ALTISITE NA3K6TI2[AL2SI8O26]CL3 A NEW HYPERALKALINE ALUMINOSILICATE FROM KOLA-PENINSULA (RUSSIA) RELATED TO LEMOYNITE - CRYSTAL-STRUCTURE AND THERMAL EVOLUTION

The crystal structure of altisite, Na3K6Ti2[Al2Si8O26]Cl3, a new hyper alkaline aluminosilicate from the Kola Peninsula (Russia), has been st udied at 20 (RT) and 700-degrees-C (HT) by single-crystal X-ray diffra ctometry [space group C2/m, Z = 2, 131 parameters: RT a = 10.363(2), b = 16.310(3), c = 9.132(2), beta = 105.34(3), R = 0.025 (1957 reflecti ons)]. At 400-degrees-C, the linear increase of the cell parameters wi th temperature suddenly shifts towards smaller values, reflecting a mi nor structural modification involving loss of alkali metal atoms; this change proves to be permanent by refinement of the structure after co oling the crystal down from 700-degrees-C (RT'). The structure consist s of (001) aluminosilicate tetrahedral slabs built up by the pentamer [AlSi4O16]13-, where an aluminum tetrahedron shares its corners with f our silicon tetrahedra. These slabs are connected by Ti octahedra and the resulting framework is crossed by three sets of infinite channels which contain the alkali atoms. Apart from the content of the channels , the same structural framework occurs in lemoynite, (Na,K)2CaZr2[Si10 O26].5-6H2O, which has the space group C2/c and a double periodicity a long [001].