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Mechanism of the SiO4 for CO3 substitution in defernite, Ca6(CO3)1.58(Si2O7 )0.21(OH)7[Cl0.50(OH)0.08(H2O)0.42]: A single-crystal X-ray study at 100 K

Abstract Defernite, Ca6(CO3)1.58(Si2O7)0.21(OH)7[Cl0.50(OH)0.08(H2O)0.42], is a carbonate mineral [space group Pnam; at 100 K: a = 17.744(2), b = 22.601(4), c = 3.633(1) Å, Z = 4] with zeolitelike structural channels. The structural framework is formed by edge- and comer-sharing CaO6-7 polyhedra, which are additionally linked by CO3 groups aligned parallel to the (001) plane. This framework has channels parallel to the c axis, confined by eight-membered rings of CaO6-7 polyhedra. The inner surface of the channels exposes H atoms of OH groups bonded to Ca. The framework carries a positive charge, which is balanced by small anions (mainly C1) in the structural channels. The crystal structure of defernite from the Kombat mine, Namibia, was refined using 3845 reflections to R = 2.6% at 100 K. The low but ubiquitous SiO2 concentrations (1-5 wt%) observed in chemical analyses of this mineral were structurally located. Defernite has two symmetrically distinct CO3 groups (C1, C2), which are repeated by a mirror plane parallel to (001). The C2 double layer of CO3 groups is partially replaced by a disilicate Si2O7 unit, where an additional O atom is accommodated between the two O triangles of the former CO3 groups. This substitution is observed only for C2. In addition, all H atoms on the surface of the structural channels were located and refined. Cl in the center of the channel is bonded to six H within 2.8 Å. Defernite may form a solid solution between Ca6 (CO3)2 (OH)7 [Cl,OH] and Ca6(CO3)1.0(Si2O7)0.5(OH)7, in which the latter end-member is characterized by a neutral framework and structural channels without charged ions.