Structural modifications induced by dehydration in yugawaralite

The dehydration behavior of yugawaralite has been studied by single crystal x-ray diffraction on three samples dehydrated under vacuum at 20, 150 and 200°C, resp. Yugawaralite held in vacuum at room temp. (r.T) - even for relatively long times - does not show noticeable structural changes with respect to untreated samples. The Ca atom remains coordinated to four framework oxygen atoms and to four water oxygens; two of these water mols. are disordered with two alternative oxygen positions. Yugawaralite heated at 150°C is characterized by a water loss as high as 54%, with all the residual water mols. in two fully occupied sites. The Ca atoms are 6-coordinated by four framework oxygens and the two residual water mols. The framework undergoes only minor modifications. The crystal heated at 200°C shows a further 50% loss of the residual water present at 150°C and a collapsing of the channel system: a sudden decrease of unit cell vol. occurs (about 13%), the cell parameter a is doubled, the space group changes from Pc to Pn, and the structure is strongly squashed. Owing to the doubling of parameter a, there are two sym. independent Ca sites; both are 6-coordinated, but now with five framework oxygens and one water mol.