Understanding "Chemical Formula"

According to the definition, minerals must have a definite chemical formula. What does that mean?

To explore this, we will use the same materials that we used to explore "naturally occurring." You will need as many of the items from the following list as you can find:

  • several mineral specimens
  • several rock specimens
  • coins, bone, teeth, sea shells, wood, nails, cloth, glass, feather, paper, water, salt, pepper, other objects made of different materials

A chemical formula tells us exactly what a substance is made of. For example, do you see any H2O in the photograph? Yes! You probably know that H2O is water. It tells us that each molecule of water is made up of two atoms of hydrogen (H2) and one atom of oxygen (O). Anyplace on Earth that you drink a glass of water, you are drinking H2O.

For some substances, the chemical formula is even more simple. On the right hand side of the photo is a roll of copper wire. Copper is an element, which means that it is made up entirely of one kind of atom. The chemical formula for copper is Cu.

Not everything has a definite chemical formula. Some things are mixtures. A good example of that is the small dish of dirt just to the left of the copper wire. The dirt is made up of tiny bits of many different substances. There are grains of sand, bits of clay, pieces of dead plant material, and many other things. Each handful of dirt has a different mixture of things. One may have a little more sand. Another may have a little less clay. There is no definite chemical formula for dirt.

Many rocks are also mixtures. Granite, limestone, basalt, and most other rocks are mixtures of different materials, and do not have a definite chemical formula. Some of the tiny bits in a piece of granite will be the mineral quartz. That tiny bit does have a definite chemical formula, SiO2. Other bits will be a type of feldspar, another mineral. There may be bits of other minerals, such as muscovite or biotite. Each tiny bit is a mineral, but the entire chunk of rock is not. This part of the definition is a critical difference between rocks and minerals.