Highly ordered, homogeneous ferrocolumbite from an anorogenic granitic pegmatite at Ivittuut (Ivigtut), SW Greenland

Accessory but widespread ferrocolumbite occurs in a coarse-grained microcli ne+quartz pegmatite overlying the Ivigtut cryolite deposit, SW Greenland. E uhedral crystals, up to 2 cm in size, are enclosed in microcline, locally i n quartz, and overgrown by late cryolite. The crystals are commonly found l oose, very likely due to weathering of the matrix. Associated minor and acc essory minerals include sulphides (galena, sphalerite, chalcopyrite, molybd enite) and Nb-rich cassiterite (with Nb/Ta-wt. = 17.2-7.5). All ferrocolumb ite samples studied are extremely homogeneous and have remarkably similar c hemical compositions; the ratio Mn/(Mn+Fe)(at.) varies from 0.22 to 0.27, w hereas Ta/(Ta+Nb)(at.) ranges from 0.004 to 0.02, and Nb/Ta-wt. = 46.1-25.1 . Minor elements include Ti and W up to 0.14 apfu, Zn up to 0.06 apfu and S n up to 0.02 apfu. The unit-cell dimensions indicate highly to fully ordere d structure. The very high Nb/Ta ratio and homogeneity of ferrocolumbite se em to be caused by very low activity of Ta relative to Nb in the parent (po ssibly mantle-derived) melt; and by near-equilibrium crystallization.