A K-DOMINANT NENADKEVICHITE FROM THE NARSSARSSUK PEGMATITE, SOUTH GREENLAND

A potassium analogue of nenadkevichite was very briefly mentioned from Gjerdingen, Norway, in 1982, analyses of nenadkevichite from Mont Sai nt-Hilaire, Quebec, Canada, with K2O > Na2O were reported in 1989 and the structure of a K-rich nenadkevichite from Vuorijarvi, Kola Peninsu la, Russia, was reported in 1994; this article presents the first desc ription of a K-dominant nenadkevichite from Greenland and the first mi neralogical description of this new member of the nenadkevichite famil y. The K-dominant nenadkevichite described is from the Narssarssuk peg matite in the westernmost margin of the Igaliko complex. It is associa ted with potassium feldspar, aegirine, eudialyte, quartz, neptunite, e lpidite, ancylite, epididymite, polylithionite and pyrochlore, forming millimetre-sized aggregates of platy/bladed, colourless, transparent, less than or equal to 0.05 mm large crystals; it is in several cases found epitaxially overgrown on elpidite in an intriguing way. Morpholo gically the crystals are orthorhombic and show the forms [010], [001], [101] and occasionally [012], the crystals are platy/bladed parallel to [001), the Narssarssuk K-dominant nenadkevichite crystals are ident ical to the individual crystals of the ''wagon wheel'' aggregates of n enadkevichite presented from Mont Saint-Hilaire. Optically this K-domi nant nenadkevichite is biaxial positive with 2V(alpha(meas.)) = 145 - 155 +/- 1 degrees, 2V(alpha(calc.)) = 148 +/- 0.5 degrees, r < v, n(al pha) = 1.6386, n(beta) = 1.6474 and n gamma = 1.7680 all +/- 0.0005. T he optical orientation of the Narssarssuk K-dominant nenadkevichite is alpha = c, beta = b and gamma = a, incontestably different. from what is generally accepted for nenadkevichite. All X-ray diffraction obser vations are consistent with an orthorhombic unit cell with the paramet ers, a = 14.72(3), b = 14.20(2) and c = 27.87(5) Angstrom, identical t o the unit cell parameters presented for the K-rich nenadkevichite fro m Vuorijarvi, Kola Peninsula, Russia. The K-dominant nenadkevichite fr om Narssarssuk shows the same absences of reflections that led RASTSVE TAEVA et al. (1994) to the conclusion that the reciprocal lattice of t he Vuorijarvi K-rich nenadkevichite ''can be considered as a superposi tion of two monoclinic diffraction patterns with a monoclinic axis par allel to axis 'b' in orthorhombically twinned lattice and with C-centr ing of each monoclinic cell''. The chemical composition of the K-domin ant nenadkevichite from Narssarssuk was determined by electron micropr obe, Na2O 3.75 (3.44-4.06), CaO 1.04 (0.96-1.09), K2O 6.53 (6.20-6.86) , BaO 1.28 (0.76-2.32), Ce2O3 0.41 (0.27-0.71),Nb2O5 35.81 (34.52-38.0 1), TiO2 2.54 (1.37-3.41), SiO2 35.48 (35.06-36.20), Al2O3 0.23 (0.18- 0.26), and H2O 13.40 (13.25-13.65) - calculated by stoichiometry, tota l 100.47 wt.% corresponding to the empirical formula (K0.47Na0.41Ca0.0 6Ba0.02Ce0.01)(Sigma 0.97) (Nb0.91Ti0.11)(Sigma 1.02) (Si1.98Al0.02)(S igma 2.00)O-6.53(OH). 2H(2)O.