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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 1.00 Mn 0.03 Na 0.08 La 0.51 Ce 1.30 Pr 0.16 Nd 0.62 Sm 0.10 Gd 0.06 Dy 0.03 Er 0.01 Y 0.06 Th 0.01 ) Σ3.97  (Al 3.21 Fe 2+  0.79 ) Σ4.00 Si 5.01 O 20 (OH) 2  for perbøeite-(Ce) [IMA 2011-55] and (Ca 1.10 Mn 0.03 Na 0.20 La 0.42 Ce 1.14 Pr 0.16 Nd 0.60 Sm 0.13 Gd 0.07 Dy 0.03 Er 0.01 Yb 0.01 Y 0.12 Th 0.02 ) Σ4.04  (Al 3.54 Fe 2+  0.40 Mg 0.02 ) Σ3.96 Si 4.99 O 20 (OH) 2  for alnaperbøeite-(Ce) [IMA 2012-54]. The respective end-member formulas are  A (Ce 3 Ca) M (Al 3 Fe 2+ )Si 2 O 7 (SiO 4 ) 3 O(OH) 2 , which requires Ce 2 O 3  45.10, CaO 5.14, FeO 6.58, Al 2 O 3  14.01, SiO 2  27.52, H 2 O 1.65, total 100.00 wt%; and  A (Ce 2.5 CaNa 0.5 ) M (Al 4 )Si 2 O 7 (SiO 4 ) 3 O(OH) 2 , which requires Ce 2 O 3  40.86, CaO 5.58, Na 2 O 1.54, Al 2 O 3  20.31, SiO 2  29.92, H 2 O 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) Å 3 , 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  P 2 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). Perbøeite-(Ce) derives from gatelite-(Ce) by the homovalent substitution [ M3 Fe 2+  →  M3 Mg]. Alnaperbøeite-(Ce) derives from perbøeite-(Ce) or gatelite-(Ce) by the coupled heterovalent substitutions [ A Na +  + 2 M3 Al 3+  →  A REE + 2 M3 (Fe 2+  or Mg)].

american mineralogist

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