Umbrianite, K_7Na_2Ca_2[Al_3Si_(10)O_(29)]F_2Cl_2, a new mineral species from melilitolite of the Pian di Celle volcano, Umbria, Italy

摘要

The new mineral umbrianite, ideally K_7Na_2Ca_2[Al_3Si_(10)O_(29)]F_2Cl_2, 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, g?tzenite, khibinskite, monticellite-kirschsteinite series minerals, westerveldite, various sulphides and peralkaline silicate glass. The empirical formula (based on Si + Al + Fe~(3+) = 13) of the holotype umbrianite (mean of 58 analyses) is (K_(6.45)Na_(0.35)(Sr,Ba)_(0.01))_(Σ6.81)(Na_(1.22)Ca_(0.78))_(Σ2.00)(Ca_(1.85)Mg_(0.13)Mn_(0.01)Ti_(0.01))_(Σ2.00)[(Fe~(3+)_(0.34)Al_(3.06)Si_(9.60))_(Σ13.00)O_(29.00)]F_(2.05)Cl_(1.91)(OH)_(0.04). The strongest lines of the X-ray diffraction powder pattern {d[? ] (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) ?, V = 1783.5(2) ?~3, Z = 2. The calculated density is 2.49 g/cm~3. 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 [Al_4(Si,Al)_2(Si,Al,Fe)_4Si_(16)O_(58)]~∞ 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 Nat (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.