Properties of minerals: Colour, streak, and hardness

This is Selma’s collection of minerals. Minerals are naturally occurring solid substances that make up the Earth’s crust. Though they occur naturally, they have never been alive and are not made up from anything that has been alive — they are inorganic. There are thousands of different kinds of minerals on Earth, and each is made up of a particular mix of chemical elements that are arranged in a very specific way. The chemical composition and structure of a mineral define its type.

But you can’t see these easily. Luckily, every type of mineral also has unique characteristics that are much easier to observe! These unique properties help us identify different types of minerals. Let’s see! One of the easiest properties of minerals to observe is colour.

The colour of a mineral is determined by the combination of elements it contains - its chemical composition. Certain minerals almost always come in a specific shade. For instance, the mineral olivine is typically green, while azurite is usually blue. However, most minerals come in a variety of colours. Calcite may be red, pink, yellow, green, blue, purple, black, or brown depending on the trace elements it contains.

Different minerals can also have the same colour — many common minerals are white, creamy, or colourless. So, although colour is the easiest to see, it is a rather unreliable property when it comes to identifying minerals. There is, however, a more reliable way to use colour to identify minerals. Let’s rub this piece of calcite on this glass tile and see what happens. A mineral rubbed against a hard material leaves a streak of fine mineral powder.

Even though the calcite appears orange, the streak it leaves on the glass is white! Small particles of a mineral tend to show colour more accurately, and impurities don’t affect the colour of the streak much. So, two pieces of the same type of mineral, even if they are different colours, will normally leave the same coloured streak. Now, let’s do the same thing with another mineral — quartz. See how there is no streak?

Instead, the quartz has left a scratch mark on the tile that doesn’t wipe off. This is because quartz is harder than glass. Hardness describes the degree to which a mineral can resist scratching. If you scratch a mineral against an object of a known hardness, such as a steel knife, or another mineral, you can tell if the mineral is softer or harder than that object. If the object scratches the mineral, the mineral is softer than that object.

If instead the object gets scratched by the mineral — the mineral is harder. People who study minerals use a scale of hardness, called the Mohs scale. The Mohs scale lists ten common minerals and assigns them a value: talc is the softest with a value of 1, and diamond is the hardest with a value of 10. Each higher-numbered mineral will scratch any mineral with a lower number. For instance, feldspar, which has a value of 6 on the Mohs scale will easily scratch gypsum, which has a value of 2, or fluorite which has a value of 4.

But it will be scratched by quartz, which has a value of 7. Testing the hardness of minerals using the Mohs scale is easy and often enough to successfully distinguish between common minerals. Colour, streak, and hardness are some characteristics of minerals that are easy to observe and test. They can help you tell the difference between minerals. The more properties of a mineral you know, the higher the chances you will be able to correctly identify it.