Most science terms have precise definitions, like:
*atoms, compounds, ionic compounds, molecular compounds
But some common words don’t have a precise meaning, like “rock.”
That word is a catch-all term for many different, unrelated, objects.
Even the word “mineral“ has multiple meanings, depending on context.
In our class we use a specific definition:
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 here.
[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)
What Is a Rock?
adapted from Andrew Alden
Everyone knows what a rock is, until you ask what it is exactly…. most people say: Rocks are hard solids, of natural origin, made of minerals. But all of those criteria have exceptions.
Rocks Are Hard
Not necessarily. Some can be scratched with your fingernail: shale, soapstone, gypsum rock, peat. Others may be soft in the ground, but they harden once they spend time in the air (and vice versa).
Rocks Are Solid
Well, some are far from completely solid. Many rocks include water in their pore spaces.
Some geodes—hollow objects found in limestone country—hold water inside them like coconuts.
Then there’s the matter of temperature. Mercury is a liquid metal at room temperature (and down to 40 below zero), and petroleum is a fluid unless it’s asphalt erupted into cold ocean water.
And ice meets all the criteria of rockhood too, in permafrost and in glaciers.
Rocks Are Natural
Not entirely. Concrete is a mixture of sand and pebbles (aggregate) and a mineral glue (cement) of calcium silicate compounds.
Concrete is a synthetic conglomerate, and it acts just like the natural rock, turning up in riverbeds and on beaches. Some of it has entered the rock cycle – to be discovered by future geologists.
Brick, too, is an artificial rock—in this case, an artificial form of massive slate.
Slag: the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, such as in road building and concrete aggregate. It too has surely found its way into sedimentary rocks already.
Rocks are made of minerals
Most are, sure, but many are not. Minerals are inorganic compounds with chemical formulas and mineral names, like quartz or pyrite.
But what about coal? Coal is made of organic material, not minerals.
And what about coquina, a rock made entirely of seashells? Shells are made of mineral matter, but they aren’t minerals any more than teeth are. Rocks like these are called biogenic rocks.
And what about obsidian, a rock glass? None of its material has gathered into crystals. It is an undifferentiated mass of geological matter. While it has no minerals in it, it is called a rock.
Lustre: Minerals and rocks reflect light in special ways, called lustre. Let’s explore – open this document: Lustre
40 common minerals and their uses