  Tricritical points separate continuous and discontinuous symmetry breaking
transitions. They occur in a variety of physical systems and their mathematical
models. A tricritical point is used to determine a liquid-solid phase
transition line in the pressure-temperature plane [Aitta, J. Stat. Mech.,
2006]. Excellent experimental agreement has been obtained for iron, the
material having the most high pressure data. This allows extrapolation to much
higher pressures and temperatures than available experimentally. One can
predict the temperature at the liquid-solid boundary in the core of the Earth
where the pressure is 329 GPa. Light matter, present as impurities in the core
fluid, is found to generate about a 600 K reduction of this temperature.
