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Alunite mineral

Alunite is the potassium aluminium hydroxysulfate mineral KAl3(SO4)2(OH)6, listed by IMA-CNMNC in September 2026 as redefined, 1987 s.p. It is a distinct species within the alunite group, not a label for every aluminium sulfate or mixed mining residue. Zhong et al. (2019) examined a natural alunite-bearing ore containing quartz and kaolinite. Their laboratory CaO-assisted thermal treatment followed by water leaching increased soluble potassium under selected conditions; the separate evaporation product remained a mixture of potassium/calcium sulfate phases. The accepted experiment is abiotic and does not establish microbial extraction, a pure potassium product, complete aluminium recovery, or industrial deployment. No constituent-element criticality is inherited.

Microbial & Biochemical Mechanisms (1)

CaO-assisted thermal decomposition followed by water leaching

recovery

Thermal treatment of a mixed alunite-bearing ore with CaO increased the fraction of initial ore potassium measured in the aqueous leachate at selected temperature and reagent ratios. Diffraction and spectroscopy support transformations to soluble and insoluble sulfate-containing phases, with aluminium coextraction dependent on conditions. The experiment measures soluble potassium relative to initial ore potassium, not a mineral-specific intrinsic rate or isolated-product yield.

Experimental Context & Analytical Limits:
Ore supplied by Zijin Mining Group in Fujian, China, was ground below 74 micrometres; the study reports 7.49 wt% K and 17.91 wt% Al. Ore and analytical CaO were dried at 110 degrees C for 24 hours. Batches used 1.0 g ore and CaO/ore mass ratios 0, 0.1, 0.2, 0.4 or 0.6, pressed at 8 MPa, then heated under high-purity nitrogen at 600-900 degrees C for two hours. Cooled material was crushed and sieved below 74 micrometres before one-hour deionized-water leaching at 30 degrees C and solid/liquid ratio 1:10 g/mL. Potassium and aluminium in filtrate were measured by ICP-AES. Leach pH, independent replicate count and error bars are not reported. A separate product experiment used CaO/ore 0.2, 700 degrees C for two hours and evaporation at 80 degrees C; its product is not assigned the 0.1-ratio optimum's leaching percentage. Figure 7 microscopy used a distinct 800-degree, 30-minute, 0.4-ratio specimen; it is not the common two-hour assay. No biological component was involved.
Curated Evidence Points:
Figure S1 identifies alunite together with quartz and kaolinite in the feed. The supplementary oxide table reports K2O 8.93 wt% and Al2O3 33.84 wt%, consistent with the elemental K/Al proportions stated in the main methods. These bulk analyses describe a mixed ore, not a phase-pure alunite composition.
Methods section 2.2 and equations 1-2 on pages 2-3 define soluble recovery relative to initial ore element inventory. Figure 1 and section 3.1 on page 4 report approximately 81% potassium in solution after the selected 700-degree, two-hour, CaO/ore 0.1 treatment, compared with approximately 27% stated for the no-CaO arm at that temperature. Higher-temperature arms differ: an improvement at 700 degrees does not establish a universal CaO benefit. Figure 2 shows less than about 1% aluminium leached at ratios below 0.2, with greater aluminium coextraction at higher ratios and temperatures.
Figure 4 on page 6 identifies a mixed crystallized product assigned K2SO4, CaSO4 and K2Ca2(SO4)3, while Table 1 on page 5 gives 40.42 wt% K2O equivalent. This is not 40.42 wt% elemental potassium, a K2SO4 purity, or the isolated mass yield of the separate 81% leaching arm. The paper calls K2Ca2(SO4)3 syngenite; the phase name and written formula are not treated here as a resolved mineral identification. Product suitability and overall potassium mass balance were not independently demonstrated.
Figures 3 and 5-8 support condition-dependent phase and spectral changes, but the proposed Ca diffusion, bond cleavage and sulfur-capture pathway is not a direct atomistic or closed gas-balance measurement. The abstract and conclusion invert tetrahedral/octahedral labels for AlO6 and AlO4 relative to section 3.2.2. Figure 6 displays multiple CaO ratios despite a caption specifying 0.4. Figure 7 and Table 2 use local EDS compositions and a separate short heating treatment; their proposed unreacted-core interpretation is not quantitative phase analysis. These inconsistencies limit detailed mechanistic reconstruction without negating the narrower soluble-potassium comparison.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026. Alunite row, PDF page 9, KAl3(SO4)2(OH)6, Rd, 1987 s.p.; definitions page 1.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-15)
  2. handbook_alunite: Handbook of Mineralogy. Alunite, complete one-page identity, natural chemistry and structure entry; solid-solution context with natroalunite.
    https://handbookofmineralogy.org/pdfs/alunite.pdf (accessed 2026-10-15)
  3. zhong2019: Zhong Y, Qiu X, Gao J, Meng L, Guo Z (2019). Recovery of Soluble Potassium from Alunite by Thermal Decomposition: Effect of CaO and Phase Transformation. Metals 9:337. Current thirteen-page v2 publisher original, sections 2-4, Figures 1-8 and Tables 1-2 inspected.
    https://doi.org/10.3390/met9030337 (accessed 2026-10-15)
  4. zhong2019_fulltext: Complete current v2 publisher PDF of Zhong2019. Older fourteen-page PDF retained separately as an earlier file version; v2 page locators are used here.
    https://mdpi-res.com/metals/metals-09-00337/article_deploy/metals-09-00337-v2.pdf (accessed 2026-10-15)
  5. zhong2019_si: Official one-page supplement to Zhong2019: Figure S1 mixed-feed diffraction and oxide-analysis table printed Table 1, announced as Table S1 in the main paper.
    https://mdpi-res.com/d_attachment/metals/metals-09-00337/article_deploy/metals-09-00337-s001.pdf?version=1553598414 (accessed 2026-10-15)

Curation History

Timestamp Curator Action Summary
2026-10-15T12:44:57.814050+00:00 codex (GPT-6; codex) CREATE Added mineral identity and qualified primary abiotic transformation evidence, preserving material provenance, controls, analytical denominators and source limitations; no microbial or inherited criticality claim.

Record source: data/records/alunite.yaml