Abstract: Arsenowagnerite-Mabc, a new polytype, occurs in association with ponomarevite, piypite, sylvite, dolerophanite, euchlorine, lammerite, johillerite, urusovite, bradaczekite, filatovite, hatertite, hematite, tenorite, wrightite, ozerovaite in the fumarole on the east side of the micrograben of the Second scoria cone of Great fissure Tolbachik eruption. The chemical composition (wt %) is MgO 41.12, CaO 0.15, As₂O₅ 52.03, V₂O₅ 0.93, P₂O₅ 0.20, SO₃ 0.18, F 8.99, –O=F 4.16, total 99.45%. The empirical formula calculated on the basis of 5 (O + F) pfu is (Mg_2.05Ca_0.01)_∑2.06(As_0.91V_0.02P_0.01)_∑0.95O₄F_0.95. The crystal structure of arsenowagnerite-Mabc was determined using single-crystal X-ray diffraction data; the space group is C2/c, a = 12.994(1) Å, b = 6.482(1) Å, c = 9.843(1) Å, β = 116.63(2)°, V = 741.1(2) ų, Z = 8, and R₁ = 5.5%.