This is one of those questions, like "how many lithospheric plates are there?" that exists to please the type of schoolteacher (or curriculum writer) who likes one pat answer. There must be one mineral that's the most common, right? Hah! It depends. And the three best answers for that question about the most common mineral are quartz, feldspar and olivine (more about that here). But that schoolteacher I mentioned won't like them because they'll encourage students to argue. Heck, I could argue that clay is the most common mineral: it covers the most ground on our planet, and clay minerals are indistinguishable except by geologists.