Altisite Na (sub 3) K (sub 6) Ti (sub 2) [Al (sub 2) Si (sub 8) O (sub 26) ]Cl (sub 3) a new hyperalkaline aluminosilicate from Kola Peninsula (Russia) related to lemoynite; crystal structure and thermal evolution

The crystal structure of altisite has been studied at 20 and 700 degrees C by single-crystal XRD to R 0.025 (a 10.363, b 16.310, c 9.132 Aa, beta 105.34 degrees ; space group C2/m; Z = 2). At 400 degrees C, the linear increase in the cell parameters with T suddenly moves to smaller values, reflecting a minor structural modification involving loss of alkali metal atoms; this change proves to be permanent by refinement of the structure after cooling the crystal down from 700 degrees C to room T. The structure consists of (001) aluminosilicate tetrahedral slabs built up by the pentamer [AlSi (sub 4) O (sub 16) ] (super 13-) , where an aluminium tetrahedron shares its corners with four silicon tetrahedra. These slabs are interconnected by Ti octahedra and the resulting framework is crossed by three sets of infinite channels containing the akali atoms. Apart from the content of the channels, the same structural framework occurs in lemoynite, (Na,K) (sub 2) CaZr (sub 2) [Si (sub 10) O (sub 26) ]<P6M><P255>5-6H (sub 2) O, which has the space group C2/c and a double periodicity along [001].