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Dissakisite-(La), ideally CaLaAl 2 MgSi 3 O 12 (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 P2 1 /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) Å 3 , 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)(211̄), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula (Ca 1.195  Mn 0.009  Sr 0.010  Na 0.002  Th 0.090  U 0.003  La 0.315  Ce 0.262  Pr  0.019  Nd 0.038  Sm 0.002  Gd 0.001  Er 0.001 ) (Al 1.816  Mg 0.622  Fe 2+  0.244  Fe 3+  0.159  Cr 0.148  Ti 0.030  Sc 0.002  V 0.008  Ga 0.001  Ni 0.010  Zn 0.015 ) (Si 2.970  Al 0.022  P 0.008 ) O 11.991  F 0.009  (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. 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.

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