Identifying Minerals: Formation and Tests

The building blocks of rocks

A mineral is a natural solid with an orderly, crystalline structure and a definite chemical make-up. Minerals are the building blocks of rocks: most rocks are simply mixtures of minerals locked together. Each mineral forms in a particular way. Some crystallise as molten rock cools, some grow when heat and pressure change an existing rock, some are left behind when salty water evaporates, and some grow from hot fluids moving through cracks in the crust. Because each mineral has its own set of physical properties, a geologist can identify an unknown sample with a few simple tests: looking at how it shines, scratching it to judge how hard it is, checking the colour of its powder, seeing how it breaks, and adding a drop of dilute acid. This module looks at how minerals form and how those tests tell them apart.

Words for minerals

Learn what each of these means before you use it.

Melt and evaporation

Two very different settings can grow a mineral, and knowing the setting helps explain where it is found.

How to identify a mineral

When you meet an unknown mineral, there is a sensible order to work through rather than guessing. Look first at how it shines and note its colour, remembering that colour alone can mislead. Scratch it against known materials to judge how hard it is. Rub it on a rough tile to see the colour of its powder. Look at how it breaks, whether along flat planes or in an uneven way. Finally add a drop of dilute acid and watch for any reaction. No single test is enough on its own, so a strong answer gathers several results and compares them against a data sheet before naming the mineral.

Match the mineral to its property

The fizz test

A student adds a drop of dilute hydrochloric acid to an unknown mineral and it fizzes, giving off a gas. Which common mineral behaves like this?

• calcite
• quartz
• galena
• halite

Testing a mineral

Select the TWO physical properties you can test to help identify a mineral.

• Its hardness
• Its streak colour
• The price of the sample
• The weather on the day it was collected

How many property tests?

There are 5 physical property tests used to identify a mineral: colour, hardness, streak, cleavage and lustre. A quick check uses just 3 of them. Subtract to find how many of the tests are not used.

Order the crystals forming

Put the stages of a mineral crystallising from magma in a sensible order.

• Magma sits as molten rock underground
• The magma slowly begins to cool
• Atoms arrange themselves into a regular pattern
• Interlocking crystals grow and form a solid rock

Complete the mineral facts

The colour of the powder a mineral leaves when rubbed on a tile is called its _____. The way a mineral breaks along flat planes is called its _____. The way a mineral shines, such as glassy or metallic, is its _____. A mineral that is a useful source of metal is called an _____.

Match the formation to its mineral

Spot the true mineral facts

Tap the TWO statements that are true.

• Calcite fizzes when dilute hydrochloric acid is added to it
• Streak is the colour of the powder a mineral leaves on a tile
• Quartz is a soft mineral that fizzes in acid
• Halite forms when magma cools deep underground

Identify the mineral

Read each case and choose the best response, then think about why.

• You find a clear, very hard mineral with a glassy shine that will scratch glass and does not fizz in acid. What is the best first identification?
• An unknown mineral leaves a reddish-brown streak on the tile. What does this most suggest?
• You need to decide whether a white mineral is calcite or quartz. Which single test settles it fastest?

Explain how minerals form

A friend is revising Geology and does not understand how minerals form or how to tell them apart. Explain how minerals form and how their physical properties are used to identify them.

• Explain what a mineral is
• Explain two ways a mineral can form, such as from a cooling melt or from evaporating water
• Explain two physical property tests, such as hardness and streak
• Explain what the dilute acid test shows and which mineral fizzes
• Finish by explaining why one test alone is often not enough to be sure