The effect of a range of inorganic and organic crystal growth modifiers on the morphology of Ca 3 Al 2 (OH) 12 has been studied. It has been found that some additives show a strong preference for a single crystal face forming polyhedral crystals whilst others are less selective, producing crystals with a less well defined morphology. In this way sulphite and dithionite preferentially interact with the {111} face producing octahedral crystals and nitrilotriacetic acid stabilises the {110} face forming rhombic dodecahedral crystals. Phosphate, however, leads to the formation of small needle-like crystals and the organic acids, N(CH 2 CO 2 H) 2 (CH 2 PO 3 H 2), N(CH 2 CO 2 H)(CH 2 PO 3 H 2) 2 , HN(CH 2 PO 3 H 2) 2 , and H 3 CN(CH 2 PO 3 H 2) 2 , produce near spherical crystals. The ability to be able to control the morphology of a material is of great importance in a variety of fields. This control is brought about by varying the crystallising conditions, typically by the addition of a crystal growth modifier to the solution. For molecular crystals, morphological modification has been brought about by the use of stereospecific inhibition of the growth of particular crystal faces. [1,2]

Molecular modeling has been used to predict both the structure and morphology of Ca3Al2(OH)12. The calculated structure is in excellent agreement with the experimentally determined one with a less than 1% decrease in the lattice parameter. The morphology of Ca3Al2(OH)12 was predicted to be the 24-sided deltoidal icositetrahedron formed from the {112} face, and while the experimental morphology of Ca3Al2(OH)12 is generally poorly defined, the smaller crystals in the sample did display this morphology. This experimental morphology of Ca3Al2(OH)12 has been shown to be strongly dependent on the presence of crystal growth modifier anions in the reaction solution with each additive studied favoring a specific crystal face to the exclusion of all others. It was found that sulfate stabilizes the {111} face, leading to the formation of crystals with an octahedral morphology, oxalate favors the {110} face, forming rhombic dodecahedral crystals, and EDTA interacts preferentially with the {210} face, generating crystals with a tetrakis hexahedral morphology. These experimental observations have been explained by molecular modeling, which demonstrated that the lowest replacement energy for each of the additive anions was for the experimentally observed crystal face.