Umbrianite, K7Na2Ca2[Al3Si10O29]F2Cl2, a new mineral species from melilitolite of the Pian di Celle volcano, Umbria, Italy

The new mineral umbrianite, ideally K₇Na₂Ca₂[Al₃Si₁₀O₂₉]F₂Cl₂, was discovered as an essential groundmass mineral in melilitolite of the Pian di Celle volcano, Umbria, Italy. It forms rectangular, lamellar or lath-shaped crystals (up to 25 × 30 × 200 μm), typically flattened on {010}, and sheaf-like aggregates (up to 200–500 μm across). Umbrianite is commonly associated with kalsilite, leucite, fluorophlogopite, melilite, olivine (Fo_>60), diopside, nepheline, Ti-rich magnetite, fluorapatite, cuspidine–hiortdahlite series minerals, götzenite, khibinskite, monticellite–kirschsteinite series minerals, westerveldite, various sulphides and peralkaline silicate glass. The empirical formula (based on Si + Al + Fe³⁺ = 13) of the holotype umbrianite (mean of 58 analyses) is (K_6.45Na_0.35(Sr,Ba)_0.01)_∑6.81(Na_1.22Ca_0.78)_∑2.00(Ca_1.85Mg_0.13Mn_0.01Ti_0.01)_∑2.00[(Fe³⁺_0.34Al_3.06Si_9.60)_∑13.00O_29.00]F_2.05Cl_1.91(OH)_0.04. The strongest lines of the X-ray diffraction powder pattern {d[Å] (I_obs)} are: 9.65(100), 6.59(97), 3.296(77), 3.118(70), 2.819(53), 2.903(52), 6.91(43). The strong bands in the Raman spectrum of umbrianite are at 525, 593, 735 and 1036 cm⁻¹. The mineral is orthorhombic, space group Pmmn, unit-cell parameters are: a = 7.0618(5), b = 38.420(2), c = 6.5734(4) Å, V = 1783.5(2) ų, Z = 2. The calculated density is 2.49 g/cm³. The crystal structure of umbrianite has been refined from X-ray single-crystal data to R = 0.0941 for 1372 independent reflections with I > 2σ(I). Umbrianite is a representative of a new structure type. Its crystal structure contains the triple-layer tetrahedral blocks [Al₄(Si,Al)₂(Si,Al,Fe)₄Si₁₆O₅₈]^∞ connected to each other via the columns of edge-shared octahedra CaO₅F to form a 3D quasi-framework with channels filled by Cl⁻, K⁺ (inside the tetrahedral blocks) and Na⁺ (between the Ca octahedral columns). Umbrianite, günterblassite and hillesheimite, containing topologically identical triple-layer tetrahedral blocks, form the günterblassite group. Umbrianite is unstable under postmagmatic hydrothermal conditions and alters to Ba-rich hydrated phases.