Calcite/Aragonite Epitaxy: A Computational Study for Understanding Mollusk Shell Formation

Understanding the selection mechanisms of CaCO3 polymorphs (vaterite, aragonite, and calcite) is pivotal for elucidating both inorganic and biogenic carbonate formation. It is peculiar that different polymorphs originate from the same organism; in addition, these polymorphs can even be epitaxially related. Here, we ask why some mollusks and gastropods develop calcite (Cc) layers at the contact with aragonite (Ar) in the outer portion of their shell, while others do not. To establish the most likely epitaxial relationships between calcite and aragonite, here, we investigated at the empirical level (and 0 K) the (001)Ar/(00.1)Cc, (110)Ar/(10.0)Cc, (010)Ar/(01.2)Cc, and (100)Ar/(11.0)Cc interfaces. Upon analyzing the (001)Ar/(00.1)Cc epitaxial and relaxed interface, we found that a hexagonal (space group, P6322) CaCO3 polymorph is generated, corresponding to the phase recently identified during the molecular dynamics study on the high-temperature (600 K) aragonite-calcite transition. This polymorph, showing a symmetric intermediate between aragonite (orthorhombic) and calcite (rhombohedral), develops as a nanometric phase and exhaustively explains the observed epitaxy, (001)Ar/(00.1)Cc. We propose that the growth of calcite at room pressure and temperature in the outer portion of the shell of mollusks and gastropods is strictly associated with the formation of this hexagonal CaCO3 polymorph. Moreover, to intensify the growth process of carbonate polymorphs, we can envisage that the findings have implications for understanding the sluggish mechanisms of aragonite-calcite transformation under ambient conditions.