The complexity of the silicate structure is a function of the temperature of crystallization. Olivine has a relatively simple structure with independent tetrahedra which crystallizes very high temperature found deep within the earth. Quartz has a complex crystalline structure which favors crystallization and melting at a much lower temperature. Crystals with a complex structures form nearer the surface because the temperature there is cooler. Peridotite is the name of the rock found in the mantle and it is brought to the surface through tectonic faulting but not through melting as a magma.
Most magmas originate deep in the crust or upper mantle.
It the magma develops by melting peridotite then it will most likely have the composition of a basalt, the type of rock commonly found in oceanic crust. If the magma forms by melting in the lower crust, it is more likely to have the composition ranging from diorite to granite. In the lower crust it is the presence of water the lowers the melting temperature so that a silica-rich magma forms. This is characteristic of extrusive rocks, those that cool on earth's surface pretty rapidly. These conditions way down in the mantle would provide very hot temps. and pressures, and would form rocks that are coarse-grained in structure.