Perboeite-(Ce) and alnaperboeite-(Ce), two new members of the epidote-tornebohmite 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), bastnasite-(Ce), and intimately inter- or overgrown by tornebohmite-(Ce). Sub- to euhedral crystals, up to 400 mu m 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 tornebohmite-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 (Ca1.00Mn0.03Na0.08La0.51 Ce1.30Pr0.16Nd0.62Sm0.10Gd0.06Dy0.03Er0.01Y0.06Th0.01)(Sigma 3.97)(Al3.21Fe0.792+)(Sigma 4.00)Si5.01O20(OH)(2) for perboeite-(Ce) [IMA 2011-55] and (Ca1.10Mn0.03Na0.20La0.42Ce1.14Pr0.16Nd0.60Sm0.13Gd0.07DY0.03Er0.01Yb0.01Y0.12Th0.02)(Sigma 4.04) (Al3.54Fe0.402+Mg0.02)(Sigma 3.96)Si4.99O20(OH)(2) for alnaperboeite-(Ce) [IMA 2012-54]. The respective end-member formulas are (A)(Ce3Ca)(M)(Al3Fe2+)Si2O7(SiO4)(3)O(OH)(2), which requires Ce2O3 45.10, CaO 5.14, FeO 6.58, Al2O3 14.01, SiO2 27.52, H20 1.65, total 100.00 wt%; and (A)(Ce2.5CaNa0.5)(M)(Al-4)Si2O7(SiO4)(3)O(OH)(2), which requires Ce2O3 40.86, CaO 5.58, Na20 1.54, Al2O3 20.31, SiO2 29.92, H2O 1.79, total 100.00 wt%. Cell parameters of perboeite-(Ce) and alnaperboeite-(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) angstrom, beta = 116.475(8) and 116.300(5)degrees, V= 794.8(1) and 796.13(7) angstrom(3), respectively. Members of the perboeite-(Ce) alnaperboeite(Ce) solid solution are topologically identical to the minerals gatelite-(Ce) and vastmanlandite-(Ce). Structural data (space group P2(1)/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). Perboeite-(Ce) derives from gatelite-(Ce) by the homovalent substitution [Fe-M3(2+) -> Mg-M3]. Alnaperboeite-(Ce) derives from perboeite-(Ce) or gatelite-(Ce) by the coupled heterovalent substitutions [Na-A(+) + 2(M3)Al(3+) -> (REE)-R-A + 2(M3)(Fe2+ or Mg)]. Tornebohmite-(Ce) associated with alnaperboeite-(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 perboeite(Ce) and alnaperboeite-(Ce) (Na0.5Al <-> REE0.5Fe2+). This could suggest, although crystallographic evidence is inconclusive, that Na incorporation in the ET polysome occurs in the E module alone (A2 or Al sites), leading to the sodian E end-member (A)(CaREE0.5Na0.5)(M)(Al-3)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.