Looking at this website, I can see the melting point of the following minerals: http://hyperphysics.phy-astr.gsu.edu/hbase/Geophys/meltrock.html
Apparently at a temperature of 1200 °C, all the minerals exist in a molten state. But, looking at the melting point for quartz on Wikipedia I find that the melting point is close to 1700 °C. What’s going on here?
It can’t be an effect of the pressure. In the interior, the pressure will be higher and thus further increase the melting point.