Nafertisite - Na3Fe10Ti2Si12(O,OH,F)(43) - from the Nanna Pegmatite, Narsaarsuup Qaava, South Greenland

Nafertisite has been identified from the Nanna Pegmatite, a lens-shaped, hi ghly complex alkaline body in the Igaliko nepheline syenite complex, South Greenland. It occurs in large amounts, associated with potassium feldspar, sodalite var. hackmanite, analcime, natrolite, aegirine, astrophyllite, cat apleiite, an unidentified Al-rich phase, and a new Ca-Y-Al-PO4-CO3-OH-H2O m ineral. Nafertisite forms asbestiform masses, with individual crystals seve ral cm long but never exceeding 10 mu m in width. It is dark to golden brow n, with a vitreous to silky lustre; its optical properties correspond accep tably to those of nafertisite from the type locality. Electron microprobe a nalysis gave: SiO2 35.34, Al2O3 3.69, TiO2 7.86, ZrO2 0.30, Nb2O5 0.96, FeO 36.75, MnO 1.94, MgO 0.37, Na2O 2.74, K2O 2.09, CaO 0.34, F 0.86, H2O 7.04 (by stoichiometry), F = O -0.36, Sum 99.92 wt.%, corresponding to the empi rical formula (Na1.50K0.75Ca0.10)(Sigma 2.35) (Fe8.67Mn0.46Mg0.16)(Sigma 9. 29) (Ti1.67Nb0.12Zr0.04)(Sigma 1.83) (Si9.96Al1.23)(Sigma 11.19)O-29(OH)(13 .23)F-0.77, based on 43 anions and (OH+F) = 14. The unit cell dimensions fr om powder data are: a 5.304(3), b 16.01(1), c 21.71(1) Angstrom and beta 94 .76(5)degrees, in close agreement with those of nafertisite from the type l ocality. TEM confirmed the space group A2/m. Almost all [001] SAED patterns showed marked streaking along b* indicating extended polysomatic disorder. [210] SAED patterns indicated polytypic disorder as well.