Multiple Twinning in Nacre and Aragonite

Abstract Twinning occurs when two crystals share a coherent interface along which their lattices are mirror symmetric. Here, twinning is investigated in biogenic and geologic aragonite (CaCO 3 ) in columnar nacre from the red abalone Haliotis rufescens , sheet nacre from the black‐lip pearl oyster Pinctada margaritifera , and geologic aragonite. All samples exhibit the expected single (110) twins, characterized by a 116.2° rotation of the aragonite a‐ axis. Surprisingly, multiple twins—double, triple, quadruple—are also observed, each involving successive 116.2° rotations of the a‐ axis around the c ‐axis. Multiple twinning is concomitant but distinct from the well‐known cyclic twinning of aragonite. Multiple twinning is most prevalent in columnar nacre, reaching up to quadruple twins, most frequently occurring between nacre tablets in the same column, whereas sheet nacre and geologic aragonite exhibit up to triple twins. The frequency of twins decreases with increasing twin multiple, consistent with a physical rather than biological origin. This interpretation is supported by observations in geologic aragonite and by simulations of columnar nacre growth, which incorporate only nucleation rate, growth rate, and basic geometric constraints, yet they reproduce the observed twinning behavior in columnar nacre, reinforcing the conclusion that multiple twinning arises from physical growth mechanisms.