Ferriperbøeite-(Ce), [CaCe3]∑=4[Fe3+Al2Fe2+]∑=4[Si2O7][SiO4]3O(OH)2, a new member of the polysomatic epidote–törnebohmite series from the Nya Bastnäs Fe-Cu-REE deposit, Sweden

Ferriperbøeite-(Ce), ideally [CaCe₃]_∑=4[Fe³⁺Al₂Fe²⁺]_∑=4[Si₂O₇][SiO₄]₃O(OH)₂, is a new mineral species from the Nya Bastnäs Fe–Cu–REE skarn deposit in the Bergslagen mining region (south-central Sweden). The mineral occurs in direct contact with ferriallanite-(Ce), cerite-(Ce) and törnebohmite-(Ce). In the type specimen, ferriperbøeite-(Ce) forms brownish black crystals, and exhibits an irregular to short-prismatic morphology elongated along [0 1 0]. The maximum crystal size is about 500 µm. The mineral is optically biaxial positive, with 2V = 65 ± 5°. It is strongly pleochroic from green, throughout orange-brown, to deep red colours. Electron microprobe analyses yields the empirical formula, calculated on the basis of 13 cations and divalent and trivalent iron partitioned to fulfill the criterion of charge balance, (Ca_0.92La_1.23Ce_1.50Pr_0.10Nd_0.27Sm_0.02Y_0.01)_∑=4.05($Al1.85Fe0.462+Fe1.133+Mg0.57Ti0.01$)_∑=4.02(Si_4.92P_0.01)_∑=4.93O₂₀(OH_1.87F_0.12Cl_0.01), which is in accord with single-crystal X-ray diffraction, infra-red, and Mössbauer spectroscopy data. The mineral is monoclinic, space group P2₁/m, with unit-cell parameters: a = 8.9320(4), b = 5.7280(3), c = 17.5549(9) Å, β = 116.030(4)°, V = 807.05(7) ų, Z = 2. Ferriperbøeite-(Ce) is as a new member of the epidote (E) − törnebohmite (T) polysomatic series, having an E module with ferriallanite-(Ce) composition. The new mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2017-037.