In the last article we looked at symmetry and as an extra note we should mention that integral symmetry of a crystal is not always obvious from its outward appearance even if the crystal is fairly well formed, it may appear squashed or have uneven side lengths. You will see this in the last diagram from part 1 and yet as also shown in this diagram you can see that the angles between the faces is constant. Thus it is usually more efficient to look at the angles between crystal faces than the faces themselves (at least in the beginning).
Another very important aspect of describing crystallography are the crystal axes. A crystallographic axis is an imaginary line through a crystal structure which indicates a repeat direction of the lattice structure. These axes are useful for describing orientations of faces, layers and even directions of symmetry axes. Many physical and optical properties are also described in relation to crystal axes.
There are seven crystal systems in which crystals are classified and recognising the system a crystal belongs to is often the first step to identifying it
.
Some crystals may also exhibit a growth pattern know as twinning, this is when there is a change in the direction the repeating crystal structure.
[caption id="attachment_526" align="aligncenter" width="235"]
Spinel twin[/caption]
[caption id="attachment_524" align="aligncenter" width="685"]
Diagram from Gem-A notes[/caption]
When describing a crystal an understanding of crystal form is helpful, this describes the shapes the crystal faces in relation to the internal crystal structure. The crystal form in turn relates to crystal habit which describes the actual physical form a crystal takes (this can include multiple crystal forms).
[caption id="attachment_522" align="aligncenter" width="600"]
Diagram from Gem-A notes
Diagram from Gem-A notes[/caption]
This is the basic theory (very basic) behind crystallography, from here a practical element is vital. There are many good gem and mineral books out there and I would recommend perusing one to start to get an idea of what gems and minerals have what shapes, crystal habits and crystal features. It is even better if you can get your hands on samples to look at at the same time. We have a number of wonderful crystal available that you can check out at http://www.lawsongems.com/gemrough/crystals
Another very important aspect of describing crystallography are the crystal axes. A crystallographic axis is an imaginary line through a crystal structure which indicates a repeat direction of the lattice structure. These axes are useful for describing orientations of faces, layers and even directions of symmetry axes. Many physical and optical properties are also described in relation to crystal axes.
There are seven crystal systems in which crystals are classified and recognising the system a crystal belongs to is often the first step to identifying it
.
Some crystals may also exhibit a growth pattern know as twinning, this is when there is a change in the direction the repeating crystal structure.
[caption id="attachment_526" align="aligncenter" width="235"] Spinel twin[/caption]
[caption id="attachment_524" align="aligncenter" width="685"]
Diagram from Gem-A notes[/caption]
When describing a crystal an understanding of crystal form is helpful, this describes the shapes the crystal faces in relation to the internal crystal structure. The crystal form in turn relates to crystal habit which describes the actual physical form a crystal takes (this can include multiple crystal forms).
[caption id="attachment_522" align="aligncenter" width="600"] Diagram from Gem-A notesDiagram from Gem-A notes[/caption]
This is the basic theory (very basic) behind crystallography, from here a practical element is vital. There are many good gem and mineral books out there and I would recommend perusing one to start to get an idea of what gems and minerals have what shapes, crystal habits and crystal features. It is even better if you can get your hands on samples to look at at the same time. We have a number of wonderful crystal available that you can check out at http://www.lawsongems.com/gemrough/crystals
Comments (0)