Altisite Na3K6Ti2[Al2Si8O26]Cl3 a new hyperalkaline aluminosilicate from Kola Peninsula (Russia) related to lemoynite: crystal structure and thermal evolution
Ferraris, Giovanni; Ivaldi, Gabriella; Khomyakov, Alexander P.
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 537 - 546
published: May 19, 1995
manuscript accepted: Jan 4, 1995
manuscript received: Oct 25, 1994
ArtNo. ESP147050703007, Price: 29.00 €
Abstract: The crystal structure of altisite, Na3K6Ti2[Al2Si8O26]Cl3, a new hyperalkaline aluminosilicate from the Kola Peninsula (Russia), has been studied at 20 (RT) and 700°C (HT) by single-crystal X-ray diffractometry [space group C2/m, Z = 2, 131 parameters: RT a = 10.363(2), b = 16.310(3), c = 9.132(2), ß = 105.34(3), R = 0.025 (1957 reflections)]. At 400°C, the linear increase of the cell parameters with temperature suddenly shifts towards 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°C (RT'). The structure consists of (001) aluminosilicate tetrahedral slabs built up by the pentamer [AlSi4O16]13- where an aluminum tetrahedron shares its corners with four 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[Si10O26].5-6H2O, which has the space group C2/c and a double periodicity along .