Feldspar is a mineral made up of some combination of calcium/sodium/potassium alumino-silicates. My mineralogy texts are down at the studio, so I can't give my "definitive" answer about the most common mineral found in rocks. Quartz is a particular crystalline form of silica, but just because silica is present in some mineral, doesn't mean that quartz is.
6. What is the most common mineral in the earth's crust? In the soil?
According to this website, feldspars are the most common mineral family found in the CRUST, while quartz is the most common mineral found in the earth's SOIL. I suppose you could make the argument that the soil, being decomposed crust, actually forms most of the earth's surface. Or perhaps you've been even more devious, and figured that since about 3/4 of the earth's surface is water, that sea salt is the most abundant mineral on the actual "surface". Care to respond?
http://soils-dev.agron.iastate.edu/classes/154/lesson06/06.Re.03.html
Feldspars are the most common mineral family in the earth's crust. Quartz is the dominant soil mineral. This is because feldspars weather more readily. By the time the parent material has weathered to form a soil many of the feldspars are gone.