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

Understanding the selection mechanisms of CaCO3polymorphs (vaterite, aragonite, and calcite) is pivotal for elucidatingboth inorganic and biogenic carbonate formation. It is peculiar thatdifferent polymorphs originate from the same organism; in addition,these polymorphs can even be epitaxially related. Here, we ask whysome mollusks and gastropods develop calcite (Cc) layers at thecontact with aragonite (Ar) in the outer portion of their shell, whileothers do not. To establish the most likely epitaxial relationshipsbetween calcite and aragonite, here, we investigated at the empiricallevel (and 0 K) the (001)Ar/(00.1)Cc, (110)Ar/(10.0)Cc, (010)Ar/(01.2)Cc,and(100)Ar/(11.0)Ccinterfaces. Upon analyzing the(001)Ar/(00.1)Ccepitaxial and relaxed interface, we found that ahexagonal (space group,P6322) CaCO3polymorph is generated,corresponding to the phase recently identified during the molecular dynamics study on the high-temperature (similar to 600 K) aragonite-calcite transition. This polymorph, showing a symmetric intermediate betweenaragonite (orthorhombic) and calcite(rhombohedral), develops as a nanometric phase and exhaustively explains the observed epitaxy, (001)Ar/(00.1)Cc. We proposethat the growth of calcite at room pressure and temperature in the outer portion of the shell of mollusks and gastropods is strictlyassociated with the formation of this hexagonal CaCO3polymorph. Moreover, to intensify the growth process of carbonatepolymorphs, we can envisage that thefindings have implications for understanding the sluggish mechanisms of aragonite-calcitetransformation under ambient conditions.