AbstractThe new mineral aluminopyracmonite, ideally (NH₄)₃Al(SO₄)₃, was found in a medium-temperature (∼250°C) intracrater active fumarole at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. It occurs on a pyroclastic breccia as colourless to white prismatic crystals up to 0.2 mm long, in association with adranosite, mascagnite, alunite and salammoniac. The mineral is identical to the synthetic compound (NH₄)₃Al(SO₄)₃. It is trigonal, space group: R (no. 148) with a = 15.0324(8), c = 8.8776(5) 5, V = 1737.3(2) ų and Z = 6. The six strongest reflections in the X-ray powder diffraction pattern are: [d_obs in Å _(I)(hkl)] 3.336(100)(131), 7.469(62)(1 1 0), 3.288(60)(122), 4.289(45)(31), 2.824(29)(51), 4.187(27) (012). The empirical formula based on 12 anions is [(NH₄)_2.89K_0.10]_{Σ 2.99}(Al_1.18Fe_0.01)_{Σ 1.19}S_2.91O₁₂, and the simplified formula (NH₄,K)₃Al(SO₄)₃. The measured density is 2.12(1) g/cm³, calculated density 2.143 g/cm³. The mineral is uniaxial(–) with ω = 1.545(3) and ε = 1.532(3) (λ = 589 nm). Using single-crystal diffraction data, the structure was refined to a final R(F) = 0.0258 for 998 independent observed reflections [I > 2σ(I)]. In spite of having unitcell parameters comparable with those of pyracmonite, the two minerals are not isostructural; the difference is related to a disordered conformation of the sulfate anions about the two independent Al³⁺ ions in aluminopyracmonite.