- The Nature of Minerals
- The Formation of Crystals
- The External Form of Crystals
- Outward Appearance Of Crystalline Material
- Distorted and Deformed Crystals and Pseudomorphs
- Crystal Systems
Everything about us- all of the substances of the earth, sea and air is composed of atoms of one or more of the 92 natural elements. Of these, only relatively few are commonly encountered, since oxygen and silicon between them account for over 74% of the weight of the rocks in the earth’s crust. Only eight elements are present in quantities equaling as much as 1% of the surface of the earth as we know it. In addition to oxygen and silicon, the remaining six are aluminum, iron, magnesium, calcium, sodium and potassium. Only a few others make up as much as 1/10 of 1 % of the earth's crust. These include hydrogen, titanium, chlorine and phosphorus. Such seemingly common metals as copper, lead and zinc together equal only 16/1000 of 1% of the earth's crust. All of the elements, with the exception of the so-called noble gases (argon, neon, krypton, etc.), which are inert chemically, have affinities for other elements in various degrees.
When compounds are dissolved and taken into solution, their components are said to IONIZE; i.e. assume positive or negative charges. Ions may be made up of single elements or groups of them, and may have single charges or two or more. If a very insoluble material is formed when oppositely charged ions meet, it will be deposited immediately. As a hot solution or molten rock cools, the minerals that can form from its components are usually deposited in order from the least soluble to the most soluble. Since the electrical charges on the ions are directional in nature, the atoms making up this material that is starting to form take regular place in the structure. A molten mass of rock behaves in the same manner as a water solution, so the same situation can take place with the growth of a crystal in a cooling molten mass. Thus when molten masses are congealing slowly or when liquids are evaporating, the atoms of which they are composed may arrange themselves, in a definite pattern one to another and form solids known as CRYSTAL or CRYSTALLINE MATERIAL. For example, as the water vapor in air cools to the freezing point, its atoms accumulate in a regular pattern to form the ice crystals we see on windows. The atoms composing water vapor in air arrange themselves into a great variety of six-sided shapes we know as snow flukes. Salt on the beaches crystallizes as sea water evaporates. However, if the formation of the solid is too hurried, such as the rapid cooling of molten glass, the atoms may not have sufficient time to arrange themselves in an orderly manner and the result will be an amorphous structure.
The affinity of atoms of one element for atoms of other elements often results in a growing process, although not in the same sense that plants and animals grow. Crystal growth may be likened to the growth of a mass of tiny iron filings attracted to a magnet. It can better be described as the attraction of like atoms forming layer upon layer from the center outward producing a definite geometric form. This growth results in a crystal form with faces so symmetrically placed and so smooth and highly reflective that it creates doubt on the part of an inexperienced observer that they have not been fashioned by man. Crystals are among the most unusual objects in nature, since they do seem to have this growth characteristic even though they are inorganic and have no life. Each crystal attracts the same kind of materials of which it is composed and arranges them with a fantastic accuracy in specific positions and holds or locks each atom into place. Almost every compound that forms in nature takes on a crystal structure as it accumulates. It is rare to encounter inorganic materials in nature without a definite internal structure, such as glass and opal. They are the exceptions rather than the rule.
Perhaps one may understand more thoroughly what is meant by this
orderly internal arrangement, or crystal structure, from the sketches
of the SPACE LATTICES of diamond (Figure 1) and fluorite (Figure 2).
These are called space lattices because they indicate the three dimensional arrangements of the atoms in a UNIT CELL (the smallest unit of a lattice that illustrates all of its characteristic geometrical properties and constituents). The tiny particles shown in these sections must be imagined to have been magnified millions of times and not to be in actual close contact with one another. The lines in the sketches connecting the atoms are imaginary and are included merely to indicate the “pattern” made by the atoms. One must imagine a piece of mineral to be composed of billions of similar sections.
From this short description, one can be expected only to visualize what is meant by crystal structure in general. Space lattices are not merely theoretical; they are based on actual X-ray photographs of the Laue type, which have revealed the internal arrangement of most minerals.
The physical properties of a mineral, such as cleavage, hardness and toughness, depend on the internal structure. These properties vary as the arrangement of the internal particles varies.