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Fetiasite mineral

Anhydrous iron(II,III)-titanium pyroarsenite (diarsenite) mineral with current September 2026 IMA-CNMNC approved formula (Fe2+,Fe3+,Ti)3O2(As3+2O5), status A, and approval number IMA 1991-019. Graeser, Schwander, Demartin, Gramaccioli, Pilati, and Reusser (1994) described cotype specimens (deposited in the Natural History Museum, Basel, and the Mineralogical Institute, University of Basel) from Alpine fissures in two-mica gneisses of the Monte Leone nappe at Pizzo Cervandone (~3000 m, Alpe Devero, Novara Province, Piedmont, Italy; Locality A) and the Lercheltini-Zone at Gorb, Binntal (~2000 m, Canton Valais, Switzerland; Localities B and C), where it coexists with quartz, magnetite, hematite, rutile, anatase, cafarsite, asbecasite, tilasite, and chernovite-(Y). ARL electron-microprobe analyses (Table 3, p. 999, with Fe2+/Fe3+ = 1.5 fixed by single-crystal charge balance and confirmed by wet-chemical KMnO4 titration of FeO) yielded for Locality A (mean of 7 analyses): As2O3 46.95, FeO 23.12, Fe2O3 17.13, TiO2 10.09, MnO 1.25, total 98.54 wt%, empirical formula (Fe2+1.38Fe3+0.92Ti0.54Mn0.08)Sigma2.92O2[As2O5]; and for Locality B (mean of 10 analyses): As2O3 46.76, FeO 24.23, Fe2O3 17.93, TiO2 11.17, MnO 0.89, total 100.98 wt%, empirical formula (Fe2+1.40Fe3+0.93Ti0.58Mn0.05)Sigma2.96O2[As2O5] (close to ideal Fe2+1.4Fe3+1.0Ti4+0.6O2As2O5). Single-crystal CAD-4 X-ray diffraction refinements in monoclinic space group P21/m (Z = 2, Tables 5-8, pp. 999-1001) gave a = 10.614(2), b = 3.252(1), c = 8.945(1) A, beta = 108.95(2) degrees, V = 291.9(2) A3, R = 0.029, wR = 0.036 for 1168 reflections, and Dcalc = 4.74 g/cm3 on Locality A, and a = 10.616(1), b = 3.242(1), c = 8.932(1) A, beta = 108.89(2) degrees, V = 290.9(2) A3, R = 0.023, wR = 0.030 for 1200 reflections, and Dcalc = 4.80 g/cm3 on Locality B (Dmeas = 4.6(1) g/cm3 by pycnometry on Locality C; VHN50 = 475 kg/mm2, Mohs hardness ~5), revealing a novel octahedral framework of three independent M(1)-M(3) (Fe,Ti)O6 sites linked by corner-sharing trigonal-pyramidal [As2O5]4- diarsenite groups with channels hosting As3+ lone-pair electrons. Distinct from paulmooreite (Pb2As2O5), schneiderhohnite, cafarsite, and elemental arsenic, titanium, or manganese; no criticality designation, microbial mechanism, or industrial recovery yield is asserted.

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Identifiers & Classification

System Identifier Label
IMA IMA:1991-019 Fetiasite

Sources & References

  1. ima_2026_09: IMA-CNMNC, The New IMA List of Minerals, September 2026, PDF page 73, Fetiasite row: formula (Fe2+,Fe3+,Ti)3O2(As3+2O5), status A, IMA No. 1991-019, country Italy / Switzerland, reference American Mineralogist 79 (1994), 996; column definitions on PDF page 1.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-23)
  2. graeser_1994_fetiasite: Graeser S, Schwander H, Demartin F, Gramaccioli CM, Pilati T, Reusser E (1994), Fetiasite (Fe2+,Fe3+,Ti)3O2[As2O5], a new arsenite mineral: its description and structure determination, American Mineralogist 79:996-1002. Complete 7-page primary PDF visually inspected: Pizzo Cervandone (Locality A) and Binntal (Localities B-C) occurrences and reflectance Tables 1-2 pp. 996-998; ARL EMPA chemistry Table 3 and powder XRD Table 4 pp. 998-999; monoclinic P21/m single-crystal refinements Tables 5-8 and Figure 3 pp. 999-1002.
    https://www.rruff.net/doclib/am/vol79/AM79_996.pdf (accessed 2026-10-23)

Curation History

Timestamp Curator Action Summary
2026-10-23T07:15:00Z antigravity CREATE Created fetiasite mineral record with verified IMA:1991-019 identity, Pizzo Cervandone and Binntal cotype EMPA chemistry, and monoclinic P21/m pyroarsenite crystal structure from the complete 1994 primary study.

Record source: data/records/fetiasite.yaml