Lileyite, Ba2(Na,Fe,Ca)3MgTi2(Si2O7)2O2F2, a new lamprophyllite-group mineral from the Eifel volcanic area, Germany

Abstract

The new Mg- and F-dominant lamprophyllite-group mineral lileyite (IMA 2011-021) was found at the Lohley quarry, Udersdorf, near Daun, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany, and named for the old name of the type locality, Liley. Associated minerals are nepheline, leucite, augite, magnetite, fluorapatite, perovskite, gotzenite. Lileyite is brown, translucent; streak is white. It forms platy crystals up to 0.1 × 0.3 × 0.5 mm in size and their clusters up to 1 mm across on the walls of cavities in an alkaline basalt. Lileyite is brittle, with Mohs hardness of 3–4 and perfect cleavage on (001). D calc is 3.776 g/cm 3 . The new mineral is biaxial (+), α = 1.718(5), β = 1.735(5), γ = 1.755(5), 2V (meas.) = 75(15)°, 2V (calc.) = 86°. The IR spectrum is given. The chemical composition is (EDS-mode electron microprobe, mean of 5 analyses, wt%): SiO 2 28.05, BaO 26.39, TiO 2 18.53, Na 2 O 6.75, MgO 4.58, FeO 4.48, CaO 2.30, SrO 2.23, MnO 1.44, K 2 O 1.41, Nb 2 O 5 0.95, F 3.88, –O=F 2 -1.63; total 99.36. The empirical formula based on 18 anions is: Ba 1.50 Sr 0.19 K 0.26 Na 1.89 Ca 0.36 Mn 0.18 Mg 0.99 Fe 0.54 Ti 2.01 Nb 0.06 Si 4.06 O 16.23 F 1.77 . The simplified formula is: Ba 2 (Na,Fe,Ca) 3 MgTi 2 (Si 2 O 7 ) 2 O 2 F 2 . The crystal structure was solved using single-crystal X-ray diffraction data ( R = 0.024). Lileyite is monoclinic, space group C 2/ m , a = 19.905(1), b = 7.098(1), c = 5.405(1) A, β = 96.349(5)°, V = 758.93(6) A 3 , Z = 2. The strongest lines of the powder diffraction pattern [ d , A ( I , %) ( hkl )] are: 3.749 (45) (31–1), 3.464 (76) (510, 311, 401), 3.045 (37) (51–1), 2.792 (100) (221, 511), 2.672 (54) (002, 601, 20-2), 2.624 (43) (710, 42–1). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 4106/1.