Twinning and homoepitaxy cooperation in the already rich growth morphology of CaCO3 polymorphs. I. Aragonite

Calcite and aragonite are the two most abundant among the CaCO 3 polymorphs and are also well known for their rich morphology and habit, to which twinning relevantly contributes. Moreover, the calcite → aragonite transformation has been debated for a long time, even though the homo-epitaxies occurring within each polymorph have been overlooked, to date, from both experimental and theoretical points of view. Twinning is common and unfortunately can be deceptive as it can be mistaken for homo-epitaxy, thus leading to confusion regarding the growth mechanisms of many crystal aggregates. Here, experimental and theoretical studies were carried out to investigate the twinning and homo-epitaxy that operate in aragonite: (i) the connection between twinning and homo-epitaxy for contact and penetration aggregates made by the lateral {010} and {110} aragonite forms; (ii) the homo-epitaxial relationships among the {001} pinacoid and both the {010} pinacoid and the prismatic {110} aragonite forms. This work attempts to provide a new approach to monitoring the growth mechanisms of aragonite in cases where it is obtained not as a single crystal but as an aggregate. An analogous problem will be explored in our forthcoming work on calcite.