Thomsenolite, NaCaAlF6•H2O: hydrogen bonding and comparison with pachnolite

Abstract

The crystal structure of thomsenolite, NaCaAlF6•H2O, has been redetermined to establish the hydrogen-bonding scheme in this mineral. Both hydrogen atoms participate in branched [Formula: see text] bonds. The hydrogen bonds link the AlF6, octahedra to form infinite chains ||b, which in turn are cross-linked to form infinite double sheets {[AlF6] + Ca}—(H2O)—{[AlF6] + Ca}||(001). The Na atoms are located exclusively in layers ||(001) which separate the double sheets. A detailed comparison of thomsenolite with its dimorph, pachnolite, shows that the structure of pachnolite is obtained in essence by interchanging the positions of one half of the Na atoms and one half of the water molecules in thomsenolite. The two-dimensional, layerlike hydrogen-bonding network in thomsenolite is thereby changed to one of a three-dimensional character in pachnolite. Other features of the two structures, including the Al—F and [Formula: see text] distances, are compared and discussed in some detail.