In the Mantle and Crust A mineral is a naturally occuring solid, with a particular crystal structure and either a particular chemical composition or a restricted range of chemical composition that can be described as a solid solution.
Rocks are solids made up of a collection of minerals.
Igneous rocks are produced by cooling of molten lava, and so are closely associated with heat transfer processes. The silica content reflects fractionation between liquid and solid composition as material melts. A low silica content reflects lower degree of fractionation, higher temperature of melting, liquid melt most like solid from which derived, i.e. less differentiated.