Gritsenkoite, CoMg2(AsO4)2(H2O)8, a new cation-ordered arsenate member of the vivianite group from the Khovu-Aksy deposit, Eastern Siberia, Russia

Abstract Gritsenkoite, ideally CoMg2(AsO4)2(H2O)8, is a new mineral (IMA 2025-052) that was first discovered at the Khovu-Aksy deposit, Eastern Siberia, Russia. It occurs as bladed crystals, up to 1 mm long, composing divergent groups on a calcite and quartz matrix. Other associated minerals include chalcopyrite, hollandite, nickelskutterudite, skutterudite, and talmessite. The new mineral is reddish-pink, transparent, with a white streak and a vitreous lustre. It is brittle, with an irregular fracture. The cleavage is perfect on {010}, good on {100}. Mohs’ hardness is ∼2½. Dmeas = 2.88(2) g cm–3 and Dcalc = 2.829 g cm–3. Optically, gritsenkoite is biaxial (+), α = 1.607(1), β = 1.617(1), γ = 1.632(1) (589 nm). 2Vmeas = 80.5° and 2Vcalc = 79.1°. Pleochroism is distinct: X∼Y 3σ(I). Gritsenkoite was also discovered in a specimen from East Oumlil Mine, Bou-Azzer District, Morocco. Its empirical formula is (Mg1.54Co1.42Mn0.02Ni0.02)Σ3.00(As1.93P0.07)Σ2.00O8(H2O)8 and its structure was refined from single-crystal X-ray diffraction data at 100K to R1 = 2.25% for 1529 reflections with I > 3σ(I). Gritsenkoite is a new, cation-ordered arsenate member of the vivianite group, with Co dominant at M1 and Mg dominant at M2.