Exploring the unusual occurrence, chemistry, and structural topology of åsgruvanite-(Ce), Ce16Ca5Al(SiO4)6(AsO3)8(CO3)2Cl3(ClF3)(OH)2 , a new rare earth element (REE) mineral from Västmanland, Sweden.

Abstract

Åsgruvanite-(Ce), ideally Ce16Ca5Al(SiO4)6(AsO3)8(CO3)2Cl3(ClF3)(OH)2, is a new mineral species (IMA–CNMNC 2025-004), from the Åsgruvan Fe-skarn deposit, Norberg, Västmanland, Sweden, which is directly related to the Bastnäs-type of REE mineralisations in the Palaeoproterozoic Bergslagen ore province. Åsgruvanite-(Ce) occurs as anhedral, occasionally elongated grains up to 400 µm. It is greyish green to nearly colourless, with a white streak and a vitreous to greasy lustre. Cleavage is distinct on {001} and less so on {100}; the mineral is brittle, and its fracture is uneven. The calculated density is 4.79(1) g·cm⁻³. Åsgruvanite-(Ce) is optically uniaxial (+), with a refractive index above 1.8; the calculated average is 1.88 (Gladstone–Dale approach). Åsgruvanite-(Ce) crystallises in the trigonal system, space group P-3m1 (Z = 1), with the following unit-cell parameters: a = 10.5728(6) Å, c = 15.0899(11) Å. Åsgruvanite-(Ce) occurs in a magnetite–REE skarn, but its formation postdates the groundmass carbonate and skarn assemblage, and it is associated with late-stage calcite, dolomite, a dollaseite-like allanite-group mineral, gadolinite-(Y/Nd), and a fluorocarbonate related to bastnäsite-(Ce), with variable F contents. The structure was refined to R1 = 6.23% for 987 reflections. It is unique and consists of two alternating layers, A and B, along the c axis. Layer A (~8.4 Å) has the composition [(Ce12Ca3)AlSi6(C1.50S0.50)S2.00O30(OH)2]15+. Layer B (~6.7 Å) corresponds to the composition [(Ce4Ca2)As3+8O24Cl4F3]15–. These layers form tunnel-like features parallel to [100], which are partially occupied by Cl atoms. Spectroscopic data (infrared and micro-Raman) support the structural model