The Formation of Crystals
“A crystal is a body bounded by flat faces that is an external expression of an internal order.”
From the Photographic Guide to Minerals of the World by Ole Johnsen
What Defines a Mineral?
A mineral is a naturally occurring element or chemical compound, formed as a result of a geological process, and is usually crystalline. Most minerals are chemical compounds (as opposed to containing just one element), and most are inorganic. Synthetically produced stones are not regarded as minerals, as they are produced in a laboratory, and should always be referred to as synthetic, or expressed as such in another way. There are approximately 4000 minerals known on Earth, and new minerals are being discovered all the time, so don’t expect to memorise every one of them! Most mineral names end in “ite” or “lite”, which comes from the Greek word lithos, meaning stone.
Crystalline, Amorphous or Organic?
- A crystal is a homogeneous body, meaning that it has the same chemical and physical properties throughout, and is formed by a regular lattice of atoms and molecules. Crystals form geometrically, and their outer shape will ideally be limited by flat surfaces, forming flat crystal faces.
- A mineral is a crystalline solid, whether or not it is limited by plane faces. Some crystals are microscopic, and not visible to the naked eye; others have a crystalline structure that does not result in plane faces, but they are still regarded as crystalline, because they possess an ordered inner structure. Some examples of this are rose quartz, aventurine and jasper. As in the case of crystals, minerals are homogeneous, and are formed through a geological process.
- Amorphous minerals are not crystalline at all, but are still of geological origin and are sufficiently homogeneous to demonstrate well-defined chemical and physical properties. The reason for lack of inner structure is usually due to rapid cooling, giving them no opportunity to crystallise, for example obsidian (volcanic glass) and tektite (glass formed by a meteorite strike).
- Organics are generally considered non-mineral, and have formed from living organisms. Examples are amber (compacted tree resin), pearl, coral, coal and jet. Some mineral books will list these as amorphous.
The Crystal Lattice
In most solid substances, atoms and molecules move about freely with a great deal of empty space around them. This empty space is vast, with far more space than matter, and the particles making up the atoms extremely small. If the nucleus of an atom were as large as 2 cm, the nearest electrons would be .5 km away, and no larger than a speck of dust. Because crystals have a strong electromagnetic field, and are often formed under intense pressure, their atoms and molecules are closely packed in a strict order during growth, according to the size of the molecules and how they organise themselves to produce the best fit. This creates what is known as the crystal lattice. The perfect symmetry of this internal structure will determine the outer shape of the crystal.