Perbøeite-(Ce) and alnaperbøeite-(Ce), two new members of the epidote-törnebohmite polysomatic series: Chemistry, structure, dehydrogenation, and clue for a sodian epidote end-member

In yttrian fluorite from pegmatites of the Tysfjord granite, Norway, grayish-green to very pale green gatelite-like crystals occur along with millimeter-size aggregates of Y-silicates as a late primary phase; they are associated with allanite-(Ce), bastnäsite-(Ce), and intimately inter- or overgrown by törnebohmite-(Ce). Sub- to euhedral crystals, up to 400 μm in size, are chemically zoned between two near-end-member compositions that imply the existence of two new members of the polysomatic gatelite group, in which ET polysomes are composed of E modules with epidote-type structure alternating with T modules of törnebohmite-type structure. The two new minerals form a continuous solid-solution series, along which two crystals of intermediate compositions served for species definition. Their electron-microprobe analyses yield the empirical formulas (Ca_1.00Mn_0.03Na_0.08La_0.51 Ce_1.30Pr_0.16Nd_0.62Sm_0.10Gd_0.06Dy_0.03Er_0.01Y_0.06Th_0.01)_∑3.97 (Al_3.21Fe²⁺_0.79)_∑4.00Si_5.01O₂₀(OH)₂ for perbøeite-(Ce) [IMA 2011-55] and (Ca_1.10Mn_0.03Na_0.20La_0.42Ce_1.14Pr_0.16Nd_0.60Sm_0.13Gd_0.07Dy_0.03Er_0.01Yb_0.01Y_0.12Th_0.02)_∑4.04 (Al_3.54Fe²⁺_0.40Mg_0.02)_∑3.96Si_4.99O₂₀(OH)₂ for alnaperbøeite-(Ce) [IMA 2012-54]. The respective end-member formulas are ^A(Ce₃Ca)^M(Al₃Fe²⁺)Si₂O₇(SiO₄)₃O(OH)₂, which requires Ce₂O₃ 45.10, CaO 5.14, FeO 6.58, Al₂O₃ 14.01, SiO₂ 27.52, H₂O 1.65, total 100.00 wt%; and ^A(Ce_2.5CaNa_0.5)^M(Al₄)Si₂O₇(SiO₄)₃O(OH)₂, which requires Ce₂O₃ 40.86, CaO 5.58, Na₂O 1.54, Al₂O₃ 20.31, SiO₂ 29.92, H₂O 1.79, total 100.00 wt%. Cell parameters of perbøeite-(Ce) and alnaperbøeite-(Ce) for these crystals are a = 8.9277(6) and 8.9110(4), b = 5.6548(6) and 5.6866(2), c = 17.587(1) and 17.5252(7) Å, β = 116.475(8) and 116.300(5)°, V = 794.8(1) and 796.13(7) ų, respectively. Members of the perbøeite-(Ce)–alnaperbøeite-(Ce) solid solution are topologically identical to the minerals gatelite-(Ce) and västmanlandite-(Ce). Structural data (space group P2₁/m) were obtained for the holotype crystals and for several crystals with intermediate composition. Structural refinements of a crystal annealed step-wise in air confirm that most of Fe in M3 is divalent before heating and show that oxidation/dehydrogenation takes place mostly in the E module (M3 and H1). Perbøeite-(Ce) derives from gatelite-(Ce) by the homovalent substitution [^M3Fe²⁺ → ^M3Mg]. Alnaperbøeite-(Ce) derives from perbøeite-(Ce) or gatelite-(Ce) by the coupled heterovalent substitutions [^ANa⁺ + 2^M3Al³⁺ → ^AREE + 2^M3(Fe²⁺ or Mg)].

Törnebohmite-(Ce) associated with alnaperbøeite-(Ce) is Na-free, whereas coexisting allanite is Na-bearing and shows the same coupled substitution between A and M sites as the one relating perbøeite-(Ce) and alnaperbøeite-(Ce) (Na_0.5Al ↔ REE_0.5Fe²⁺). This could suggest, although crystallographic evidence is inconclusive, that Na incorporation in the ET polysome occurs in the E module alone (A2 or A1 sites), leading to the sodian E end-member ^A(CaREE_0.5Na_0.5)^M(Al₃)Si₂O₇(SiO₄)O(OH). In any event, this new epidote end-member is needed to account for up to ca. 10 mol% of the composition of allanite-group minerals, in which Na₂O contents may reach 0.3 wt%. Sodium must be analyzed in epidote-supergroup and gatelite-group minerals.