New Mn- and rare-earth-rich epidote-group minerals in metacherts: manganiandrosite-(Ce) and vanadoandrosite-(Ce)

Abstract

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+0.60Ca0.40]Σ=1 A2[(Ce0.46La0.23Nd0.12Sm0.01)ΣREE ≥ 0.82Sr0.07Ca0.02]Σ ≥ 0.91 M1[Mn3+0.63Fe3+0.23Ti0.10Mg0.04]Σ=1 M2Al1.00 M3[Mn2+0.96Mn3+0.04]Σ=1 Si2O7SiO4O(OH), ideally A1Mn2+ A2Ce M1Mn3+ M2Al M3Mn2+ Si2O7SiO4O(OH). Monoclinic, space group P2 1/ m, a 8.901(2) A, b 5.738(1) A, c 10.068(2) A, β 113.425(3)°, V 471.81 A3, Z = 2. Biaxial positive, 2V = 80.6(1.5)°, n(calc) = 1.80, strong pleochroism: α light yellow, β orange-brown, γ red-brown. Vanadoandrosite-(Ce) has dominant V3+ in M1. It occurs at the Vielle Aure mining district, central Pyrenees, 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 Ca0.38)Σ1.00 A2[(Ce0.39La0.15Nd0.10Sm0.02)ΣREE=0.66Ca0.21Sr0.11]Σ0.98 M1(V3+ 0.80Al0.16Mg0.03Ti0.01)Σ1.00 M2Al1.00 M3(Mn2+0.36V3+0.31Fe2+0.23 Fe3+0.10)Σ1.00 Si2O7SiO4O(OH), ideally A1Mn2+ A2Ce M1V3+ M2Al M3Mn2+ Si2O7SiO4O(OH). Monoclinic, space group P2 1 /m, a 8.856(3) A, b 5.729(2) A, c 10.038(4) A, β 113.088(5)°, V 468.5 A3, 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.