The solid solution between ettringite (Ca6Al2(SO4)(3)(OH)(12). 26H(2)O) and carbonate ertringite (Ca6Al2(CO3)(OH)(12). 26H(2)O) has been investigated by X-ray powder diffraction (XRPD) full pattern fitting. A single solid-solution phase was found when SO42- was the majority anion, while two phases existed when CO32- predominated. It is proposed that the formation of two phases is due to a crystallographic incompatibility between the two end members and hence, no mechanism for a continuous solid-solution series from one to the other is possible. Interpreting the data using the variation in unit cell volume as a function of chemical composition and through a unit cell parameter map, a more detailed examination of the crystallography of the phases was possible. This showed that the solid-solution mechanisms at the atomic level lead to complex yet consistent changes in the unit cell shape and volume that are readily followed by modem powder diffraction methods. (C) 2001 Elsevier Science Ltd. All rights reserved.