New data on eudialyte decomposition minerals from kakortokites and associated pegmatites of the Ilimaussaq complex, South Greenland

A suite of samples with eudialyte and eudialyte decomposition minerals from the kakortokite and associated pegmatites of the Ilimaussaq complex in South Greenland has been investigated by electron microprobe analysis. Extensive decomposition of eudialyte has resulted in the formation of catapleiite as host for a number of rare and hitherto unknown REE minerals besides known minerals such as monazite and kainosite. Mineral A1 is present in very small amounts in nearly all eudialyte decomposition aggregates and comprises two varieties: Ca-rich A1 with composition HCa₃REE₆(SiO₄)₆(Fsquare) and presumed apatite structure, and Ca-poor A1 with composition (Fe,Mn,Ca)_1.5REE₆Si₆FO₂₂ and unknown structure. Mineral A2 with composition (Ca,Fe)_1.2 REE₄Si₆O_19-y(OH)_2y^.nH₂O is indistinguishable from A1 in EMP-backscattered light and has only been found at a limited number of localities. Mineral A2 also occurs as a primary mineral at one locality. Additional rare and new REE-minerals are mineral A3 with composition Na_0.2Ca_0.6Fe_0.2Mn_0.5Al_0.5REE_2.8Si₆F_0.5O)_18-y(OH)_2y ^. nH₂O; mineral Uk2 with composition REE_2.00F_1.50_O2.25-y(OH)_2y ^. nH₂O; mineral Uk3 with composition CaREE₄O_7-yOH)_{2y ^.} nH₂O; and mineral Y1 with composition Na₂Ca₄Y_2.7REE_1.3F₁₈ (OH)₄. The Ce:(Y+La+Pr+Nd+Sm+Gd) molar ratio for A1, A2, A3, Uk2, Uk3 and monazite is close to 1:1. Characteristic for A1, A2 and monazite are substantial solid solutions between La and (Pr+Nd+Sm+Gd) with slowly increasing content of Ce as the content of La increases. A similar pattern does not exist for the REE in fresh eudialyte. Kainosite, identified in one decomposition aggregate, has not previously been found in the Il maussaq complex.