Structural modifications induced by dehydration in yugawaralite.

Abstract

The dehydration behaviour of yugawaralite has been studied by single crystal X-ray diffraction on three samples dehydrated under vacuum at 20, 150 and 200°C, respectively. Yugawaralite held in vacuum at room temperature(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 molecules 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 molecules in two fully occupied sites. The Ca atoms are 6-coordinated by four framework oxygens and the two residual water molecules. 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 volume 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 symmetrically independent Ca sites; both are 6-coordinated, but now with five framework oxygens and one water molecule.