  The stability, structure and properties of carbonate minerals at lower mantle
conditions has significant impact on our understanding of the global carbon
cycle and the composition of the interior of the Earth. In recent years, there
has been significant interest in the behavior of carbonates at lower mantle
conditions, specifically in their carbon hybridization, which has relevance for
the storage of carbon within the deep mantle. Using high-pressure synchrotron
X-ray diffraction in a diamond anvil cell coupled with direct laser heating of
CaCO$_{3}$ using a CO$_{2}$ laser, we identify a crystalline phase of the
material above 40 GPa $-$ corresponding to a lower mantle depth of around 1,000
km $-$ which has first been predicted by \textit{ab initio} structure
predictions. The observed $sp^{2}$ carbon hybridized species at 40 GPa is
monoclinic with $P2_{1}/c$ symmetry and is stable up to 50 GPa, above which it
transforms into a structure which cannot be indexed by existing known phases. A
combination of \textit{ab initio} random structure search (AIRSS) and
quasi-harmonic approximation (QHA) calculations are used to re-explore the
relative phase stabilities of the rich phase diagram of CaCO$_{3}$. Nudged
elastic band (NEB) calculations are used to investigate the reaction mechanisms
between relevant crystal phases of CaCO$_{3}$ and we postulate that the mineral
is capable of undergoing $sp^{2}$-$sp^{3}$ hybridization change purely in the
$P2_{1}/c$ structure $-$ forgoing the accepted post-aragonite $Pmmn$ structure.
