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New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO₂ system is more complex than previously thought. We characterized the BaCa(CO₃)₂ alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V₀ = 1608(2) ų, B₀ = 60(3) GPa, B′₀ = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO₃)₂ polymorphs was investigated. Authors thank the financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN) and the Agencia Estatal de Investigación under projects MALTA Consolider Ingenio 2010 network (MAT2015-71070-REDC) and PGC2018-097520-A-I00 (co-financed by EU FEDER funds), and from the Generalitat Valenciana under project PROMETEO/2018/123. D.S.-P. and A.O.R. acknowledge the financial support of the Spanish MINECO for the RyC-2014-15643 and RyC-2016-20301 Ramon y Cajal Grants, respectively. C.P acknowledges the financial support from the Spanish Ministerio de Economia y Competitividad (MINECO project FIS2017-83295-P). Authors also thank Dr. Nicolescu and the Mineralogy and Meteroritic Department of the Yale Peabody Museum of Natural History for providing the mineral samples, and ALBA-CELLS synchrotron for providing beamtime under experiment 2019093741. A.O.R. thanks the MALTA Consolider supercomputing centre and Compute Canada for computational resources.