We often look at the beautiful shapes and colours of crystals but how many of us have ever wondered how they are formed?  What happens underground to produce such interesting shapes and colours with each one looking different to any other?

We hope to give you some insights into some of the processes that produce these wonders on the natural world.  If this sparks your interest to study them at a deeper, scientific level then this can only enhance your appreciation of the beauty of the crystals that you have in your home.

The Planet Earth and minerals 

The Earth is composed of thousands of minerals and crystals, although many of these are very small and not immediately obvious to us.  The Crystals and minerals make up a large proportion of the soil, clay and sand in our gardens, and deeper down the solid rock.  Even squelchy clay is composed of minerals and crystals.  Clay minerals form very small crystals, too small to be seen with the naked eye.  One clay mineral (kaolinite) is used to make finest bone China.  

Some of the rock names we all know, such as granite and marble are composed of minerals and crystals.  Granites often include quartz and feldspars and marbles contain calcite.

Shap Granite (from the Lake District, England) is a decorative rock which can be found in use in buildings all over the world.  It shows orthclose feldspar (large, well formed pinky crystals, rich in potassium), plagioclase feldspar (off-white crystals, rich in calcium and sodium), quartz (dull grey colour) and various other oxide minerals and crystals (black).

Processes within the Earth lead to the development of these crystals.  In the case of the Shap granite above, a magma chamber composed of melted rock started to cool.  As the magma cooled, probably from approximately 600oC, minerals started to crystalise.  Chemically, this process is no different to crystals of sugar forming in molten sugar as it cools or ice crystals forming on our windows in winter. 

Look at the magnified ice crystal. Notice the same pattern is repeated six times.  Now look at an amethyst, quartz or citrine crystal.  Can you see any similarities?

Ice crystal showing hexagonal crystal form. 

What gives crystals their shape and colour?

A mineral is a naturally formed, inorganic, substance which possesses a definite chemical composition and a definite chemical structure.  Minerals are made up of atoms in a regular 3 dimensional pattern, significant to each mineral.  It is as a consequence of the orderly, 3 dimensional structure which can lead to the formation of well developed crystals, including where crystals are bounded by plane surfaces, for example the smooth faces of a quartz crystal.

Mineral Chemistry
    
The chemistry of a mineral is important for the formation of crystals.  For example, halite (commonly known as rock salt), has the chemical formula NaCl.  This nice and simple mineral forms perfect cubes due to the simple 3 dimensional structure.  As the bonds between the atoms within a mineral become more complicated due to the composition of the minerals containing a greater number of atoms and/or more complicated bonds between them, then the type of crystals, as well as the strength will change.

During a crystal’s formation, minor amounts of other atoms may become incorporated into the crystal.  These may not be present in a large enough quantity to change the mineral to a different one, but they may affect the colour.  A good example of this is quartz, amethyst and citrine.  If a quartz crystal incorporates small amounts of iron, whilst it is growing, this may turn the crystal purple.  If heat is subsequently applied to the amethyst crystal, this may change the composition of the iron within the amethyst which changes the colour, resulting in citrine.

However, there is much more to minerals and crystals than just the chemical composition.  Look at the two minerals below.  Can you see any difference?

A cut diamond                                     Graphite pencils

Diamond and graphite would appear to be worlds apart.  One is the hardest naturally occurring substance, and graphite is very soft and is used as a lubricant, a factor which results in it’s use in pencils.  However, chemically, they are exactly the same!  Both made of carbon.

Graphite had carbon bonded into sheets, with weak bonds between the sheets.  Try rubbing a thick layer of graphite pencil onto a piece of paper.  Now run you finger over it and see how silky it is.  This is the sheets of carbon gliding past each other

Diamond structure               Granite structure

In diamond, the carbon atoms are bonded in 3 dimensions.  The results in strong, covalent bonds in all directions.  Because all the bonds between the same atoms are the same, there are no inherent weaknesses within diamond. This gives diamond it’s unique strength properties.

Crystal Formation

So far we have briefly looked at what minerals are and how the chemical composition will effect the crystals produced, including colour and shape. However, we have seen that minerals can have the same chemistry, but vastly different properties.  So why might this be?

Different minerals are stable in different environments within an on the Earth.  For example, diamonds are produced in extremely deep magma chambers where extremely high temperatures and pressures exist.  Here carbon, when it occurs in high enough concentrations is stable in the form of diamond.  Graphite on the other hand can easily be produced at the Earth’s surface.  Other examples of stability may be gypsum and anhydrite.  Both of these minerals have very a similar chemistry.  Gypsum is commonly produced by evaporation of seawater, leading to different minerals becoming saturated and precipitating.  Gypsum is used in manufacturing plaster.  It is heated slightly, and this drives off loosely bound water from within the crystal matrix forming anhydrite.  When water is added again, gypsum crystals start to form, and when they are formed the plaster becomes hard with gypsum crystals.

Role of Bacteria

Believe it or not, but bacteria can have a key role in the formation of minerals and crystals such as fools gold (pyrite) and many of the sulphide minerals.  Incredible but true.  In fact we are only just starting to discover the real importance of bacteria in the Earth’s processes.

Have you seen nature programmes in the deep oceans of ‘black smokers’?

What looks like smoke actually is not.  This is minerals and crystals forming very quickly before your eyes.  Due to the very short time period, there is no time for nice, large, well formed crystal to develop.  As the warm, metal rich water hit the cold sea, it cools which helps lead to minerals becoming saturated.  In addition, there is bacteria in the water which does not use sunlight for energy and they use sulphate to (SO4 2-), which is abundant in the oceans.  The bi-product is dissolved ‘sulphite’ in the water, which forms extremely insoluble metal sulphite minerals, such as pyrite, chalcopyrite and galena.  The minerals formed would depend on what metals were dissolved in the water.

There are so many ways that crystals can form, but the type and size will depend on:
•    Chemistry
•    Time
•    Space – i.e. room to grow
•    Pressure

So, next time you buy a crystal or find a pebble on the beach, look at it and see the wonder that is our beautiful planet.  What processes have formed that crystal?  How long did it take to grow?  How hot was it when is was made?  These are just the start of the story of our beautiful planet and the many wonderful processes that have shaped, and continue to shape, it’s surface as well as all the places beneath our feet which we cannot see.