Calcite Twinning in Mollusk Shells and Carrara Marble

• Published in: Advanced functional materials 2024-08, Vol.34 (35), p.n/a
• Main Authors: Castillo Alvarez, Cristina, Grimsich, John L., Schmidt, Connor A., Lisabeth, Harrison, Voigtländer, Anne, Gilbert, Pupa U. P. A.
• Format: Article
• Language: English
• Subjects: Aragonite; biominerals; Calcite; Crystal structure; Deformation; EBSD; Geological mapping; Image contrast; Marble; MATERIALS SCIENCE; PEEM; PIC mapping; Prisms; Shells; Twinning; twins
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202304288

Mollusk shells protect the animals that form and inhabit them. They are composites of minerals and organics, with diverse mesostructures, including nacre, prismatic calcite, crossed‐lamellar aragonite, and foliated calcite. Twins, that is, crystals mirror symmetric with respect to their coherent interface, occurring as formation or deformation twins, are observed in all mollusk shell mesostructures but never within calcite prisms. Here, nanotwins and microwins within single calcite prisms are observed in different shells. Using Polarization‐dependent Imaging Contrast (PIC) mapping with 20–60 nm resolution, twins are observed to be 0.2–3 µm thick layers of differently oriented and colored crystals with respect to the main prism crystal. Multiple twins are interspersed with the prism crystal, parallel to one another, and similarly oriented. When comparing images of calcite prisms and twins obtained by PIC mapping and by Electron Back‐Scattered Diffraction (EBSD), the images correspond precisely. All twins are e‐twin types, with 127° angular distance between c‐axes. E‐twins are the most common deformation twins in geologic calcite, as also observed here in Carrara marble. Location of all twins near the outer surface of all shells and e‐twin type both suggest that twins within calcite prisms in mollusk shells result from deformation twinning. Deformation twinning occurs in Carrara marble and appears in polarized light microscopy as parallel lines in calcite crystals (bottom left). Twins are known to harden and toughen calcite crystals. Twins are never observed within mollusk shell calcite prisms (top left). Here, nano‐ and micro‐twins within calcite prisms and in Carrara marble (top right, bottom right) are found.