  We present a generalized Landau-Brazovskii free energy for the solidification
of chiral molecules on a spherical surface in the context of the assembly of
viral shells. We encounter two types of icosahedral solidification transitions.
The first type is a conventional first-order phase transition from the uniform
to the icosahedral state. It can be described by a single icosahedral spherical
harmonic of even $l$. The chiral pseudo-scalar term in the free energy creates
secondary terms with chiral character but it does not affect the thermodynamics
of the transition. The second type, associated with icosahedral spherical
harmonics with odd $l$, is anomalous. Pure odd $l$ icosahedral states are
unstable but stability is recovered if admixture with the neighboring $l+1$
icosahedral spherical harmonic is included, generated by the non-linear terms.
This is in conflict with the principle of Landau theory that symmetry-breaking
transitions are characterized by only a \textit{single} irreducible
representation of the symmetry group of the uniform phase and we argue that
this principle should be removed from Landau theory. The chiral term now
directly affects the transition because it lifts the degeneracy between two
isomeric mixed-$l$ icosahedral states. A direct transition is possible only
over a limited range of parameters. Outside this range, non-icosahedral states
intervene. For the important case of capsid assembly dominated by $l=15$, the
intervening states are found to be based on octahedral symmetry.
