Original paper

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

Sharygin, Victor V.; Pekov, Igor V.; Zubkova, Natalia V.; Khomyakov, Alexander P.; Stoppa, Francesco; Pushcharovsky, Dmitry Yu.

Abstract

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 ?m), typically flattened on {010}, and sheaf-like aggregates (up to 200?500 ?m 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 (K6.45Na0.35(Sr, Ba)0.01)? 6.81(Na1.22Ca0.78)? 2.00(Ca1.85Mg0.13Mn0.01Ti0.01)? 2.00[(Fe3+ 0.34Al3.06Si9.60)?13.00O29.00]F2.05Cl1.91(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 cm1. 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) Å3, 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 > 2?(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.

Keywords

crystal structuredelhayelitehillesheimiteitalykamafugitemelilitolitenew mineralnterblassitephyllosilicatepian di celle volcanoumbrianite