TEM characterization of synthetic hillebrandite: Structure, microstructure, and dehydration

Abstract

Hillebrandite (Ca2SiO4•H2O) occurs in nature as a white, fibrous mineral. Synthetic hillebrandite was prepared hydrothemally from calcia and silica. On heating, hillebrandite dehydrated to β-Ca2SiO4 which is stable at room temperature. The crystal structure, microstructures and the dehydration mechanism of the synthetic hillebrandite prepared by Ishida et. al were investigated by conventional TEM, HREM, and in situ hot-stage TEM.Most hillebrandite fibers showed their fiber axis to be parallel to the b axis and tended to lie on {001} cleavage planes. SADP’s frequently displayed continuous streaking, and corresponding dark field images revealed stacking disorders perpendicular to the fiber axis (Fig. 1). The observed [001] SADP (Fig. 2) was very different from the expected [001] SADP based on published XRD data (space group, P21/a). For all hk0 reflections, (i) if k = odd, no individual spots are shown but weak continuous streaking exists, (ii) if k = 2, reflections of h = even are extinct, and (iii) if k = 4, reflections of h = odd are extinct.