The word ‘ Crystal’ is from the Greek for ‘frozen ice’, krystallor. The two characteristics of a crystal are its transparency and its geometry. An early law of crystallography was first stated by Romé de l’Isle, as follows: -
In all crystals of the same substance the angles between corresponding faces have the same values.
A further law was laid down by the Father of Crystallograhy, René-Juste Haüy: -
Crystals are built up from large numbers of units, all of which have the same shape, and are called integrant molecules.
It was worked out mathematically how many differently shaped ‘bricks’ or space lattices could exist. The space lattices or nets were defined by lines joining the centre points of adjacent identical atoms. There were fourteen distinguishable lattices, and these could be grouped according to shape, into seven systems.
Colours
There is a tendency in each system for the different prismatic structurings to give rise to certain colours, though there is no prism with a pentagonal or heptagonal base, so some crystals lack colour:
RED-PURPLE, for example Garnet, a cubic crystal.
GREEN, for example Beryl and Emerald, hexagonal.
RED, for example, Cinnabar, trigonal.
ORANGE-RED, for example, Wulfenite, Tetragonal.
GOLDEN-ORANGE, for example Topaz, orthorhombic
BLUE, for example, Azurite, monoclinic
PURPLE, for example, Axinite, Triclinic.
RED-PURPLE, for example, Garnet, a cubic crystal.
GREEN, for example, Beryl and Emerald, hexagonal.
RED, for example, Cinnabar, trigonal.
ORANGE-RED, for example, Wulfenite, Tetragonal.
BLUE, for example, Azurite, monoclinic.
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Malachite botryoidal (bunch of grapes) |
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The structures and lattices of crystals are described simply with good diagrams and photographs in colour in:
MICHELE, V. de (1973) Crystals. Symmetry in the Mineral Kingdom. London: Orbis.
JORDAN, J. (c.1860) Elementary Crystallography. London: Thomas Murby
Simplified nets for contructing models of the crystals were published by the Victorian mining Record Officer, James B. Jordan. Some of his constructional diagrams are added above.
D.P.
