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Rocks and Minerals: Definitions and Differences

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  • 0:08 Rock or Mineral?
  • 1:12 Minerals
  • 2:45 Rocks
  • 4:10 Classification Differences
  • 6:26 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

Can you tell the difference between a rock and a mineral? What makes a mineral so much different from a rock? Explore these questions in depth, and learn how we classify rocks and minerals.

Rock or Mineral?

They say that a diamond is a girl's best friend. Rubies, sapphires and other precious gemstones are also popular ways to impress a lady. Mankind has always been infatuated with rare crystals. Diamonds, in particular, have become a symbol of love and commitment. Young ladies can't wait to show off when they get a big shiny rock from the groom-to-be. But, should we really be using the word 'rock' to describe a diamond?

You may have heard the phrase 'rocks and minerals' at some point in your science schooling. The reason we say 'rocks and minerals' is because they are two different things. Rocks are not minerals, and minerals are not rocks.

As we'll soon discover, diamonds are actually minerals. Some people are under the false impression that minerals are things we eat, as in the phrase 'essential vitamins and minerals.' But really, minerals encompass a broad range of natural substances, some of which you eat, some of which you forge into tools and some of which you give to your future spouse. Let's talk about the differences between rocks and minerals so we can get all our facts straight. We'll start right now with minerals.

Minerals

A mineral is a solid, inorganic, naturally-formed substance that has a crystalline structure and specific chemical composition. I know that's a lot to take in, so let's break down this definition piece by piece.

First of all, minerals are solid and formed naturally in the earth. Natural solids can be familiar things like sand, granite, salt and wood. Our definition says that minerals are inorganic, which means that they do not consist of tissues from living things. So, that means that wood is not a mineral.

The atoms of salt are arranged in a specific lattice.
image showing salt structure

It also says that minerals have a crystalline structure. That means that the component atoms of the substance have a repeating, 3-dimensional arrangement. Above is a picture showing how the atoms of salt are arranged. Salt is also called sodium chloride because it is made of two types of atoms: sodium and chlorine. The sodium and chlorine atoms are put together in a very specific, 3-dimensional lattice that repeats over and over until you get to the end of the salt crystal. Yes, it's a crystal! I know you typically don't think of salt as being a crystal. But it is, and so is sand. Sand is just tiny chunks of quartz crystal, made by a pattern of silicon and oxygen atoms.

What about our last substance, granite? It's solid. It's formed naturally in the earth. It's inorganic. But, does it have a specific chemical composition? Does it have a regular, repeating atomic arrangement? No, granite may be made up of minerals, but it's not a mineral itself. It's a rock.

Rocks

A rock is a solid, inorganic, naturally-formed substance without a particular atomic structure or chemical composition. It's probably easier to just remember that rocks are made up of two or more minerals. Examples of rocks include granite, limestone, marble, pumice, obsidian, sandstone, shale and slate. Each of these rocks consists of several different minerals, which are mixed up inside the rock through a variety of geologic processes.

Let's take granite, for example. Granite is mostly composed of three minerals: quartz, feldspar and mica. Each of these minerals can be found alone in nature, but here, they are mixed up inside of the rock. Sometimes you see large chunks of one of these minerals inside of the granite. But, when you take the stone as a whole, you have to call it a rock.

Other rocks are much finer grained than granite, so it's not easy to spot the different minerals. Slate is a rock that was made from clay, and clay is composed of tiny, tiny particles. Those particles can be minerals like quartz, pyrite, apatite, muscovite, feldspar, kaolinite, biotite, tourmaline - the list goes on! But, you can't see these minerals inside of a slate rock. The slate looks all the same color and texture. Still, it's not a mineral because it has no regular atomic structure or chemical composition. Rocks are made of minerals, and minerals stand alone.

Classification Differences

Another way to distinguish between rocks and minerals is by their classification systems. It makes sense that they would be classified differently, since they are different things. But, many students get confused; they try to describe a rock as though it were a mineral or describe a mineral as though it were a rock.

At the most basic level, rocks are classified by the geologic processes that formed them. The three main groups are igneous, metamorphic and sedimentary rocks. Minerals are not classified this way. You can't say that quartz is igneous, because it's not even a rock!

Minerals are classified by their properties and chemical classes. Chemical classes of minerals include groups like silicates, sulfates and carbonates. We group the minerals based on the atoms and chemical bonds that make them up. Silicates include minerals like mica and feldspar. Carbonates include calcite and hematite. Some minerals, called the native elements, have only one type of atom: these include copper, gold and silver minerals.

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