Question

Determine whether quartz or plagioclase feldspar is more likely to form a well shaped crystal in an igneous rock

Answer 1

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.