Orpiment mineral
Orpiment is the monoclinic As2S3 mineral species, grandfathered in the September 2026 IMA list, whose original-description year is unspecified. It is distinct from elemental arsenic, realgar and amorphous As2S3. The observation below concerns an XRD-assigned secondary component of arsenopyrite leaching residues, not an isolated orpiment feed or recovered product. No criticality designation is inherited from arsenic.
Microbial & Biochemical Mechanisms (1)
Orpiment-assigned secondary phase during arsenopyrite culture leaching
redox transformation
Microbial
Associated Organisms:
Acidithiobacillus ferrooxidans (NCBITaxon:920)
During laboratory arsenopyrite leaching in Acidithiobacillus ferrooxidans culture, the authors assigned orpiment among the crystalline components of washed residual solids using XRD and Rietveld phase analysis. They proposed formation through ferric-mediated arsenopyrite oxidation in a system with biological iron oxidation. This is a culture-associated secondary-phase observation with an inferred chemical pathway, not a demonstration that cells are necessary for orpiment formation. It is the same Liu 2019 experiment curated for the distinct arsenopyrite owner; treating orpiment as its own material does not add an independent experiment.
Experimental Context & Analytical Limits:
The source used high-purity natural Yaogangxian, Hunan arsenopyrite, XRF composition Fe1.00As0.99S0.97 with trace Si, Co and Ni. Wet-milled particles were over 90% below 74 micrometres and loaded at 10 g/L. Cultures in 250 mL flasks contained 100 mL 9K medium plus 15 mL inoculum, at 30 degrees C and 160 rpm, initially pH 1.8 adjusted with approximately 3 M sulfuric acid. Medium per litre contained 44.8 g FeSO4.7H2O, 3.0 g (NH4)2SO4, 0.50 g K2HPO4.3H2O, 0.1 g KCl, 0.5 g MgSO4.7H2O and 0.01 g Ca(NO3)2.4H2O. Bacteria had been acclimated with arsenopyrite for three months, but strain accession and initial cell density are unspecified. Solids were filtered, washed at least ten times with deionized water and vacuum-dried overnight at 35 degrees C. Separate approximately 15 by 15 by 5 mm cubes were polished and cleaned with acid, methanol and water for microscopy, then fixed, rinsed and ethanol-dehydrated. The paper does not specify replicate count or error-bar definition, matched sterile or matched-pH kinetic controls, or a selective film-removal test. No orpiment-specific isolated yield, arsenic mass balance or industrial deployment is established.
The source used high-purity natural Yaogangxian, Hunan arsenopyrite, XRF composition Fe1.00As0.99S0.97 with trace Si, Co and Ni. Wet-milled particles were over 90% below 74 micrometres and loaded at 10 g/L. Cultures in 250 mL flasks contained 100 mL 9K medium plus 15 mL inoculum, at 30 degrees C and 160 rpm, initially pH 1.8 adjusted with approximately 3 M sulfuric acid. Medium per litre contained 44.8 g FeSO4.7H2O, 3.0 g (NH4)2SO4, 0.50 g K2HPO4.3H2O, 0.1 g KCl, 0.5 g MgSO4.7H2O and 0.01 g Ca(NO3)2.4H2O. Bacteria had been acclimated with arsenopyrite for three months, but strain accession and initial cell density are unspecified. Solids were filtered, washed at least ten times with deionized water and vacuum-dried overnight at 35 degrees C. Separate approximately 15 by 15 by 5 mm cubes were polished and cleaned with acid, methanol and water for microscopy, then fixed, rinsed and ethanol-dehydrated. The paper does not specify replicate count or error-bar definition, matched sterile or matched-pH kinetic controls, or a selective film-removal test. No orpiment-specific isolated yield, arsenic mass balance or industrial deployment is established.
Curated Evidence Points:
Support
Source:
liu2019
Sections 3.2-3.3 and 4.1.2, Figure 2a-b, identify orpiment alongside realgar, sulfur, jarosite and residual arsenopyrite in a time series including days 1, 3, 5, 9 and 19. The source describes early orpiment formation and declining relative content as leaching progresses. The normalized solid-phase proportions do not quantify absolute orpiment production or destruction. Source phase assignment is based on the diffraction analysis, not inferred here from As2S3 formula or EDS alone.
Partial
Source:
liu2019
Figures 4-5 and Table 1 show a dense As/S-rich early surface film on separate cubes, with cracks attributed to drying, and a later porous jarosite-associated overlayer. The proposed orpiment-containing passivating film combines particle XRD with cube morphology/EDS; it is not an orpiment-specific causal intervention. Figure 6 and Table 2 are a proposed pathway and thermodynamic reactions, not directly measured reaction fluxes. The 25 degrees C equilibrium calculation differs from the 30 degrees C culture and the authors acknowledge disagreement with early-stage phase observations. Neither a unique passivating role nor a microbial increment over abiotic orpiment formation is isolated.
Context_only
Source:
newman1997
The distinct Desulfotomaculum auripigmentum precipitation experiment is not accepted as crystalline orpiment evidence. Page 2023 explicitly reports electron diffraction showing an amorphous precipitate; elemental proportions and Figure 2 support As2S3 composition without establishing the orpiment crystal structure. Its microbial precipitation and aqueous arsenic-removal measurements are not transferred to this mineral-specific mechanism.
Sources & References
-
ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026. PDF page 160, Orpiment row; page 1 column definitions. Retained original rechecked.
https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-14) -
handbook_orpiment: Handbook of Mineralogy. Orpiment, version 1 (2001-2005), one-page species entry with monoclinic cell and X-ray powder data.
https://handbookofmineralogy.org/pdfs/orpiment.pdf (accessed 2026-10-14) -
liu2019: Liu, Li, Zhang, Yang, Xu and Jiang (2019). Formation Process of the Passivating Products from Arsenopyrite Bioleaching by Acidithiobacillus ferrooxidans in 9K Culture Medium. Metals 9, 1320. Complete original, sections 3 and 4, Figures 1-6, Tables 1-2 and Appendix A; previously acquired publisher PDF and XML rechecked.
https://doi.org/10.3390/met9121320 (accessed 2026-10-14) -
newman1997: Newman, Beveridge and Morel (1997). Precipitation of Arsenic Trisulfide by Desulfotomaculum auripigmentum. Applied and Environmental Microbiology 63, 2022-2028. Complete author-deposited original; Identification of precipitate, page 2023, and Figure 2 establish amorphous-product scope rather than crystalline orpiment.
https://doi.org/10.1128/aem.63.5.2022-2028.1997 (accessed 2026-10-14) -
ncbi_ferrooxidans: NCBI Taxonomy 920. Acidithiobacillus ferrooxidans, species-level entry; no experimental strain identifier is supplied by Liu et al.
https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=920&mode=Info (accessed 2026-10-14)
Curation History
| Timestamp | Curator | Action | Summary |
|---|---|---|---|
2026-10-14T13:09:55.723654+00:00 |
codex (GPT-6; codex) | CREATE | Created orpiment owner for shared-study secondary-phase evidence, retaining missing controls and excluding amorphous bacterial As2S3 as crystal-structure proof. |
Record source: data/records/orpiment.yaml