Mafic minerals are the olivines, pyroxenes, amphiboles, and the micas biotite and phlogopite. They are dark minerals. To the right is basaltic olivine. There are gemstones called peridot that are a form of olivine. It is named for it's dark green color. 
Felsic minerals are light in color. Below is Felspar. Each of these minerals are formed under dense heat from Earth itself. Of course heat at depth is far hotter than in more shallow water. The ocean waters eventually cool the lava to form these rocks.
Earth is literally a oven that creates its' own distribution of these elements.
In Earth's deep ovens these elements don't exist. Rocks are manifestations of surface conditions. If you want to know more study geology. It is a fascinating planet.
It is stated the innor core of Earth is nickel and iron. Why isn't there any Felspar? As lava moves upward to the surface of Earth, rocks will form according to the mixture of elements that arise.
