2Steenstrupine-(Ce) from two localities, Taseq and Mellemelv, both in the Ilímaussaq complex of South Greenland has been studied. At Taseq ussingite veins contain idiomorphic steenstrupine-(Ce) crystals with decomposed central parts, marginal zones of very thin rhythmically/oscillatory precipitated layers exceptionally rich in Th (up to 10.6 wt% ThO2) and rim zones with distinctly lower Th values (min. 5.5 wt% ThO2). The low content of Na (between 0.9 and 3.8 wt% Na2O) in the mineral has presumably been caused by protonation exchange of Na+ by H+. Two types of late- to post magmatic alteration have attacked the crystals. Decomposition is strictly confined either to distinct zones inside the crystals or to their cores and has resulted in the formation of extremely fine-grained crystalline phases. The other alteration product cuts into steenstrupine-(Ce) crystals as irregularly shaped botryoidal bodies, composed presumably of amorphous products. Hexagonally shaped crystals of britholite-(Ce) and presumably a new mineral species labelled A4 with square outlines and the composition Na1.41(Ca0.14Fe0.39Mn0.24)∑0.77(REE5.39Th0.22)∑5.61(Si5.36P0.64)∑6.00F0.55O22.02−n(OH)2n, are enclosed as idiomorphic crystals in many steenstrupine-(Ce) crystals. Britholite-(Ce) also forms radiating aggregates in adjacent silicate minerals. Britholite-(Ce) is characterized by exceptionally high contents of REE (73.0 wt% REE2O3) and low contents of CaO (2.2 wt%).

At the Mellemelv locality, both fresh and completely decomposed steenstrupine-(Ce) occurs in a complex naujaite pegmatite. Fresh steenstrupine-(Ce) differs in composition from steenstrupine-(Ce) at the Taseq locality. It has a lower (Si+P):REE ratio = 2.19 compared to 3.29 for steenstrupine-(Ce) from Taseq. It may represent a new steenstrupine-(Ce) variety. Two secondary and chemically well-defined minerals, labelled A5, (Na0.02K0.01)∑0.03(Ca0.70Fe0.07Mn0.03)∑0.80(REE4.79Th0.18)∑4.97(Nb0.01Ti0.01Zr0.02)∑0.04(Si1.97P4.00Al0.03)∑6.00F0.39Cl0.02O22.22−n(OH)2n, and A6, (Na0.10K0.03)∑0.13(Ca0.36Fe0.65Mn0.19)∑1.20(REE3.25Th0.34)∑3.59(Ti0.09Zr0.25)∑0.34Nb0.03(Si4.81P0.56Al0.63)∑6.00F0.57Cl0.05O19.88−y(OH)2y·nH2O, appear to be new mineral species. They constitute major alteration products in the decomposed steenstrupine-(Ce) crystals; A5 also occurs closely associated with the unaltered steenstrupine-(Ce) variety.

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