Th-rich Ioparite from the Khibina alkaline complex, Kola Peninsula: isomorphism and paragenesis

Th-rich (up to 18.4 wt.% ThO2) loparite occurs as an accessory phase in foyaite pegmatites at Mr. Eveslogchorr, Khibina complex, Russia. It is associated with aegirine, astrophyllite, eudialyte, lorenzenite, lamprophyllite, magnesio-arfvedsonite and gerasimovskite. Loparite crystals are zoned from niobian loparite (core) to niobian thorian and thorian niobian loparite (rim). Th-enrichment is accompanied by a decrease in Na, LREE, Sr and increase in A-site vacancies. The most Th-rich composition approaches (Nao.39LREE0.19ThoA2Cao.05Sr0.02)ro.77(Tio.76Nb0.27):Cl.O303. The mineral is partly or completely metamict and after annealing gives an X-ray diffraction powder pattern similar to that of synthetic NaLaTi206 and naturally occurring loparite of different composition. For the Th-rich rim sample, the five strongest diffraction lines o(A) are: 2.72 (100) 110, 1.575 (60) 211, 1.925 (40), 1.368 (30) 220, 1.222 (20) 310; a = 3.867(2) A. The X-ray diffraction patterns do not exhibit peak splitting or other diffraction lines typical of low-symmetry and ordered perovskite-type structures. Composition determinations, infrared transmission spectroscopy and X-ray diffractometry show that thorian loparite is partly replaced by betafite with LREE and Th as dominant A-site cations ('ceriobetafite'). Some loparite samples also exhibit thin replacement mantles of belyankinite with high LREE203 and ThO2 contents. Both 'ceriobetafite' and belyankinite were formed due to metasomatic alteration of loparite. KEYWORDS" loparite, betafite, belyankinite, nepheline-syenite pegmatite, Khibina alkaline complex, Kola Peninsula, Russia.