Minerals: Properties & Identification

What is a mineral?

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly internal crystal structure. That definition rules out things people sometimes call minerals: ice from a freezer isn't natural, glass has no crystal structure, and coal is organic. A rock, by contrast, is a mixture of one or more minerals.

Physical properties used to identify minerals

• Hardness — resistance to scratching, ranked 1–10 on the Mohs scale (talc = 1, diamond = 10). A harder mineral scratches a softer one.
• Cleavage — the tendency to break along smooth, flat, parallel planes (mica peels into sheets; halite breaks into cubes).
• Fracture — breaking along rough, curved, or irregular surfaces instead of flat planes (quartz shows curved "conchoidal" fracture).
• Luster — how the surface reflects light: metallic (like polished metal) or nonmetallic (glassy, pearly, dull, earthy).
• Streak — the color of the mineral's powder when rubbed on a tile; often more reliable than the mineral's outward color.
• Color — easiest to see but least reliable, since impurities change it.
• Crystal shape, density, magnetism, and reaction to acid can also help (calcite fizzes in acid).

Cleavage vs fracture — the classic distinction

This pairing shows up constantly. Cleavage produces smooth, flat surfaces because the mineral's bonds are weakest along certain planes — so it splits cleanly there every time. Fracture produces rough or curved surfaces because the bonds are roughly equal in all directions, so it breaks unevenly. Mica = cleavage (sheets); quartz = fracture (curved chips).

How minerals form

Minerals crystallize when magma cools, when water rich in dissolved ions evaporates, or under heat and pressure deep in the crust. Slow cooling gives atoms time to arrange into large crystals; fast cooling gives small crystals or even glass.