Transitional calcite-aragonite stacking as a signature of hexagonal aragonite in biogenic and inorganic deposits

Aragonite abundantly crystallizes in marine and continental environments and occurs ubiquitously in biominerals, despite being metastable under ambient conditions. To resolve this apparent contradiction, we studied the crystal structure of aragonite that grew on calcite of biogenic (Berger Károly cave; Hungary) and inorganic (Sonnenberg; Italy and Erzberg; Austria) origin, in a range of natural settings and temperatures. Electron microscopy measurements show (001) stacking faults (SFs) in the first-grown aragonite. Structural modelling combined with theory-based optimization suggests that the crystal structure of this new type of SF can be explained by the presence of (000l) calcite-like layers within aragonite. Abundant (0001) calcite twins occur in the last-grown calcite, and hexarag, the hypothetical hexagonal polymorph of aragonite, was identified across the calcite twin interfaces. Oxygen isotope data indicate that both the Berger-Károly and Sonnenberg samples formed under non-equilibrium conditions, whereas the Erzberg aragonite formed near equilibrium, possibly explaining the defect-free structure. The presence of (001) SFs and (0001) calcite twins testifies to the formation of transitional calcite-aragonite structures, suggests the occurrence of a hexarag precursor phase and provides new insights into the long-debated mechanism of metastable aragonite formation in dynamically evolving (bio)geologic environments.