Mafic and Felsic Silicate Minerals

For general petrological discussion, the common silicate minerals are divided into two broad groups — mafic and felsic — based on their major cation content. This distinction is especially useful because the two groups differ systematically in color, density, and the rock types in which they occur.

Mafic Silicate Minerals

Mafic silicates contain magnesium and/or iron as major constituents. The term is a portmanteau: “ma” from magnesium and “f” from the Latin word for iron, ferrum. The common mafic silicates are biotite, amphiboles, pyroxenes, and olivine. These minerals tend to be fairly dark colored — dark green, brown, or black — and rocks in which they are abundant are correspondingly dark. Some exceptions exist: certain pyroxenes are pale green, and some amphiboles are colorless to pale, but dark coloration is the general rule. ^[1]

Felsic Silicate Minerals

Felsic silicates lack Mg and Fe as major constituents. The name is derived from “feldspar,” the most abundant mineral in this group. The felsic silicates include feldspars (K-feldspar and plagioclase), quartz, muscovite, and feldspathoids (nepheline, leucite, sodalite). Because they do not carry iron or other chromophore elements in major structural positions, felsic minerals are generally light colored — white, gray, or pink — and felsic-dominated rocks are correspondingly pale. ^[1]

Silicon Abundance Controls Polymerization Degree

As a general rule, a high degree of tetrahedral polymerization is possible only if silicon is relatively abundant in the rock. Framework silicates such as quartz and K-feldspar therefore occur only in rocks that contain fairly abundant SiO₂ — granites, rhyolites, and related rocks. Orthosilicates such as olivine are restricted to rocks that are relatively poor in Si but rich in Mg and Fe — ultramafic and mafic rocks such as peridotite, dunite, and komatiite. ^[1]

The Aluminum Substitution Exception

An important complication to the Si-control rule is that Al³⁺ can substitute for Si⁴⁺ in tetrahedral sites in many silicate minerals. This substitution allows framework silicates to form even in relatively Si-poor rocks, provided that Al is available to fill some of the tetrahedral positions. The most geologically important examples are plagioclase and nepheline, both framework silicates that are common in Si-poor rocks such as gabbro and syenite. In these rocks there is insufficient Si to fill all the tetrahedral sites with silicon alone, but Al substitution compensates and permits the fully polymerized framework structure to exist. ^[1]

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