How does silica affect the color of igneous rocks?

Question

Audrey Adams · Accepted Answer

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. Pure quartz and glass are both colorless; the more silica present in a rock, the paler the rock generally appears to be. Felsic rocks are high in silica because they contain large amounts of both quartz and feldspar, a different mineral that contains silicon and oxygen along with metal atoms from sodium, potassium, and calcium. For example, mafic rocks (low silica content) are dominated by coloured minerals such as olivine and micas; these rocks include basalt, which is a very dark-coloured rock. Therefore, the degree of paleness in a rock can be a good indicator of its silica content (high silica) or low silica content (mafic).

Wesley Alvey · Answer

undefined Silica affects the color of igneous rocks by influencing the crystallization process. Higher silica content leads to slower cooling and the formation of lighter-colored minerals like quartz and feldspar, resulting in lighter-colored igneous rocks such as granite. Lower silica content promotes faster cooling and the formation of darker-colored minerals like olivine and pyroxene, resulting in darker-colored igneous rocks like basalt.