Carolyn N. Kinder
Sometimes when a mineral is forming in the earth’s crust, it grows into a particular geometric shape. The shape of a crystal results from the way the atoms or molecules of a mineral come together as the mineral is forming. So, each mineral has its own crystal shape. This solid body have a characteristic internal structure and is enclosed by symmetrically arranged plane surfaces, intersecting at definite and characteristic angles.
Fashioned snowflakes, flawless diamonds with glittering facets, the almost perfect cubes of salt grains are all fine examples of crystals bodies with a pattern of flat surfaces that meet at definite angles. The universe is full of almost all nonliving substances in the solid state form crystals. Crystals are ice, snow, sugar, salt and sulfur; in metals like gold, silver, copper, iron and mercury; in precious stone like zircon, emerald, topaz, ruby, and sapphire.
A specific crystal is a collection of fundamental building blocks with atoms and molecules arranged in a unique and always repeated regular space arrangement. Nature has grown crystals over a long span of geological time. The smooth, hard surfaces of a crystal are not shaped by the tools of man.
The Inner Structure
The external differences between crystals are based on differences in internal structure. The particles of matter within a crystal are arranged in a framework called a crystal lattice.
There are four types of structural units in crystal lattices. They are small molecules, giant molecules, ions or electrically charged molecules and atoms.
Crystals Made Up Of Small Molecules
In substances like ice, iodine and solid carbon dioxide or dry ice, the structural units of the crystal lattice are small molecules. These are held together by rather weak electrical forces. There is much space between the molecules and the crystals are light in weight. That is why ice is lighter than liquid water, though both substances are built up of the very same water molecules. It is important to know that ice is unique because if ice would sink, life in the ocean would be at stake. Fish and other aquatic life will freeze.
Usually, crystals in which small molecules are the structural units have low melting points; they are good insulators and are relatively soft. In some cases the bonds between the molecules are so weak that the solid will change into a gas without first becoming a liquid. This is what happens in the case of dry ice which is solid carbon dioxide.
Crystals Made Up Of Giant Molecules
Some crystals consist of giant molecules. These may be built up in one, two, or three dimensions.
Asbestos is a good example of a substance that forms one dimensional giant molecule. The asbestos giant molecule consists of a long chain of atoms; this accounts for the fibrous structure of the mineral. The molecules are set side by side; they are linked together by weak forces of attraction.
The giant molecules of graphite, made up entirely of carbon atoms, are two-dimensional; they are joined together in flat hexagonal plates which lie parallel to each other. See figure 4. The bonds between layers are weak in comparison with those within the hexagons; hence one layer slips easily over the one beneath it. That is why graphite is one of the best lubricants known.
The diamond is a giant molecule built up in three dimensions. Diamond consists exclusively of carbon atoms. Each atom is bonded to four neighboring atoms, which are grouped about it at equal distances. See figure 5, for example, the carbon atom A is bonded to carbon atom B,C,D, and E. B,C,D, and E are each bonded to other atoms in the same way. Since the distances between the atoms in this type of giant molecule are equal, the bonds are of equal strength. The results is a very rigid formation. The diamond is the hardest substance known and it is very difficult to cleave or split it. It has a high melting point; is a good insulator and is transparent.
(figure available in print form
Figure 4. Show the giant molecules of graphite, made up of carbon atoms, form parallel layers of flat, joined hexagonal plates. The bonds between layers are shown by dotted lines in the diagram.
(figure available in print form)
Figure 5. Shows how the atoms of giant diamond molecules are grouped. A,B,C,D and E are carbon atoms.
Crystals Made Up Of Ions
In salts, the unit making up the crystal is an ion, an electrically charged molecule or atom. Each atom has a nucleus or central core made up chiefly of protons, each with a positive electrical charge, and neutrons, which have no charge. Around this central core revolve the electrons, each of which has a negative charge. Normally the charge of an atom is neutral; which means that there will be as many negative charges as there are positive charges. If an atom loses an electron, it has one excess positive charge; it becomes a positive ion. If an atom gains an electron, it has one excess negative charge; it becomes a negative ion.
Look at what happens when sodium, normally a metal, and chlorine, normally a gas react to form the solid called sodium chloride, NaCl, which is table salt. Each sodium atom transfers an electron to a chlorine atom. The sodium atom becomes a positive ion since it now has an excess positive charge. Each chlorine atom acquires a single excess negative charge; it is now a negative ion.
Ions with unlike charges attract each other, the chlorine ions will attract the sodium ions; but will hold off the other chlorine ions since ions with like charges repel each other. As a result of the attraction between the oppositely charged particles, each chlorine ion will surround itself with six sodium ions; each sodium ion will surround itself with six chlorine ions. See figure 6
This pattern will be repeated throughout the crystal. Figure 7 shows positive ions (Na
+
) and negative chlorine ions (cl-), closely packed together in a crystal lattice of table salt.
Substances that have the ionic type of lattice have moderate insulating properties and high melting points. They are hard, but they can be split along definite lines.
(figure available in print form)
Figure 6. Shows ions (NA+) and negative (clÐ), closely packed together in the crystal lattice table salt.
(figure available in print form)
Figure 7.
Crystals Made Up of Electrically Neutral Atoms
In metals, the atom is the structural unit in the formation of a crystal. The atoms may be thought of as spheres having the same diameter and packed together as closely as possible. To illustrate one arrangement, let us imagine that fifteen billiard balls are racked up to form the base, or foundation layer. See figure 8. Six more are set on top of the first layer of balls; then another ball is placed on the second layer. This shows the closest packing possible in a cube. Iron, lead, gold, silver, and aluminum assume this kind of pattern. There are several other arrangements of atoms in metallic crystal lattices. Lattices of this kind are opaque; they have moderate harness; they have high melting points; they are the best conductors of heat and electricity. These qualities make metals very useful.
(figure available in print form)
Figure 8. Shows how atoms (viewed from above) are packed in the crystal lattice of various metals, such as iron, lead, gold, silver and aluminum.
The Internal Structure Of A Crystal Affects Its Properties
The variation in internal structure shown by different crystals have a direct bearing upon their properties. Different crystals have different lines of cleavage, which are lines along which they split most readily. They conduct heat at different rates. They react differently to magnetic and electrical forces.
A few crystals, like those of the mineral Iceland spar, allows only light waves that vibrate in parallel planes to pass through them. This effect is called plane polarized light. For example, try to pass a knife blade between the pages of a closed book. This will be possible only if the knife blade is held parallel to the pages. The book in this case would correspond to the Iceland spar crystal; the knife would correspond to one of the parallel planes in which the light would vibrate.
If a light is allowed to pass through a selected crystal of quartz, the plane of polarized light is twisted to the plane to the same angle to the left. Crystal of the first type are called right-handed, those of the second type, left handed.
The fact that different crystals will rotate the plane of polarized light in different directions forms a reliable means of identifying certain substances. For example, sugars in solution will rotate the planes of light through different angles; the angle of rotation will identify each sugar in question.