Vielleaureite-(Ce), a new epidote of the dollaseite group, coexisting with Mn-dominant hellandite in Mn deposits of the central Pyrenees

The new rare-earth element (REE) bearing epidote-supergroup mineral vielleaureite-(Ce), Mn2CCe(MgAlMn2C)(Si2O7)(SiO4)F(OH), is described from a Paleozoic stratiform Mn ore deposit at the Coustou mine above the village of Vielle-Aure, Hautes Pyrénées, France. It occurs as isolated submillimetric prisms in veinlets crosscutting the microcrystalline rhodochrosite ore, associated with rhodochrosite, friedelite, spessartine, “Mn-humite”, baryte, and numerous sulfides. The dark-brown “allanite”-looking prisms are patchily zoned from vanadoandrosite-(Ce) or vanadoakasakaite-(Ce) to a near-endmember vielleaureite-(REE) composition, with a strict correlation between F and Mg contents. The empirical formula of the crystal studied is, on an eight-cation basis, A1(Mn2C 0:85Ca0:14Na0:01)61:00 A2(Ce0:66La0:32Nd0:02Er0:01)61:00 M1(Mg0:71V0:14Mn2C 0:08Mn3C 0:02Fe3C 0:06)61:01M2Al1:00 M3(Mn2C 0:98Mg0:02)61:00 T (Si2:94Al0:04B0:01)62:99O11(F0:85Cl0:01O0:14)61:00 (OH). The mean refractive index calculated for the holotype composition is n D 1:79. The calculated density is 4.33 gcm􀀀3. Pleochroism is strong: very light yellow < light brown < brown //b. Monoclinic unit-cell parameters refined from single-crystal X-ray diffraction data are a D 8:8238(13) Å, b D 5:7131(9) Å, c D 10:0034(16) Å, D 112:823(6)°, V D 464:80(13)Å3, Z D 2. The anisotropic model of the P21=m structure converged to 0.0575 for 1565 reflections with Fo > 4 (Fo) and 126 refined parameters; it confirms the Mn2C dominance at the A1 and M3 sites and the charge compensation of fluorine incorporation at O4 by Mg atM1, pointing to a new member of the dollaseite group, the A1Mn2C equivalent of khristovite-(Ce). In the same rocks, “hellandite” was discovered as tiny oriented relict inclusions within vielleaureite-(Ce), locally associated with micrometre-size fluorite and “parisite” inclusions and more rarely as ghost crystals in rhodochrosite.With a Mn=(MnCCa) ratio in the range 0.49 to 0.61, this hellandite-group phase is the Mn2C equivalent of hellandite-(Y).