Malladrite Na2SiF6 is one of the few minerals that contains silicon but is not a silicate. Most of the silicon-bearing minerals without oxygen are either elemental alloys, carbides, nitrides or halides.
For a mineral with a simple formula it has a somewhat irregular structure. Because of the high charge of silicon and the small center to center distance between silicon and fluorine, the silicon atoms are octahedrally coordinated with the fluorine and the octahedra are quite regular. Also, fluorine is smaller than oxygen, favoring octahedral coordination. The sodium ions can't compete in either size or charge and they occupy voids between the silicon octahedra, with irregular octahedra.
Above: View down the c-axis. Silicon polyhedra are purple, sodium yellow, and fluorine atoms (smaller than scale) are green.
Above: view perpendicular to the c-axis.
Created 22 April 2013, Last Update 13 Mar 2016
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