The key to molding a well-shaped crystal is that the mineral
needs to have space to grow. The minerals that freeze out of a melt first (these
are the ones with high melting points) form nice crystals, while the ones with
lower melting points can't make nice crystals since they run into the crystals
of other minerals that already froze out. The answer can be found in the Bowen Reaction
Series. In here, you will see that that quartz crystallizes at a lower
temperature than plagioclase.
