Most terms used in science have precise definitions, like:
*compounds, ionic compounds, molecular compounds
But some terms are common English terms that turn out not to have a precise definition. Rock is one of these terms. It is really a catch-all term for many different, often unrelated, objects. Even the word mineral has multiple meanings, depending on the context. In our class, we will use a specific definition of mineral:
Hobart M. King, Ph.D. Mansfield University, writes:
To meet the definition of “mineral” used by most geologists a substance must meet five requirements:
definite chemical composition
ordered internal structure
“Naturally occurring” means that people did not make it. Steel is not a mineral because it is an alloy produced by people. “Inorganic” means that the substance is not made by an organism. Wood and pearls are made by organisms and thus are not minerals. “Solid” means that it is not a liquid or a gas at standard temperature and pressure.
“Definite chemical composition” means that all occurrences of that mineral have a chemical composition that varies within a specific limited range. For example: the mineral halite (known as “rock salt” when it is mined) has a chemical composition of NaCl. It is made up of an equal number of atoms of sodium and chlorine.
“Ordered internal structure” means that the atoms in a mineral are arranged in a systematic and repeating pattern. The structure of the mineral halite is shown : Halite is composed of an equal ratio of sodium and chlorine atoms arranged in a cubic pattern.
[However] the word “mineral” is used inconsistently in geology. In mining, anything obtained from the ground and used by man is considered to be a “mineral commodity” or a “mineral material”. These include: crushed stone, which is a manufactured product made from crushed rocks; lime, which is a manufactured product made from limestone or marble (both composed of the mineral calcite); coal which is organic; oil and gas which are organic fluids; rocks such as granite that are mixtures of minerals; and, rocks such as obsidian which do not have a definite composition and ordered internal structure.
Other examples of ordered internal structures:
Quartz crystal (silicon dioxide, SiO2)