Introduction
Mineralogists have catalogued roughly 5,500 distinct mineral species. That is an impressive number — but geology does not require you to know all of them. The overwhelming majority of Earth's crust and mantle is built from fewer than 30. If you narrow further to the minerals that account for most of the crust's volume, you get a list of perhaps a dozen species. This lesson covers that short list — the rock-forming minerals — and by the end of it you will be able to look at most common rocks and read them the way a geologist does.
In Module 1.1, we described Earth's interior using shorthand: "silicate rock" for the crust and mantle, "iron-nickel alloy" for the core. In Lesson 1.1.2, we introduced mafic and felsic as descriptions of the two crust types — but without explaining what those words mean at the mineral level. The mafic–felsic spectrum is a compositional gradient running from rocks rich in magnesium and iron at one end to rocks rich in silicon and aluminium at the other. Mafic rocks are darker, denser, and dominate the ocean floors; felsic rocks are lighter, less dense, and make up the bulk of the continents. Understanding what drives this contrast — and why it matters for everything from volcanic hazard to the stability of the continents — requires moving one level deeper, into the minerals themselves.
"Silicate rock" means rock built from silicate minerals — and there are only a handful of silicate mineral groups that do most of the building. Each end of the mafic–felsic spectrum has its own signature minerals: olivine and pyroxene define the mafic end; quartz and orthoclase feldspar define the felsic end. Identifying these minerals by eye is the foundation of field geology, and it is where the vocabulary introduced in earlier lessons becomes concrete and useful.
This lesson is organised into two parts. First, the silicates — the dominant group, making up roughly 90% of the crust by volume — with the six mineral families you will encounter repeatedly throughout this course. Second, the non-silicates — carbonates, oxides, sulphides, and evaporites — which are less abundant in the crust overall but include some of the most geologically and economically important minerals on Earth.
Key Terms
A mineral whose fundamental structural unit is the silica tetrahedron (SiO₄) — one silicon atom bonded to four oxygen atoms. Silicates make up roughly 90% of Earth's crust and the vast majority of the mantle. They are the most abundant mineral group by far because silicon and oxygen are the two most abundant elements in the crust.
The SiO₄ building unit: one silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron. These units link together in different ways — sharing corners, edges, or forming chains, sheets, or frameworks — producing the structural diversity of the silicate mineral family.
Silicate minerals rich in magnesium and iron: olivine, pyroxene, and amphibole. They are dark-coloured (black, dark green, or dark brown), relatively dense, and dominant in oceanic crust and the upper mantle. The term 'mafic' was introduced in Lesson 1.1.2 to describe oceanic crust; here it applies at the individual mineral level.
Silicate minerals rich in silicon and aluminium: quartz, feldspars, and most micas. They are light-coloured (white, grey, pink, or colourless), less dense than mafic minerals, and dominant in continental crust. The term 'felsic' was introduced in Lesson 1.1.2 to describe continental crust.