Element, Mineral, or Rock? What Geologic Materials Are

Think of language. Letters combine to make words, and words combine to make sentences. Geologic materials work the same way:

• An element is a pure substance made of one kind of atom — defined by how many protons it has. Oxygen, silicon, iron, and gold are elements. There are about 90 that occur naturally on Earth (technetium and promethium are essentially absent).
• A mineral is a naturally occurring solid with a definite chemical composition (a fixed recipe of elements) and an orderly, repeating internal arrangement of atoms — a crystalline structure.
• A rock is a naturally occurring solid made of one or more minerals (sometimes also mineral-like glass or organic matter) bound together.

So elements make minerals; minerals make rocks. A rock is a mixture; a mineral is a single, precisely defined substance. That is the whole game.

Geologists are picky: not every solid lump counts as a mineral. To be a mineral, a material must meet five criteria:

1. Naturally occurring — formed by geologic processes, not manufactured. (Synthetic diamonds are real diamond material but are not, strictly, 'minerals.')
2. Solid and stable at ordinary surface conditions.
3. Crystalline — its atoms sit in a fixed, orderly, repeating 3-D pattern. This is what gives minerals their characteristic crystal shapes and cleavage.
4. Definite chemical composition — a fixed recipe, like SiO₂ for quartz or NaCl for halite (some allow a little variation, but within set limits).
5. Generally inorganic — not built as the shell or bone of a living thing, though this criterion has fuzzy edges we examine in Module 3.

Fail one and you usually do not have a mineral. Glass has no orderly crystal structure (it is amorphous), so obsidian — natural volcanic glass — is a rock, not a mineral. Coal is organic, with no fixed composition or crystal structure, so coal is a rock, not a mineral.

Earth's crust is overwhelmingly built from just a few ingredients. By mass, eight elements make up about 98% of the continental crust — and two of them, oxygen and silicon, account for roughly three-quarters of it:

• Oxygen ~46.6%  ·  Silicon ~27.7%  ·  Aluminium ~8.1%  ·  Iron ~5.0%
• Calcium ~3.6%  ·  Sodium ~2.8%  ·  Potassium ~2.6%  ·  Magnesium ~2.1%

With oxygen and silicon so dominant, it is no surprise that the most common minerals are the silicates — minerals built around a silicon–oxygen building block — and that they, in turn, make up most rocks. We name and classify those minerals in Module 3 and the rocks they form in Modules 4–6.

A few solids sit outside the tidy hierarchy and are worth flagging now. Glassy materials like obsidian solidified too fast for crystals to form, so they have no crystalline structure — they are rocks but contain no minerals. Organic solids like coal come from accumulated plant matter; coal is a rock but not a mineral (no fixed composition, no crystal structure). And mineraloids like opal sit at the edge — near-minerals that fall short on structure. We sort all of these out precisely in Module 3.