Large Nb-Rutile Polyhedra from Chamber Pegmatites of Volyn Region, Ukraine

Abstract

The crystal morphology, chemical composition and mineral inclusions of large Nb-rutile polyhedra from chamber pegmatites of Volyn region were studied. X-ray data for one niobium rutile crystal were also obtained. The sizes of twinned black crystals of the mineral are from 10 to 35 mm. They are interbedded with clevelandite and rare relict microcline. Simple forms of crystals: tetragonal prisms {110} and {100}, dipyramid {111} and sometimes pinacoid {001}. The habit of the crystals is dipyramidal and dipyramidal-prismatic, the shape of the crystals is isometric and short-prismatic. The faces of the prisms are covered with parallel hatching in the vertical belt [001], the faces of the dipyramids are covered with trigon overgrowths. Crystals are twins according to (101), some of them are irregular growths of twins. The unit cell parameters for the studied niobium rutile crystal are a = 4.6159 Å, c = 2.9715 Å. The chemical composition of the mineral, the content of the main components, in mass. %: TiO2 — average 76.16, Nb2O5 — up to 20.45 (average 12.77), Ta2O5 — up to 4.92 (average 3.49) and FeO — up to 6.61 (average 4.56). Mineral inclusions in Nb-rutile crystals are columbite, ilmenite, cassiterite, Fe-phase, W-phase, and quartz. A comparison of the morphology of Nb-rutile from crystalline rocks of Ukraine was made and the dependence of their shape on their chemical composition was analyzed. The probable temperature of the mineral formation lies within the crystallization parameters of clevelandite — approximately 300-400°С.