribution to the originally named inoculum. Ferric-mediated oxidation, sulfur-derived acidity and individual microbial roles remain proposed rather than separately isolated mechanisms in this assay.
Partial
Source:
johnson2020_supplement
Supplementary Figure S1 retains skutterudite peaks in leached Bou Azzer material; S2 and S5 show mixed residual arsenides and new iron-arsenate precipitates. No complete skutterudite disappearance or mineral-specific conversion fraction is demonstrated. The supplementary XRD labels use Bou Azzer experiment 1/2, whereas the main methods number its reactor/flask experiments 4/5; this numbering mismatch limits exact label-level crosswalks and is not treated as proof of an additional independent experiment.
Support
Source:
johnson2020
Section 2.5 experiment 5 pre-established an active culture with 5% w/v sterile pyrite and 5 mM ferrous sulfate for five days before adding 1% w/v nonsterile concentrate. Sections 3.3-3.4 and Figures 4-7 identify scorodite in the final flask residue using XRD, FTIR and SEM-EDX; it was not identified in the original concentrate or non-pyrite reactor residue by those analyses. The original trace Co-Ni-Ca-rich arsenate was compositionally distinguished from the new iron arsenate, also illustrated in supplementary Figure S5. This is concurrent secondary precipitation during mixed-feed processing, not a separate replication or a purified product yield.
Partial
Source:
johnson2020
The pyrite flask and non-pyrite reactor differ in geometry, conditioning and operating conditions, so they do not isolate a pyrite treatment effect. The specific preconditioning recipe does not separately state a zero-valent-sulfur dose. Soluble arsenic and iron decline during the flask experiment, and iron can also derive from added pyrite; the authors therefore cannot calculate total cumulative leaching accurately. Discussion and Figures 8-9 infer arsenic oxidation states from redox chemistry and propose pyrite's catalytic, nucleation and iron-supply roles. Leachate As(III)/As(V) was not directly measured; the separate arsenite/arsenate titration is not a matched leach control. Scorodite stability, detoxification, isolated-metal recovery and industrial deployment were not tested.
Sources & References
-
ima_2026_09: IMA-CNMNC (September 2026). The New IMA List of Minerals. PDF page 201, Skutterudite row; page 1, status and year definitions.
https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-15) -
handbook_skutterudite: Handbook of Mineralogy. Skutterudite, CoAs2-3. One-page entry, symmetry, composition, powder pattern and relation to nickelskutterudite.
https://www.handbookofmineralogy.org/pdfs/skutterudite.pdf (accessed 2026-10-15) -
johnson2020: Johnson DB, Dybowska A, Schofield PF, Herrington RJ, Smith SL and Santos AL (2020). Bioleaching of arsenic-rich cobalt mineral resources, and evidence for concurrent biomineralisation of scorodite during oxidative bio-processing of skutterudite. Hydrometallurgy 195, 105395. Complete version of record, sections 2.2-2.8 and 3.1, 3.3-3.5, Table 1 and Figures 3-9; official supplementary Figures S1-S5.
https://doi.org/10.1016/j.hydromet.2020.105395 (accessed 2026-10-15) -
johnson2020_supplement: Johnson et al. (2020), official Supplementary Data DOCX. Figures S1-S5, original diffraction profiles, SEM-EDX maps and captions. S3-S4 concern Iron Mask ore and are not transferred to the Bou Azzer experiment.
https://ars.els-cdn.com/content/image/1-s2.0-S0304386X20302851-mmc1.docx (accessed 2026-10-15) -
ncbi_thiooxidans: NCBI Taxonomy, Acidithiobacillus thiooxidans, species taxon 930. Species-level grounding only; no strain equivalence is inferred.
https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=930 (accessed 2026-10-15) -
ncbi_thermosulfidooxidans: NCBI Taxonomy, Sulfobacillus thermosulfidooxidans, species taxon 28034. Species-level grounding only; no strain equivalence is inferred.
https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=28034 (accessed 2026-10-15)
Curation History
| Timestamp | Curator | Action | Summary |
|---|---|---|---|
2026-10-15T08:32:54.424781+00:00 |
codex (GPT-6; codex) | CREATE | Created IMA-grounded skutterudite with complete Johnson2020 main and official supplement evidence for microbial processing of mixed natural concentrate; preserved phase, control, replication and speciation limits. |
Record source: data/records/skutterudite.yaml