Polysomatism and modules of gehlenite composition and structure in the Hanic phase (Ca5Al6MgSiO17)

Discrete, well-ordered, single-phase crystals (the Hanic phase) form when p owders the bulk composition of Ca5Al6MgSiO17 are allowed to react in the ab sence of a melt. If, on the other hand, either the powders or crystals of C a5Al6MgSiO17 are molten and then cooled, complex intergrowths form. These i ntergrowths consist of modules having the composition and structure of Ca3A l4MgO10 and gehlenite (Ca2Al2SiO7), forming a polysomatic series in which t he modules occur in varied proportions. High-resolution transmission electr on microscope (HRTEM) images reveal the details of these intergrowths. Mass balance is achieved by the formation of this polysomatic series together w ith Mg-substituted mayenite (Ca12-xMgxAl14O33). The tetrahedral sheets in g ehlenite have close structural similarities to those of Ca5Al6O14. Based on our HRTEM images, we hypothesize chemical substitutions between gehlenite and Ca5Al6O14, resulting in gehlenite-like modules that are depleted in Si and correspondingly enriched in Al and Ca.