Introduction
Pick up a piece of granite and look at it closely. You will see distinct specks of different colours: pale pink crystals, glassy translucent grains, and tiny plates of shimmering black. The granite itself is not a single substance — it is a mixture. Each speck is a different material with its own chemistry, its own internal structure, and its own set of physical properties. Those specks are minerals. Granite is a rock. The distinction matters more than it might seem.
In Module 1.1, we described Earth's crust as "silicate rock" and the mantle as "solid silicate rock" without ever defining what those materials are at the molecular level. Now we can zoom in. Silicate rock is built from silicate minerals — specific substances with specific chemical formulas. The crust's most abundant mineral, feldspar, is a silicate. So is quartz, the glassy grain you just spotted in that granite. The mantle is dominated by olivine and pyroxene — also silicates. Every layer we studied in Module 1.1 resolves, at closer inspection, into a collection of minerals.
But not everything that looks mineral-like is actually a mineral. Coal looks like a rock, forms underground, and is solid — yet it is not a mineral. Pearl is a hard, shiny, natural solid — yet it is not a mineral. Obsidian is a naturally occurring black glass that fills museum display cases — yet it is not technically a mineral either. The difference in each case comes down to one or more of five specific criteria that every mineral must meet.
This lesson establishes those five criteria precisely, explores the edge cases that make the definition interesting, and introduces a handful of common minerals by name and formula — the building blocks that will reappear throughout this course.
Key Terms
A naturally occurring, inorganic, solid substance with a definite chemical composition and an ordered crystalline structure. All five criteria must be met simultaneously. There are over 5,500 known mineral species, but fewer than 30 make up the bulk of Earth's crust and mantle.
A solid in which atoms are arranged in a regular, repeating three-dimensional pattern — a crystalline lattice. This internal order is what gives every sample of a given mineral the same consistent properties: the same hardness, the same way it breaks, the same optical behaviour, regardless of where it formed.
A shorthand notation specifying which elements are present in a substance and in what ratio. Quartz is always SiO₂ (one silicon, two oxygen). Halite is always NaCl (one sodium, one chlorine). A definite formula is one of the five criteria a mineral must have.
A naturally occurring, inorganic solid that resembles a mineral but lacks a fully ordered crystalline structure — its atoms are arranged randomly rather than in a repeating lattice. Obsidian (volcanic glass) and opal are the most common examples. Mineraloids pass four of the five criteria but fail the crystalline test.