Introduction to minerals and crystals, including links. Suitable for new A level Geology OCR specification (UK)

1.  Although most minerals are solids at the Earth’s surface (except mercury), they
often grow from liquids.
 For example, the elements forming the mineral quartz (Si and O) are present in
magma, a liquid from which crystals grow and combine to make rocks.
 Ions (charged particles) of these, and all other elements, behave differently when
liquid or solid.
 They move at high velocity in liquid as they have higher thermal energy. They are
disorganised and do not form a regular pattern.
 Slow cooling of a liquid reduces the thermal energy and gives ions more time to
form into regular patterns.
 These organised patterns give the internal structure and external appearance of
crystals.
 In the case of Si and O, they would form crystals of the mineral quartz.
 If liquids cool rapidly, they freeze the disorganised arrangement of a liquid and a
natural glass is formed. In a silicon and oxygen-rich liquid the glass is called
obsidian.
Minerals and crystals

2. Arrangement of atoms in liquids,
crystalline solids and glass

3. Size and shape of crystals
 Mineral crystals grow from the centre outwards by adding to their
faces. Although the faces may be of different sizes, the angle between
them is always constant for any given mineral (eg quartz)
 This is due to the regular arrangement of the atoms in their structure.
 If a liquid is cooled slowly, fewer crystals begin to form than if it is
cooled more quickly.
 Slower cooling gives this smaller number of crystals more space to
grow, so slowly cooled liquids produce larger crystals.
 Eventually, the crystals meet each other, and the grain boundaries
between them are irregular.
 If they grow into a cavity they will then have good crystal shape.
 One point to remember is that crystals of a particular mineral do not
always grow into the same shape- Calcite can form rhombs (as in
Iceland spar) or more pointed crystals (dogtooth spar). However, they
share the same symmetry and other properties..

4. Different crystal forms of
calcite
Iceland spar
Dogtooth spar and
other varieties

5. Tasks and homework
 1. Describe one difference between a rock and a
mineral.
 2. Explain why a naturally formed glass (eg opal
or obsidian) is not a mineral
 3. Why are silicates the most important rock-
forming minerals?
 For next tutorial: Research the seven crystal
systems and compare their shape and symmetry.
See:
http://www.crystalage.com/crystal_information/seven_crystal_s

6. Tasks and homework
 1. Describe one difference between a rock and a
mineral.
 2. Explain why a naturally formed glass (eg opal
or obsidian) is not a mineral
 3. Why are silicates the most important rock-
forming minerals?
 For next tutorial: Research the seven crystal
systems and compare their shape and symmetry.
See:
http://www.crystalage.com/crystal_information/seven_crystal_s