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

¹ Dipartimento di Scienze Chimiche e Ambientali, Università degli Studi dell’Insubria, Via Lucini 3, I-22100 Como, Italy
² Laboratoire de Géosciences marines, Institut de Physique du Globe, 4, place Jussieu, F-75252 Paris, France
³ Dipartimento di Mineralogia e Petrologia, Università degli Studi di Padova, C.so Garibaldi 37, I-35100 Padova, Italy
⁴ Ufficio Geologia e Prove Materiali, Provincia Autonoma di Bolzano, Via Val d’Ega 48, I-39053 Cardano (Bolzano), Italy
⁵ Université Montpellier II, place Eugène Bataillon, F-34095 Montpellier, France

Correspondence: ^* E-mail: simone.tumiati{at}uninsubria.it

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, ß = 115.193(6)°, V = 475.30(7) ų, Z = 2. The strongest X-ray powder diffraction lines are: [d(Å) (I)(hkl)] 2.926(100)(11), 2.860(53)(020), 2.553(51)(202), 3.526(49)(21), 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_0.090U_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 Fe²⁺ 0.244 Fe³⁺ 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/cm³; D_calc = 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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