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Defernite, Ca 6 (CO 3 ) 1.58 (Si 2 O 7 ) 0.21 (OH) 7 [Cl 0.50 (OH) 0.08 (H 2 O) 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 CaO 6-7  polyhedra, which are additionally linked by CO 3  groups aligned parallel to the (001) plane. This framework has channels parallel to the c axis, confined by eight-membered rings of CaO 6-7  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 SiO 2  concentrations (1-5 wt%) observed in chemical analyses of this mineral were structurally located. Defernite has two symmetrically distinct CO 3  groups (C1, C2), which are repeated by a mirror plane parallel to (001). The C2 double layer of CO 3  groups is partially replaced by a disilicate Si 2 O 7  unit, where an additional O atom is accommodated between the two O triangles of the former CO 3  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 Ca 6  (CO 3 ) 2  (OH) 7  [Cl,OH] and Ca 6 (CO 3 ) 1.0 (Si 2 O 7 ) 0.5 (OH) 7 , in which the latter end-member is characterized by a neutral framework and structural channels without charged ions.

american mineralogist

Mechanism of the SiO 4  for CO 3  substitution in defernite, Ca 6 (CO 3 ) 1.58 (Si 2 O 7  ) 0.21 (OH) 7 [Cl 0.50 (OH) 0.08 (H 2 O) 0.42 ]: A single-crystal X-ray study at 100 K