Abstract

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. Mohs hardness is 6.5–7; the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group P21/m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) Å, β = 115.193(6)°, V = 475.30(7) Å3, Z = 2. The strongest X-ray powder diffraction lines are: [d(Å) (I)(hkl)] 2.926(100)(113̅), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211̅), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula (Ca1.195 Mn0.009 Sr0.010 Na0.002 Th0.090U0.003 La0.315 Ce0.262 Pr 0.019 Nd0.038 Sm0.002 Gd0.001 Er0.001) (Al1.816 Mg0.622 Fe2+ 0.244 Fe3+ 0.159 Cr0.148 Ti0.030 Sc0.002 V0.008 Ga0.001 Ni0.010 Zn0.015) (Si2.970 Al0.022 P0.008) O11.991 F0.009 (OH). The La/(La + Ce) 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. Dmeas = 3.79(15) g/cm3; Dcalc = 3.84 g/cm3. 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.

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