Why does an increase in silica content result in a decrease in rock density?
The simple reason is that when SiO2 content increases, everything else (most of which is a higher density elements) decreases. SiO2 forms tetrahedra that extend off into space. Silicon itself is not a massive element, so the unit density of the tetrahedron is generally low. When there is not a lot of SiO2 relative to the total mass, more dense elements like iron fit in between the tetrahedra without causing the tetrahedra to split apart much. Small dense ions fit into the open spaces. When silicate content gets high, there are not enough other dense elements to fit into the available open space, so you tend to get less dense minerals like and micas, which have no or fewer ions in those available open spaces. Just not enough cations to fit into those available holes, so they stay empty. Empty is less mass than something, basically, so the density drops. I didn’t discuss how the silica tetrahedra tend to also form different spacial structures but that also has an impact, even though most
