Is it a Rock or Mineral?
The terms “mineral” and “rock” are often used interchangeably, but they aren’t one and the same. The difference between the two is small, but important. While rocks are made of minerals, minerals aren’t made of rocks. It may not seem like a big deal, but incorrectly calling a mineral a rock can make some geology-nazis quite angry.
The formal definition of a mineral is: a naturally occurring, inorganic solid that has a crystalline structure and a definite chemical composition.
In simpler words, this means:
- A mineral must form in nature; therefore, industrial made diamonds and gems are not considered true minerals.
- It must be made from non-living material; it can’t contain fossils.
- The molecules within the mineral must be arranged in a pattern to form a crystal-like shape.
- It must be made of only one element or compound. (Generally, there are a few exceptions which we’ll discuss.)
A mineral can either be made of a single element or, more commonly, of compounds from chemically combined elements. Gold is an example of a single element mineral, also known as a “native” mineral, and sodium chloride (NaCl) is an example of compound mineral, also known as the mineral halite and rock salt.
Typically, a mineral has only one chemical composition. Silicate (SiO4), for example, will always be the only compound that forms the mineral quartz and will always have one silicon (Si) atom for every two oxygen (O) atoms. Olivine, on the other hand, can be formed from either Fe2SiO4 or Mg2SiO4. Why both? The size and the charge of magnesium (Mg) and iron (Fe) atoms are relatively the same, so they can easily substitute for each other in a compound. For this reason, you may sometimes see the chemical formula of olivine expressed as (Mg,Fe)2SiO4.
Even without knowing the chemistry behind the exception to the one-element-or-compound rule, it’s easy enough to understand if you look at how the ratios of atoms stay the same: whether magnesium or iron is used, there will always be two magnesium or iron atoms for every one silicate (SiO4) molecule.
Olivine in a meteorite. Picture by: Doug Bowman via Flickr
The definition of a rock is simpler. They’re basically all of the naturally occurring solids that don’t fit the criteria for minerals. They can be a mixture of minerals, fossils, and other remains of living things like coal and amber. If you still have a hard time seeing what the difference is, just remember that a mineral is made of either a single element or compound, while a rock is a blend of a bunch of things.
Basically, the mineral was a thing before the rock was a thing. How hipster.