Jarosite-type KFe₃(SO₄)₂(OH)₆ has a hexagonal crystal structure containing 2-D networks of corner-connected FeO₆ octahedra in the a-b plane, stacked and separated from one another along the c-axis.

(a) This compound exhibits antiferromagnetic correlations between nearest-neighbour Fe ions. Explain, with orbital and/or energy level diagrams as necessary, the magnetic exchange mechanism responsible for these correlations.

(b) This compound adopts a complex long-range-ordered antiferromagnetic state below 46 K. However, a fit of the Curie-Weiss law to the linear region of the inverse magnetic susceptibility yields a Weiss temperature (θ) of -600 K. Explain the discrepancy between these two values in terms of the crystal structure and magnetic exchange interactions