In previous high-pressure experiments, the emergence of sp3-hybridized tetrahedral carbonate anions was shown for CaCO3 above 100 GPa by Raman spectroscopy. Recent ab initio calculations, however, have shown that much lower pressures of ~20 GPa could be required for the synthesis of sp3-orthocarbonates Ca3CO5 and Ca2CO4. The latter phase was recently successfully synthesized, though the structure and stability limits of the former one have not been assessed experimentally yet. The main objective of the proposal is to investigate the formation and crystal structure of novel sp3-orthocarbonate Ca3CO5 and the reaction of its decomposition into CaCO3 and another sp3-orthocarbonate Ca2CO4 by means of angle-dispersive in situ x-ray diffraction in multianvil press.