Christian Chopin

The new rare-earth element (REE) bearing epidote-supergroup mineral vielleaureite-(Ce), Mn²⁺Ce(MgAlMn²⁺)(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(Mn${}_{\mathrm{0.85}}^{\mathrm{2}+}$ Ca_0.14Na_0.01)_Σ1.00 ^A2(Ce_0.66La_0.32Nd_0.02Er_0.01)_Σ1.00 ^M1(Mg_0.71V_0.14Mn${}_{\mathrm{0.08}}^{\mathrm{2}+}$ Mn${}_{\mathrm{0.02}}^{\mathrm{3}+}$ Fe${}_{\mathrm{0.06}}^{\mathrm{3}+}$ )_Σ1.01^M2Al_1.00 ^M3(Mn${}_{\mathrm{0.98}}^{\mathrm{2}+}$ Mg_0.02)_Σ1.00 ^T(Si_2.94Al_0.04B_0.01)_Σ2.99O₁₁(F_0.85Cl_0.01O_0.14)_Σ1.00 (OH). The mean refractive index calculated for the holotype composition is n=1.79. The calculated density is 4.33 g cm⁻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=8.8238(13) Å, b=5.7131(9) Å, c=10.0034(16) Å, β=112.823(6)°, V=464.80(13) ų, Z=2. The anisotropic model of the P2${}_{\mathrm{1}}/m$ structure converged to 0.0575 for 1565 reflections with F_o>4σ(F_o) and 126 refined parameters; it confirms the Mn²⁺ dominance at the A1 and M3 sites and the charge compensation of fluorine incorporation at O4 by Mg at M1, pointing to a new member of the dollaseite group, the ^A1Mn²⁺ 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 $\mathrm{Mn}/(\mathrm{Mn}+\mathrm{Ca}$ ) ratio in the range 0.49 to 0.61, this hellandite-group phase is the Mn²⁺ equivalent of hellandite-(Y).