Crystal structure of the K, Ti analogue of ilímaussite-(Ce), (Ba,K,Na,Ca)11–12(REE,Fe,Th)4(Ti,Nb)6(Si6O18)4(OH)12·4.5 H2O: revision of structure model and structural formula

Abstract

Abstract The crystal structure of the K, Ti analogue of il¡maussite-(Ce), (Ba,K,Na,Ca)11–12(REE,Fe,Th)4(Ti,Nb)6(Si6O18)4(OH)12·4.5 H2O, from the Yukspor mountain, Khibiny massif, Kola peninsula, Russia, has been solved by direct methods and refined to R 1 = 0.077 on the basis of 1762 reflections with |F o| ≥ 4σ F. The mineral is rhombohedral, R32, a = 10.708(2), c = 60.073(11) Å, V = 5965.2(17) Å3. The structure is based upon a heteropolyhedral framework that consists of layers of corner-linked polyhedra. The SiO4 tetrahedra share corners to form 6-membered rings which are further interlinked via REEO6 trigonal prisms and Tiφ 6 octahedra (φ = O, OH). The obtained structure model is different from that recently reported by Rastsvetaeva et al. (2003) which is considered incorrect.