ON POSSIBLE REASONS OF SULFATIC SODALITE MODULATIONS. V.G.Evsyunin, Institute of Earth's Crust, Irkutsk; A.N.Sapozhnikov, Institute of Geochemistry, Irkutsk; R.K.Rastsvetaeva, Institute of Crystallography, Moscow; A.A.Kashaev, Teacher Training Institute, Irkutsk

Sulfur containing sodalite minerals are often characterized by satellite reflections on their diffraction pattern, causing commensurate and incommensurate structures. As it is known the great amount of potassium atoms (>1 per cell) prevents the arising of satellite reflections in hauyne, we have carried out X-ray single crystal analysis of the potassium-rich (1.8 atoms) sam- ple from Arissia (Italy). The structure was refined up to R=0.032 under the P-43n space group, 164 reflections. As a result we established that framework oxygen atoms occupy only one 24(i) position, and SO₄ group has two statistical orientations. K, Na and Ca ions are distributed statistically in three 8(e) close positions. The large K atom occupies one of these together with Na and its position is firmly fixed between three framework oxygen atoms and three oxygen atoms from SO₄ group, that prevents framework distortion. On the contrary Ca and Na atoms are suitable for the sodalite cavities but they are too small for the hauyne framework cavities. This is one of the reasons for the structure modulation of some sulfatic sodalites containing small amount of K-atoms.