Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps): A new end-member of the epidote group

Dissakisite-(La), ideally CaLaAl2MgSi3O12(OH), has been found in the Hochwart peridotite, Ulten zone, Italy. The mineral occurs as centimeter-sized black to very dark brown anhedral nodules and smaller grains. Associated minerals are: olivine, spinel, amphiboles, clino- and orthopyroxenes, and minor clinochlore, uraninite, thorite, thorianite, phlogopite, zircon, apatite, calcite, dolomite, pentlandite, and copper sulfides. The streak is gray-greenish and the luster is vitreous. Mobs hardness is 6.5-7-1 the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group P2(1)/m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) Alpha, beta = 115.193(6)degrees, V= 475.30(7) angstrom(3), Z = 2. The strongest X-ray powder diffraction lines are: [d(angstrom) (I)(hkl)] 2.926(100)(11(3) over bar), 2.860(53)(020),2.553(51)(202), 3.526(49)(21(1) over bar),2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula (Ca1.195Mn0.009Sr0.010 Na0.002Th0.090U0.003La0.315Ce0.262Pr0.019Nd0.038Sm0.002Gd0.001Er0.001) (Al1.816Mg0.622Fe0.2442+Fe0.1593+Cr0.148 Ti0.030Sc0.002V0..008Ga0.001Ni0.010Zn0.015) (Si2.970Al0.022P0.008) O11.991F0.009 (OH). The La/(La + Cc) ratio is 0.545(16) in the type analysis and 0.543(18) in an average of 70 analyses of the type sample A4310. Ce >= La was not observed in any analysis. D-meas = 3.79(15) g/cm(3); D-calc = 3.84 g/cm(3). Radioactivity is appreciable. The optical properties and Raman spectrometry have also been investigated. The mineral formed by hydration and enrichment in LILE and LREE of a peridotite body, in relation to HP-migmatization of the surrounding gneisses during the Variscan orogeny.