Mechanism of the SiO4for CO3substitution in defernite, Ca6(CO3) (sub 1.58) (Si2O7) (sub 0.21) (OH)7[Cl (sub 0.50) (OH) (sub 0.08) (H2O) (sub 0.42) ]; a single-crystal X-ray study at 100 K

Abstract

Defernite, Ca6(C03)L5s(Si207)o.21(OHMClo.5o(OH)o.os(H20)od, is a carbonate mineral [space group Pnam; at 100 K: a = 17.744(2), b = 22.601(4), c = 3.633(1) A, Z = 4] with zeolitelike structural channels. The structural framework is formed by edge- and comer-sharing Ca06_7 polyhedra, which are additionally linked by C03 groups aligned parallel to the (001) plane. This framework has channels parallel to the c axis, confined by eight-membered rings of Ca06_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 Cl) 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 Si02 concentrations (1-5 wt%) observed in chemical analyses of this mineral were structurally located. Defernite has two symmetrically distinct C03 groups (Cl, C2), which are repeated by a mirror plane parallel to (001). The C2 double layer of C03 groups is partially replaced by a disilicate Si207 unit, where an additional 0 atom is accommodated between the two 0 triangles of the former C03 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 A. Defernite may form a solid solution between Ca6 (C03)2 (OH)7 [Cl,OH] and Ca6(C03)l.o(Si207)oAOH)7, in which the latter end-member is characterized by a neutral framework and structural channels without charged ions.