The new mineral gajardoite-(NH4), ideally (NH4)As3+4O6Cl2[(Ca0.5□0.5)(H2O)5], is found at the Khovu-Aksy Ni-Co deposit, Republic of Tyva, Russia. Gajardoite-(NH4) occurs as tiny lamellar coarsely hexagonal or irregular curved and divergent crystals up to 0.01 mm in size. The crystals are combined in groups, rosette-like clusters or spherulitic aggregates up to 0.2 mm, which are intimately intergrown with annabergite, arsenolite, and pharmacolite on a matrix of skutterudite, safforite, and other minerals. The new mineral is colorless, white in aggregates, transparent with a white streak and a vitreous lustre. It is brittle, with a perfect cleavage on {001}. The Mohs hardness is ~1½. The calculated density (Dcalc ) is 2.583 g/cm3.Gajardoite-(NH4) is optically non-pleochroic, uniaxial (–), ω = 1.745(10), ε = 1.558(5) (589 nm). The chemical composition determined by electron microprobe (wt. %, H2O content calculated by stoichiometry) is as follows: (NH4)2O 3.17, Na2O 0.40, K2O 1.07, CaO 5.28, As2O3 67.25, Cl 12.21, H2O 15.30, O=Cl –2.76, total 101.92. The empirical formula based on four As and 11 O atoms per formula unit is [(NH4)0.72K0.13Na0.08]Σ0.93Ca0.55As3+4O6Cl2.03(H2O)5. Gajardoite-(NH4) is hexagonal, space group P6/mmm; the unit-cell parameters are as follows: a = 5.263(3), c = 16.078(5) Å, V = 385.8(5) Å3, Z = 1. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] include 16.08 (34) (001), 5.36 (34) (003), 4.565 (41) (100), 3.466 (23) (103), 2.637 (100) (110), 2.360 (25) (113). Gajardoite-(NH4) is an ammonium analog of gajardoite KCa0.5As3+4O6Cl2 • 5H2O. Their structural identity is confrmed by powder X-ray diffraction and infrared and Raman spectroscopy.

Keywords: gajardoite-(NH4), new mineral, chemical composition, powder X-ray diffraction, IR spectrum, Raman spectrum, gajardoite, Khovu-Aksy deposit, Eastern Siberia.

Funding. The IR spectroscopy study and the interpretation of the Raman spectrum were supported by state contract no. FFSG-2024-0009.

Acknowledgements. We thank Elena V. Belogub and Irina Yu. Melekestseva for their constructive comments that allowed us to improve the manuscript. Maria D. Milshina and Yulia D. Gritsenko are acknowledged for their help with photography and useful information on the Khovu-Aksy deposit, respectively.

Confict of interest. The authors declare that they have no conficts of interest.

Author contribution. A.V. Kasatkin – conceptualization, investigation, visualization, writing – review & editing; A.V. Kasatkin, V.V. Gurzhiy, N.V. Chukanov, A.A. Agakhanov, R. Škoda, D.I. Belakovskiy – analytical/experimental works; A.V. Kasatkin, V. V. Gurzhiy, N.V. Chukanov – writing – original draft. All the authors approved the fnal version of the manuscript prior to publication.

For citation: Kasatkin A.V., Gurzhiy V.V., Chukanov N.V., Agakhanov A.A., Škoda R., Belakovskiy D.I. Gajardoite-(NH₄), (NH₄)As³⁺₄O₆C_l2[(Ca_0.5□_0.5)(H₂O)₅], a new mineral from the Khovu-Aksy deposit, Eastern Siberia, Russia. Mineralogy, 10 (1), 5–15. DOI: 10.35597/2313-545X-2024-10-1-1.

Received 07.01.2024, revised 26.01.2024, accepted 10.02.2024

A.V. Kasatkin, Fersman Mineralogical Museum RAS, Leninskiy pr. 18/2, Moscow, 119071 Russia;  anatoly.kasatkin@gmail.com

V.V. Gurzhiy, Institute of Earth Sciences, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russia;

N.V. Chukanov, Institute of Earth Sciences, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russia;

A.A. Agakhanov, Fersman Mineralogical Museum RAS, Leninskiy pr. 18/2, Moscow, 119071 Russia;

R. Škoda, Masaryk University, Kotlářská 2, Brno, 611 37 Czech Republic;

D.I. Belakovskiy, Fersman Mineralogical Museum RAS, Leninskiy pr. 18/2, Moscow, 119071 Russia

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