Ca L23 spectrum in amorphous and crystalline phases of calcium carbonate.

The calcium L(23) absorption edge is very sensitive to changes in local symmetry. Distinct spectra from calcite and aragonite as well as two different amorphous phases of calcium carbonate have been reported. Multiplet calculations using the CTM4XAS code, taking account of the local symmetry and the crystal field, suggest that the coordination octahedra are distorted by a change in length along one axis, the reduction in symmetry being coupled with tetrahedral displacements of the 4 atoms in the perpendicular plane. Examination of the polarization dependence implies that the two different amorphous phases arise from different levels of ordering of the oxygen coordination polyhedra around calcium. The change in the ordering suggests a pathway for the transformation to single crystal calcite. Although the code is unable to directly simulate the very low symmetry environment of Ca in aragonite, some degree of agreement is obtained with the same distortion of the coordination octahedron and greater tetrahedral displacements.