FERSMITE FROM THE ILIMAUSSAQ ALKALINE COMPLEX, SOUTH GREENLAND

The relatively rare mineral fersmite has been found for the first time in Greenland, in a set of microcline veins in augite syenite at Nakal aq (Nakkaalaaq) in the Ilimaussaq alkaline complex, South Greenland. T he Ilimaussaq fersmite forms several mm-large pinkish aggregates of in tergrown fans and bunches of needle-shaped crystals no more than 0.lmm in length. Or, when grown in some of the numerous cavities, it forms several mm-large free fans and sprays, which are yellowish with an ora nge hue. The needleshaped or fibrous crystals constituting the fans an d sprays are whitish or colourless, most often completely transparent, and rarely exceed 1 mm in length. Associated minerals include microcl ine, aegirine, bavenite, bertrandite, epididymite, helvite, genthelvit e, galena, titanite and calcite. The fersmite belongs to the latest mi nerals formed; it is primary and unrelated to columbite/tantalite, as is otherwise common in such ''pegmatitic'' occurrences of fersmite. Th e Ilimaussaq fersmite is biaxial positive, the acicular crystals have parallel extinction and positive elongation, n(gamma) = 2.19 and n(alp ha) = 2.10 both +/- 0.02. The optical orientation of fersmite, partly determined on additional material from Valle Vigezzo, Italy, is alpha = b, beta = a and gamma = c. The orthorhombic unit cell dimensions are , a = 5.760, b = 14.910, c = 5.241 Angstrom all +/- 0.005 Angstrom. Bo th optical and Xray data agree well with those of fersmite from other localities. The chemical composition, determined by electron microprob e, is: Nb2O5 = 71.50, Ta2O5 = 0.42, SiO2 = 1.39, TiO2 = 5.90, Al2O3 = 0 06. MnO = 0.17, FeO = 0.92, CaO = 17.70, Total = 98.06, correspondin g to the empirical formula, Ca-0.98 (Nb1.67Ti0.23Si0.07Fe0.04Ta0.01Mn0 .01)(Sigma 2.03) (O-5.62 (OH)(0.38))(Sigma 6.00).