  Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays
a crucial role in various fields. Of the four polymorphs, calcite, aragonite,
vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an
ongoing debate regarding its structure that has persisted for nearly a century.
In this work, based on systematic transmission electron microscopy
characterizations, elaborate crystallographic analysis and machine learning
aided molecular dynamics simulations with ab initio accuracy, we reveal that
vaterite can be regarded as a polytypic structure. The basic phase is a
monoclinic lattice possessing pseudohexagonal symmetry. Direct imaging and
atomic-scale simulations provide evidence that a single grain of vaterite can
have three orientation variants. Additionally, we find that vaterite undergoes
a second-order phase transition. These atomic scale insights provide a
comprehensive understanding of the structure of vaterite and offer new
perspectives on the biomineralization process of calcium carbonate.
