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New Mn- and rare-earth-rich epidote-group minerals in metacherts

¹ Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, 24 rue Lhomond, 75005 Paris, France
² Now at: Centre de Géochimie de la surface – EOST, 1 rue Blessig, 67083 Strasbourg Cedex, France
³ Laboratoire de Tectonique – UMR 7072 du CNRS, Université Pierre et Marie Curie, Case 129,4 place Jussieu, 75252 Paris Cedex 05, France
⁴ Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, 3012 Bern, Switzerland
⁵ Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

^* Corresponding author, e-mail: chopin{at}geologie.ens.fr

Two new rare-earth-bearing members of the epidote group are described from carbonate pods in Mn-rich metacherts. According to the structure refinements, Mn occurs mainly in the divalent state in both of them and occupies both the large A1 site instead of Ca and the large octahedral M3 site instead of Al or Fe³⁺, charge-compensating the substitution of Ca²⁺ by REE³⁺ in the largest, A2 site. The two minerals are therefore members of the androsite series.

Manganiandrosite-(Ce) has dominant Mn³⁺ in the M1 site; it occurs at the Praborna manganese mine, Saint-Marcel, Aosta valley, Italy, in a Mesozoic eclogite-facies ophiolitic unit of the western Alps. Associated minerals are rhodochrosite and Mn-pyroxenoid, with minor calderite, spessartine, hematite and pyrophanite. The structural formula is ^A1[Mn²⁺_0.60Ca_0.40]₌₁ ^A2[(Ce_0.46La_0.23Nd_0.12Sm_0.01)_{REE 0.82}Sr_0.07Ca_0.02] _0.91 ^M1[Mn³⁺_0.63Fe³⁺_0.23Ti_0.10Mg_0.04]₌₁ ^M2Al_1.00 ^M3[Mn²⁺_0.96Mn³⁺_0.04]₌₁ Si₂O₇SiO₄O(OH), ideally ^A1Mn^{2+ A2}Ce ^M1Mn^{3+ M2}Al ^M3Mn²⁺ Si₂O₇SiO₄O(OH). Monoclinic, space group P2₁/m, a 8.901(2) Å, b 5.738(1) Å, c 10.068(2) Å, ß 113.425(3)°, V471.81 ų, Z = 2. Biaxial positive, 2V = 80.6(1.5)°, n(calc) = 1.80, strong pleochroism: light yellow, ß orange-brown, red-brown.

Vanadoandrosite-(Ce) has dominant V³⁺ in M1. It occurs at the Vielle Aure mining district, central Pyrénées, France, in and around quartz-rhodochrosite-sulphide veinlets cross-cutting the rhodochrosite ore in greenschist-facies Lower Carboniferous radiolarite. Other associated minerals are vuorelainenite, chalcopyrite, vanadian spessartine, and friedelite. The structural formula of the most V-rich crystal is ^A1(Mn²⁺_0.62 Ca_0.38)_1.00 ^A2[(Ce_0.39La_0.15Nd_0.10Sm_0.02)_REE=0.66Ca_0.21Sr_0.11]_0.98 ^M1(V³⁺ _0.80Al_0.16Mg_0.03Ti_0.01)_1.00 ^M2Al_1.00 ^M3(Mn²⁺_0.36V³⁺_0.31Fe²⁺_0.23 Fe³⁺_0.10)_1.00 Si₂O₇SiO₄O(OH), ideally ^A1Mn^{2+ A2}Ce ^M1V^{3+ M2}Al ^M3Mn²⁺ Si₂O₇SiO₄O(OH). Monoclinic, space group P2₁/m, a 8.856(3) Å, b 5.729(2) Å, c 10.038(4) Å, ß 113.088(5)°, V 468.5 ų, Z = 2. Biaxial, n(calc) = 1.82, strong pleochroism: yellow-brown < red-brown < dark greenish brown //b. The root-name applies to any epidote-group mineral in which REE are dominant in A2, Mn²⁺ in A1, V³⁺ in M1, Al in M2, and in which Mn²⁺ is the dominant charge-compensating (i.e. divalent) cation in M3.

The structural study of these new androsites suggests that khristovite-(Ce) also has significant Mn²⁺ on A1 and should be reanalysed. Other epidote-group minerals from the Pyrenean deposits represent two new potential species, with the end-members CaCe AlAlMn²⁺ Si₃O₁₂(OH), i.e. the ^[M3]Mn²⁺ analogue of allanite and dissakisite, and Mn²⁺Ce MgAlMn²⁺ Si₃O₁₁F(OH), i.e. the ^[A1]Mn analogue of khristovite. Even in these Mn-rich systems, incorporation of F on O4 is balanced by Mg and not Mn incorporation. The variety of mineral assemblages in Mn-V-REE-rich systems is discussed on the basis of oxidation state.

Key-words: epidote group, rare-earth elements, manganiandrosite-(Ce), vanadoandrosite-(Ce), new mineral, crystal structure, khristovite-(Ce).

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