Calcium carbonate, CaCO₃, contains planar trigonal CO₃²⁻ ions and spherical Ca²⁺ ions (radius = 114 pm). In the most common polymorph (calcite), each Ca²⁺ is coordinated by six oxygen atoms from six different CO₃²⁻ ions.
Consider a simplified model in which the CO₃²⁻ ion is represented as an equilateral triangle of three point-charge oxygens surrounding a central carbon atom (which may be ignored electrostatically).
(a) Sketch a three-dimensional fragment containing one Ca²⁺ and the nearest six CO₃²⁻ groups. Indicate the orientation of each planar CO₃²⁻ ion relative to the Ca²⁺ centre.
(b) Describe in words how the orientation you selected in part (a) helps to maximise attractive interactions while minimising repulsive O²⁻–O²⁻ contacts.
(c) Suppose all CO₃²⁻ ions flipped 90° so that their planes became perpendicular rather than parallel to the Ca²⁺–O contacts. Predict the qualitative effect of this reorientation on lattice enthalpy, and justify your answer.
(d) Real calcite exhibits slight distortions from ideal geometry. Propose one electrostatic reason, other than ion size mismatch, that could lead to such distortions, and explain how it influences the final lattice.