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

Abstract

The new mineral umbrianite, ideally K 7 Na 2 Ca 2 [Al 3 Si 10 O 29 ]F 2 Cl 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, gotzenite, 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 ) ∑6.81 (Na 1.22 Ca 0.78 ) ∑2.00 (Ca 1.85 Mg 0.13 Mn 0.01 Ti 0.01 ) ∑2.00 [(Fe 3+ 0.34 Al 3.06 Si 9.60 ) ∑13.00 O 29.00 ]F 2.05 Cl 1.91 (OH) 0.04 . The strongest lines of the X-ray diffraction powder pattern {d[A] ( 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) A, V = 1783.5(2) A 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) 4 Si 16 O 58 ] ∞ connected to each other via the columns of edge-shared octahedra CaO 5 F to form a 3D quasi-framework with channels filled by Cl − , K + (inside the tetrahedral blocks) and Na + (between the Ca octahedral columns). Umbrianite, gunterblassite and hillesheimite, containing topologically identical triple-layer tetrahedral blocks, form the gunterblassite group. Umbrianite is unstable under postmagmatic hydrothermal conditions and alters to Ba-rich hydrated phases.