Are you a mineral?
There used to be a TV commercial that said "you are what you eat." If that's true - and to some extent it is - then you are a mineral. Nearly all of our food is salted, and what is salt but the mineral halite? You also wear minerals, play with and on minerals, and admire the beauty of minerals. However, a mineral by definition cannot be organic, so despite what you heard on TV, you aren't what you eat!
What is a Mineral?
Minerals are everywhere! Scientists have identified more than 4,000 minerals in Earth’s crust, although the bulk of the planet is composed of just a few.
A mineral possesses the following qualities:
- It must be solid.
- It must be crystalline, meaning it has a repeating arrangement of atoms.
- It must be naturally occurring.
- It must be inorganic.
- It must have a specific chemical composition.
Minerals can be identified by their physical properties, such as hardness, color, luster (shininess), and odor. The most common laboratory technique used to identify a mineral is X-ray diffraction (XRD), a technique that involves shining an X-ray light on a sample, and observing how the light exiting the sample is bent. XRD is not useful in the field, however.
The definition of a mineral is more restricted than you might think at first. For example, glass is made of sand, which is rich in the mineral quartz. But glass is not a mineral, because it is not crystalline. Instead, glass has a random assemblage of molecules. What about steel? Steel is made by mixing different metal minerals like iron, cobalt, chromium, vanadium, and molybdenum, but steel is not a mineral because it is made by humans and therefore is not naturally occurring. However, almost any rock you pick up is composed of minerals. Below we explore the qualities of minerals in more detail.
Minerals are "crystalline" solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern. Notice that in Figure below the green and purple spheres, representing sodium and chlorine, form a repeating pattern. In this case, they alternate in all directions.
Sodium ions (purple balls) bond with chloride ions (green balls) to make table salt (halite). All of the grains of salt that are in a salt shaker have this crystalline structure.
Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock, but is not a mineral.
Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral, but a diamond made in a laboratory by humans is not. Be careful about buying a laboratory-made “diamond” for jewelry. It may look pretty, but it's not a diamond and is not technically a mineral.
Nearly all (98.5%) of Earth’s crust is made up of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals.
All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds. Each mineral has its own chemical formula. Table salt (also known as halite), pictured in Figure above, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms (red) bonded to a silicon atom (grey), represented by the chemical formula SiO2 (Figure below).
Quartz is made of two oxygen atoms (red) bonded to a silicon atom (grey).
In nature, things are rarely as simple as in the lab, and so it should not come as a surprise that some minerals have a range of chemical compositions. One important example in Earth science is olivine, which always has silicon and oxygen as well as some iron and magnesium, (Mg, Fe)2SiO4.
Some minerals can be identified with little more than the naked eye. We do this by examining the physical properties of the mineral in question, which include:
- Color: the color of the mineral.
- Streak: the color of the mineral’s powder (this is often different from the color of the whole mineral).
- Luster: shininess.
- Density: mass per volume, typically reported in "specific gravity," which is the density relative to water.
- Cleavage: the mineral’s tendency to break along planes of weakness.
- Fracture: the pattern in which a mineral breaks.
- Hardness: which minerals it can scratch and which minerals can scratch it.
How physical properties are used to identify minerals is described in the concept "Mineral Identification."
- A mineral is an inorganic, crystalline solid.
- A mineral is formed through natural processes and has a definite chemical composition.
- Minerals can be identified by their characteristic physical properties, such as crystalline structure, hardness, density, breakage, and color.
- Is coal a mineral? Why or why not?
- Is a diamond made in a laboratory a mineral? Why or why not?
- How does the internal structure of a mineral reflect in its physical appearance?
Use this resource to answer the questions that follow.
- If you look carefully at a piece of granite, what can you see in it?
- What are diamonds used for?
- What is the mineral fluorite used for?
- What five criteria must be met for a substance to be a mineral?
- What are the majority of minerals on Earth's surface? What are they made up of and in what form?
- Is ice a mineral? Why or why not?
- What do all of the physical properties of a mineral come from?
- Why does the element carbon create such different minerals as diamond and graphite?