AbstractThe new mineral campostriniite, (Bi³⁺,Na)₃(NH₄,K)₂Na₂(SO₄)₆·H₂O, was found in an active fumarole (fumarole FA, temperature ∼350°C) at La Fossa Crater, Vulcano, Aeolian Islands, Sicily, Italy. It occurs on a pyroclastic breccia as white prismatic crystals up to 0.2 mm long, in association with adranosite, demicheleite-(Br), demicheleite-(I), argesite and sassolite. The mineral is monoclinic, space group: C2/c (no. 15) with a = 17.748(3), b = 6.982(1) c = 18.221(3) Å, β = 113.97(1)°, V = 2063(1) ų and Z = 4. The six strongest reflections in the powder X-ray diffraction pattern are: [dobs in Å (I)(hkl)] 6.396(100)(110), 7.507(75)(202), 2.766(60)(316), 3.380(57)(312), 5.677(55)(111), 3.166(50)(4 0 2). The empirical formula (based on 25 anions p.f.u.) is Bi_2.41N_1.52Na_2.41K_0.48 S_6.07H_8.08O₂₅. The calculated density is 3.87 g cm^–3. Using single-crystal diffraction data, the structure was refined to a final R(F) = 0.051 for 3025 independent observed reflections [I > 2σ(I)]. Campostriniite is isostructural with görgeyite and belongs to the 7.CD group of the Strunz classification system. The structure contains two independent nine-fold coordinated sites, one of them located on a two-fold axis (M1) and the other one in general position (M2) essentially occupied by Bi³⁺ atoms and minor amounts of Na⁺ ions, an eight-fold coordinated site fully occupied by Na +ions and another eight-fold coordinated site occupied by NH⁺₄ and K⁺ ions; three independent sulfate anions in a general position and a water molecule coordinated to M1 and located on a two-fold axis are also present.