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

Authors: Holtstam, Dan; Kolitsch, Uwe; Andersson, Ulf B.

Source: European Journal of Mineralogy, Volume 17, Number 1, January 2005 , pp. 129-142(14)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

Västmanlandite-(Ce), ideally (Ce,La)₃CaAl₂Mg₂[Si₂O₇][SiO₄]₃F(OH)₂, is a new mineral species from the Västmanland county, Bergslagen region, Sweden. Together with more Fe-rich, F-poor members, it forms solid solutions that are important for lanthanide sequestration in the Bastnäs-type skarn deposits in Västmanland. At the type locality (Malmkärra, 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 ⁵⁷Fe Mössbauer spectroscopy yield: La₂O₃ 13.65, Ce₂O₃ 23.90, Pr₂O₃ 2.07, Nd₂O₃ 6.28, Sm₂O₃ 0.42, Gd₂O₃ 0.15, Y₂O₃ 0.18, CaO 4.65, FeO 1.14, Fe₂O₃ 2.69, MgO 5.51, Al₂O₃ 8.58, TiO₂ 0.04, P₂O₅ 0.05, SiO₂ 26.61, F 1.06, H₂O_calc 1.61, O = F -0.45, sum 98.14. Västmanlandite-(Ce) is monoclinic, P2₁/m, with a = 8.939(1), b = 5.706(1), c = 15.855(2) Å, β = 94.58(1)°, V = 806.1(2) Å³, and Z = 2 (single-crystal data). The eight strongest lines in the X-ray powder diffraction pattern are [d (in Å) (I/I₀)(hkl)]: 15.81(16)(001), 4.65(10)(111), 3.50(20)(210), 2.983(100)(114), 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 'observed' reflections with F_o > 4σ(F_o). Västmanlandite-(Ce) is nearly isotypic to gatelite-(Ce), ideally Ce₃ CaAl₂AlMg[Si₂O₇][SiO₄ ]₃O(OH)₂, and can be described as a regular dollaseite-(Ce)-törnebohmite-(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 törnebohmite-(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.

Keywords: VASTMANLANDITE-(CE); NEW MINERAL SPECIES; CRYSTAL STRUCTURE; POLYSOME; GATELITE-(CE)

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2005/0017-0129

Publication date: 2005-01-01

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