Vastmanlandite-(Ce) a new lanthanide- and F-bearing sorosilicate mineral from Vastmanland, Sweden: description, crystal structure, and relation to gatelite-(Ce)

Abstract

Vastmanlandite-(Ce), ideally (Ce,La) 3 CaAl 2 Mg 2 [Si 2 O 7 ][SiO 4 ] 3 F(OH) 2 , is a new mineral species from the Vastmanland county, Bergslagen region, Sweden. Together with more Fe-rich, F-poor members, it forms solid solutions that are important for lanthanide sequestration in the Bastnas-type skarn deposits in Vastmanland. At the type locality (Malmkarra, Norberg district) it occurs as anhedral grains 0.2–3 mm across, in association with fluorbritholite-(Ce), tremolite, a serpentine mineral, magnetite, dollaseite-(Ce) and dolomite. The mineral is allanite-like in appearance; it is dark brown, translucent, with vitreous luster, and has good cleavage parallel to {001}, uneven to conchoidal fracture, and a yellowish gray streak; D calc = 4.51(2) g·cm −3 and Mohs hardness ≈ 6. Optically it is biaxial (-), with α = 1.781 (4), β calc = 1.792, γ = 1.810(4) and 2V α = 75(5)°. Chemical analysis by electron-microprobe and 57 Fe Mossbauer spectroscopy yield: La 2 O 3 13.65, Ce 2 O 3 23.90, Pr 2 O 3 2.07, Nd 2 O 3 6.28, Sm 2 O 3 0.42, Gd 2 O 3 0.15, Y 2 O 3 0.18, CaO 4.65, FeO 1.14, Fe 2 O 3 2.69, MgO 5.51, Al 2 O 3 8.58, TiO 2 0.04, P 2 O 5 0.05, SiO 2 26.61, F 1.06, H 2 O calc 1.61, O ≡ F −0.45, sum 98.14. Vastmanlandite-(Ce) is monoclinic, P2 1 / m , with a = 8.939(1), b = 5.706(1), c = 15.855(2) A, β = 94.58(1)°, V = 806.1(2) A 3 , and Z = 2 (single-crystal data). The eight strongest lines in the X-ray powder diffraction pattern are [ d (in A) ( I/I 0 )( hkl )]: 15.81(16)(001), 4.65(10)(111), 3.50(20)(210), 2.983(100)(014), 2.685(13)(121), 2.678(11)(205), 2.625(19)(311), and 2.187(15)(402). The crystal structure was determined using single-crystal X-ray diffraction data and refined to R1( F ) = 1.73% for 2864 9observed9 reflections with F o > 4σ( F o ). Vastmanlandite-(Ce) is nearly isotypic to gatelite-(Ce), ideally Ce 3 CaAl 2 AlMg[Si 2 O 7 ][SiO 4 ] 3 O(OH) 2 , and can be described as a regular dollaseite-(Ce)-tornebohmite-(Ce) polysome. The occurrence of extremely weak, continuous streaking in the diffraction patterns, and the presence of two pairs of mirror-related atoms in the tornebohmite-(Ce) module indicate that the structure model represents an average structure, unlike the situation in gatelite-(Ce) where there are no continuous streaks but instead, due to atomic ordering, sharp superstructure reflections resulting in a different unit cell.