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Rhomboclase mineral

Rhomboclase is an orthorhombic acidic ferric sulfate hydrate mineral, (H5O2)Fe(III)(SO4)2·2H2O, recognized as a grandfathered species in the September 2026 IMA list. It is distinct from generic ferric sulfate solutions and other sulfate hydrates. The accepted original supports abiotic evaporative preparation and acid-solution calorimetry of a structurally characterized synthetic sample with excess sulfate and protons relative to ideal stoichiometry. No microbial mechanism or isolated metal-recovery yield is established.

Microbial & Biochemical Mechanisms (2)

Rhomboclase crystallization from an acidic ferric sulfate solution

other

Slow room-temperature evaporation of an acidic ferric sulfate solution produced a pale yellow rhomboclase sample. Powder diffraction and refinement support phase identity, while chemical analysis indicates a composition different from the ideal formula.

Experimental Context & Analytical Limits:
Materials and methods, printed page 176 (PDF page 2), mixes 2.5 g hydrated ferric sulfate, 5 mL deionized water and 1.36 mL sulfuric acid (96% by mass, density 1.84 g/cm3). The specified ferric sulfate was the first, X-ray-amorphous supplier batch with about 21% water. The solution evaporated slowly at room temperature. Evaporation duration, isolated-product mass, synthesis replicate count and an absolute phase-purity estimate are not reported for this preparation. No organism or biomass was used.
Curated Evidence Points:
The preparation appears on printed page 176 (PDF page 2). Table 1, printed page 177 (PDF page 3), reports Pnma; Table 2, printed page 178 (PDF page 4), supplies the refined atomic coordinates and disordered interlayer oxygen occupancies. Table 5, printed page 179 (PDF page 5), reports refinement statistics. These results identify a synthetic rhomboclase sample without quantifying a recovered iron product.
Table 8 and Chemical analysis, printed page 181 (PDF page 7), derive (H3O)1.34Fe(SO4)2.17(H2O)3.06, rather than ideal (H3O)Fe(SO4)2(H2O)3, which is compositionally equivalent to the authority formula (H5O2)Fe(SO4)2·2H2O. The authors propose that excess sulfate and protons could be accommodated in the interlayer; their argument against trapped free acid is not a direct proof of the proposed accommodation mechanism.
Materials and methods, printed page 176 (PDF page 2), distinguishes a second commercial ferric sulfate batch with a copiapite-group phase, minor kornelite, coarser grains and about 28% water, with which not all syntheses succeeded. The supplier label therefore does not establish interchangeable starting material. The study does not report a rhomboclase yield comparison between the batches.

Sample-specific dissolution in an acid calorimetric cycle

other

Rhomboclase dissolution in an acid-solution calorimetric cycle at 298.15 K supplied a sample-specific heat measurement. This is thermochemical evidence rather than a quantified mineral-leaching recovery, kinetic assay or microbial transformation.

Experimental Context & Analytical Limits:
Methods, printed page 179 (PDF page 5), uses an IMC-4400 calorimeter and 25 mL standardized 5 N HCl. Pellets across the studied materials weighed 12-18 mg, chosen for thermochemical stoichiometry; the source does not specify a single rhomboclase loading. MgO was dissolved first, followed by the ferric sulfate sample in the same solvent. Samples were stored at 295 K and 55% relative humidity; daily XRD and weight-loss checks found no changes during the experiments, without establishing indefinite stability or a reported overall storage duration.
Curated Evidence Points:
Thermochemistry and Table 10, printed page 182 (PDF page 8), define the cycle. Table 11, printed page 183 (PDF page 9), reports delta H6 = +15.07 ± 0.16 kJ/mol from seven measurements; the uncertainty is two standard deviations of the mean. The molar basis is the analyzed Table 8 sample formula (H3O)1.34Fe(SO4)2.17(H2O)3.06, not one mole of iron or an ideal mineral endmember. Repeated calorimetry does not establish repeated independent syntheses.
Discussion, printed pages 184-185 (PDF pages 10-11), and Figure 1 use estimated entropy, calculated Gibbs energy and specified activities to model phase fields. The ferricopiapite-rhomboclase boundary changes under sulfate versus bisulfate assumptions and the plotted boundary is described as metastable. It is not an experimentally measured field pH limit. No biological comparator, metal-recovery mass balance or industrial deployment is demonstrated.

Sources & References

  1. ima_2026_09: IMA-CNMNC. The New IMA List of Minerals, September 2026; Rhomboclase row on PDF page 185 and definitions on pages 1-2. Formula (H5O2)Fe3+(SO4)2·2H2O; status G, original-description year 1891, not an IMA approval number.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-15)
  2. handbook: Handbook of Mineralogy. Rhomboclase, version 1 (2001-2005), complete one-page entry; orthorhombic Pnma and acidic ferric sulfate hydrate.
    https://handbookofmineralogy.org/pdfs/rhomboclase.pdf (accessed 2026-10-15)
  3. majzlan2006: Majzlan, J., Navrotsky, A., McCleskey, R. B. and Alpers, C. N. (2006). Thermodynamic properties and crystal structure refinement of ferricopiapite, coquimbite, rhomboclase, and Fe2(SO4)3(H2O)5. European Journal of Mineralogy 18(2), 175-186. DOI 10.1127/0935-1221/2006/0018-0175. Complete 12-page original, Tables 1-14 and Figure 1 inspected.
    https://doi.org/10.1127/0935-1221/2006/0018-0175 (accessed 2026-10-15)

Curation History

Timestamp Curator Action Summary
2026-10-15T13:54:49Z codex (GPT-6; codex) CREATE Created rhomboclase from current authorities and the complete abiotic synthesis and calorimetry original, retaining nonideal composition, unreported synthesis yield and sample-specific thermochemical scope.

Record source: data/records/rhomboclase.yaml