The crystal systems of minerals
Crystallography is a huge topic and indepth coverage of this subject is far beyond the scope of this site. Here we have included basic definitions of common crystal forms with some examples. It is important to note that minerals with the same chemical structure can show remarkable differences at the crystal level. Probably the most extreme example being diamond and graphite. The crystal structures of graphite and diamond are shown below. The graphite consists of hexagonal sheaths which are parallel and do not have chemical bonds between them. As a result the graphite is soft, brittle and easily breakable. Diamond, on the other hand, is a cube with an octahedral cleavage. It has strong chemical bonds between all carbon atoms, ensuring rigid and perfect crystals.
GRAPHITE
 DIAMOND

Common crystal forms
 Cubic
 Orthorhombic
 Monoclinic
 Triclinic
 Trigonal
 Hexagonal
Cubic 
 contains three axes of equal length. All axes are positioned at rightangle to each other. A cube is also referred to as a closed form and can itself be a crystal.



 Gold
 Copper
 Diamond
 Lazurite
 Galena
 Pyrite

Orthorhombic 
 contains three axes which are of different lengths. All axes are positioned at rightangles to each other.



 Sulphur
 Stibnite
 Topaz
 Chrysoberyl
 Aragonite
 Baryte

Monoclinic 
 contains three axis of unequal length. Two axes are not at right angels. The third axis is at a right angle to the plane containing the other two axes and it is often referred to as the symmetry axis.



 Wolframite
 Manganite
 Malahite
 Azurite
 Borax
 Gypsum

Triclinic 
 contains three axes. All axes are of different length and none at a right angle to the others



 Ulexite
 Chalcanthite
 Turquoise
 Kyanite
 Rhodonite
 Heulandite

Trigonal 
 contains four axes with three of equal length, all three arranged in the horizontal plane. The forth axis is perpendicular to that plane and is of a different length to the others. Axes and angles in this system are similar to the Hexagonal System. In the crosssection of a Trigonal crystal there will be three sides.



 Tourmaline
 Dolomite
 Hematite
 Corundum
 Calcite
 Rhodochrosite

Hexagonal 
 contains four crystallographic axes consisting of three equal horizontal, or equilateral axes at 120 degrees to each other, as well as one vertical axis which is perpendicular to the other three. This vertical axis can be longer or shorter than the horizontal axes.



 Graphite
 Nickeline
 Molibdenite
 Apatite
 Vanadinite
 Beryl



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