Inorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous crystalline 3-dimensional structure. The defined chemical composition can vary between compositional end members within a mineral system as observed in the silicate pyroxene mineral system. Mineral-like substances that don’t strictly meet the definition (especially if the crystal structure is amorphous), such as opal and obsidian, are classified as mineraloids. Liquids and gases are not minerals. Substances like coal that are derived from organic matter are also not minerals.

However, the definition is not without some interesting examples that provide better insight into the the definition of the word mineral. The following discussion is derived from the article “Is Water a Mineral? Is Ice a Mineral?

Is Water a Mineral?

If we compare water’s properties to requirements above, we find that it fails to qualify as a mineral because water is not a solid and does not have a crystalline structure. However, at temperatures below 32 oF or 0 oC (at standard atmospheric pressure), water solidifies into ice.

Is Ice a Mineral?

Micrograph of 6-point “fern” snowflake. Source: Quanta Magazine & Kenneth Libbrecht
Micrograph of 6-point plate snowflake. Source: Quanta Magazine & Kenneth Libbrecht
Micrograph of 12-point star snowflake. Source: Quanta Magazine & Kenneth Libbrecht

If we compare the properties of ice to the four requirements of the mineral definition, we find that it clearly meets the last three. However, requirement (1) does present a problem. A natural snowflake would be considered to be a mineral because it forms naturally in Earth’s atmosphere. However, an ice cube made in a refrigerator would not be considered a mineral because it was produced by the actions of people.