Andrew Usher wrote:

Every solid save water ice and a few semi-metals is denser than the
liquid of the same composition, and the same temperature and pressure.

Bull****. Silicon, bismuth, clathrates... have denser liquids than
solids. Ditto silicate melts vs. zeolites. Metal-organic
frameworks. It's a trivial thing to do given crystal structure.

This means that a liquid other than water will not exist beneath a
solid of the same composition in a planet. A magma ocean therefore can
only freeze from the bottom up,

Since it can only cool from the top down you might want to rethink
that. Hawaii is frozen at the top and liquid at the base.

except for the granitic crust which,
owing to its composition, is lighter than the liquid mantle. The
amount of granitic crust which could exsolve is pretty high, if the
mean composition is that of albite perhaps 20% of the mantle in the
extreme - but this would not be reached until far below the solidus.

Freezing from the bottom up is a much faster process than freezing
from the top down, as the heat flux from below continues to decrease
(yes, there is convection, but the effective viscosity is much higher
in a solid than a liquid), and therefore should go to completion.

So sad.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2