What Is a Mineral?
A mineral is any substance with all of four specific qualities.
1. Minerals Are Natural: substances that form without any human help.
(more content follows the advertisement below)
A D V E R T I S E M E N T
2. Minerals Are Solid: substances that don't droop or melt or evaporate.
3. Minerals Are Inorganic: substances that aren't carbon compounds like those found in living things.
4. Minerals Are Crystalline: substances that have a distinct recipe and arrangement of atoms.
Until the 1990s, mineralogists could propose names for chemical compounds that formed during the breakdown of artificial substances, things found in places like industrial sludge pits and rusting cars (although iron rust is the same as the natural minerals hematite, magnetite and goethite). That loophole is now closed, but there are minerals on the books that aren't truly natural.
Traditionally, native mercury is considered a mineral, even though the metal is liquid at room temperature. At about 40 degrees below zero, mercury solidifies and forms crystals like other metals. So there are parts of Antarctica where mercury is unimpeachably a mineral.
For a less extreme example, consider the mineral ikaite, a hydrated calcium carbonate that forms only in cold water. It degrades into calcite and water above 8 degrees Celsius. It is significant in the polar regions, the ocean floor and other cold places, but you can't bring it into the lab except in a freezer.
Ice is a mineral, even though it isn't listed in the mineral field guide. But when ice collects in large enough bodies, it flows in its solid state—that's what glaciers are. And salt (halite) behaves similarly, rising underground in broad domes and sometimes spilling out in salt glaciers. Indeed, all minerals, and the rocks they are part of, slowly deform given enough heat and pressure. That's what makes plate tectonics possible. So in a sense, no mineral is really solid except maybe diamond.
Other minerals that aren't quite solid are instead flexible. The mica minerals are the best-known example, but molybdenite is another. Its metallic flakes can be crumpled like aluminum foil. And of course the asbestos mineral chrysotile is stringy enough to weave into cloth.