The constituent atoms of a quartz molecule (SiO2) are silicon (Si) and oxygen (O), both of which are light elements all by themselves. When you pair the two oxygen atoms with the silicon atom, you get a molecule made of light elements. So that's a light mass. Now, if you look at the exact atomic structure, a silicon molecule takes up a fair amount of space. Since density is mass over volume... you get a low density mineral: 2.62 g/cm^3
