Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps) : A new end-member of the epidote group
Articolo
Data di Pubblicazione:
2005
Citazione:
Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps) : A new end-member of the epidote group / S. Tumiati, G. Godard, S. Martin, P. Nimis, V. Mair, B. Boyer. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - 90:7(2005), pp. 1177-1185.
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 sulfi des. 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)(113–), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211–), 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.
Tipologia IRIS:
01 - Articolo su periodico
Elenco autori:
S. Tumiati, G. Godard, S. Martin, P. Nimis, V. Mair, B. Boyer
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