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

 Discrete, well-ordered, single-phase crystals (the Hanic phase) form when powders the bulk composition of Ca₅Al₆MgSiO₁₇ are allowed to react in the absence of a melt. If, on the other hand, either the powders or crystals of Ca₅Al₆MgSiO₁₇ are molten and then cooled, complex intergrowths form. These intergrowths consist of modules having the composition and structure of Ca₃Al₄MgO₁₀ and gehlenite (Ca₂Al₂SiO₇), forming a polysomatic series in which the modules occur in varied proportions. High-resolution transmission electron microscope (HRTEM) images reveal the details of these intergrowths. Mass balance is achieved by the formation of this polysomatic series together with Mg-substituted mayenite (Ca_12−xMg_xAl₁₄O₃₃). The tetrahedral sheets in gehlenite 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. Received: 2 August 2000 / Accepted: 18 January 2001