Metaettringite, a decomposition product of ettringite.

Dehydroxylation of ettringite in an atmosphere of constant. partial water vapour pressure (30-400 Torr) and controlled temperature (55-95 degreesC) yields an X-ray amorphous product containing 11-13 H2O per ettringite formula unit. The X-ray amorphous product does, however, give electron diffraction patterns similar to those of ettringite but with a considerably reduced from similar to1.123 nm (ettringite) to 0.85 nm in the partially dehydroxylated product, termed metaettringite. The structure of metaettringite is closely related to that of fleischerite, Pb3Ge[(OH)(6)](SO4)(2)(.)3H(2)O and the isostructural despujolsite, Ca3MnIV(SO4)(2)(OH)(6)(.)3H(2)O. These minerals contain columnar units. The columnar structure is like that of etttingite but with closer packing of columns in the a direction, resembling metaettringite.

The mechanism of collapse of ettringite to metaettringite, involving loss of water and motion of columns, cannot be achieved without scissoring and possibly rotation of individual columns; this and other defect-producing mechanisms result in loss of crystallinity of the metaettringite product. (C) 2004 Elsevier Ltd. All rights reserved.