Melting point of minerals 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.
Start with my answer to this very highly related question here:

https://earthscience.stackexchange.com/a/2742/725



The melting point of minerals in isolation, or a pure substance is higher than mixtures of minerals. For example - a (well-mixed) mixture of quartz and pyroxene will melt at a lower temperature than pure quartz or pure pyroxene. The exact temperature is dependent on the proportions.

In general, the more different kind of components (i.e. "things": minerals, elements, molecules, etc) you add to a mixture, the lower the melting point of the mixture as a whole.

For example, sea water melts at -2 °C because it's a mixture of water (melts at 0 °C) and all kinds of salts. Steel melts at a lower temperature than pure iron because it's a mixture of iron and carbon.