First occurrence of moskvinite-(Y) in the Ilímaussaq alkaline complex, South Greenland – implications for rare-earth element mobility

Abstract

Moskvinite-(Y), Na 2 K(Y,REE)Si 6 O 15 , is a rare mineral, which until now has only been described from its type locality Dara-i-Pioz, Tajikistan. At Ilímaussaq moskvinite-(Y) was discovered in a drill core from Kvanefjeld, where it occurs as a replacement mineral associated with a mineral belonging to the britholite group. The composition was determined by a combination of electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry analyses. The empirical formula based on 15 oxygens is Na 1.94 K 0.99 (Y 0.94 Yb 0.03 Er 0.03 Dy 0.03 Ho 0.01 Gd 0.01 ) ∑1.05 Si 5.98 O 15 . The coexistence of an almost pure Y and a light rare-earth element (REE) mineral is interpreted as fractionation of REE and Y during the replacement of an earlier formed REE mineral. Theoretical calculations of the observed replacement of feldspathoids by natrolite show that the generated fluid would have pH > 8, which inhibits large scale mobility of REE. In addition, a K-Fe sulfide member of the chlorbartonite-bartonite group is for the first time observed in Ilímaussaq where it occurs where sodalite is replaced by natrolite and arfvedsonite by aegirine. The sulfide incorporates the S and some of the Cl generated by the alteration of sodalite, whereas the K and Fe originates from the replacement of arfvedsonite by aegirine.