Fig. 2.

Polymorph-selective nucleation of BaCO₃ on different CaCO₃ polymorphs. (A) Backscatter electron microscopy images were used to highlight the difference between BaCO₃ (appearing light) on different polymorphs of CaCO₃ (appearing dark). The lattice mismatches were estimated by measuring the crystallographic alignment of BaCO₃ on calcite, aragonite, and vaterite (SI Appendix). (A, i) Different polymorphs of CaCO₃. (A, ii) BaCO₃ crystals on top of the different polymorphs of CaCO₃. (A, iii–v) TEM imaging and corresponding selected-area electron diffraction at the interfaces revealed crystallographic alignment of BaCO₃ on top of calcite and aragonite, whereas no crystallographic alignment was found on vaterite. (B–D) Three representations of statistical data of CaCO₃ overgrowth. (B) Total number of various polymorphs of CaCO₃ crystals as a function of depth: calcite (left bar), aragonite (middle bar), and vaterite (right bar). The number of the overgrown CaCO₃ crystals in each category are shown as filled bars: calcite-BaCO₃ (blue), aragonite-BaCO₃ (green), and vaterite-BaCO₃ (red). (C) Polymorph ratio based on data in Fig. 2B, highlighting that close to the meniscus BaCO₃ nucleation only occurs on calcite and aragonite, and deeper in the solution––only on vaterite. (D) CDF of the overgrown crystals as a function of depth.