How does silica affect the color of igneous rocks?

1 Answer
Dec 20, 2015

Rocks that are high in silica tend to be pale in colour.


Pure silica has the chemical formula #SiO_2# - silicon dioxide. Glass is silica that has an amorphous structure, which means the silicon and oxygen atoms aren't in a repeating lattice. Quartz is the form that pure silica takes in nature. It has a tectosilicate structure, which means the atoms form a lattice of tetrahedra bonded together.

Both glass and quartz are colourless when pure. The more silica you have in a rock, the paler it tends to be. Felsic rocks have high silica content, because they contain lots of quartz and another mineral called feldspar. (In feldspar, there are metal atoms like sodium, potassium and calcium as well as silicon and oxygen.)

Consider mafic rocks, which have low silica content and tend to be dominated by coloured minerals like olivine and micas. These are rocks like basalt, which is very darkly coloured. So how pale a rock is can be a very good indication of whether it's felsic (high in silica) or mafic (low in silica).