Synthesis and proposed crystal structure of a disordered cadmium arsenate apatite Cd5(AsO4)3Cl1−2x−yOx□xOHy

Abstract

During a study into the synthesis of minerals composed of mining wastes aimed at improving their immobilisation, a cadmium arsenate apatite has been prepared by hydrothermal methods. The structure of this apatite was analysed by single crystal X-ray diffraction, and was found to consist of a standard apatite framework based on Cd₅(AsO₄)₃X, where X represents an anion resident on the (0,0,0.25) site. The framework is hexagonal with the space group P6₃/m (no 176), a = 9.9709(8), c = 6.4916(4) Å. The X ion site is predominantly occupied by Cl⁻ ions; however due to significant shortening of the c axis exhibited by all cadmium containing apatite phases, a pure chlorapatite is not possible without a significant cation deficiency. No evidence of the necessary deficiency was found in the crystal structure. For larger bromo- and iodo-apatites significant modulations along the c-axis are required to accommodate the halide. This paper examines a number of compensation mechanisms and proposes that a minor disorder of chloride, oxide and hydroxide located on the X ion site provides the required charge compensation mechanism. This is contrary to previous complex modulations proposed in the literature. The proposed chemical formula is Cd₅(AsO₄)₃Cl_1−2x−yO_x□_xOH_y where □ represents a vacancy.