Crystal (mineral)

I

Introduction

Crystal (mineral), homogeneous portion of matter that has a definite, orderly atomic structure, and an outward form bounded by smooth, plane surfaces, symmetrically arranged. Crystals are produced whenever a solid is formed gradually from a fluid, whether the formation results from the freezing of a liquid, the deposition of dissolved matter, or the direct condensation of a gas into solid form. The angles between corresponding faces of any two crystals of the same substance, regardless of size or superficial difference of form, are always identical.

Most solid matter displays orderly atomic arrangement and is of crystalline structure. Solids that have no crystalline structure, such as glass, are called amorphous. In structure they show greater similarity to liquids than to solids.

II

Conditions for Formation

The same liquids that gradually freeze deep within the earth to form granite are sometimes ejected at the surface as volcanic lava and cool quickly, forming a glassy rock called obsidian. If the cooling is slightly slower, a rock called felsite is formed; it is crystalline, but the crystals are too small to be seen with the naked eye. Such a structure is called cryptocrystalline, or aphanitic. Still slower cooling results in a rock of porphyritic structure, in which some of the crystals are large enough to be visible; this rock, which may be of identical composition with obsidian, felsite, or granite, is called rhyolite.

Granite, rhyolite, and felsite are not homogeneous and therefore cannot be single crystals, but they are crystalline rocks. Each of the constituent minerals in these rocks is present in the form of small, but homogeneous crystals. Those substances that first solidified during the cooling of molten rock exhibit a normal arrangement of crystal faces. Those that, because of lower freezing points, solidified later were forced to occupy the remaining interstices, so that their external appearance is deformed.

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The same tendency that causes homogeneous crystals to form from mixed liquids can be used to purify many crystalline substances. Chemists frequently use this method, and organic chemicals in particular are almost invariably purified by recrystallization.

In some mineral groups, ions of one element may substitute for ions of another, leaving the crystal structure unchanged but forming what is, in effect, a series of solid solutions. Such groups, in which there is a complete gradation of chemical composition from one pure end member to another, are called isomorphous. An example is the variety of feldspar called plagioclase, which forms a complete series ranging in composition from pure sodium aluminosilicate (albite) to pure calcium aluminosilicate (anorthite). Other mineral groups that form isomorphous series include apatite, barite, calcite, and spinel.

Crystal growth is attained when a minute crystal that has formed abstracts more of the same constituent from its environment. Sometimes, in the absence of this first minute crystal, or seed, crystallization does not take place, and the solution becomes supersaturated, just as a liquid below its freezing point becomes supercooled. When a new organic chemical is prepared, it is often difficult to make the first crystal unless an isomorphous substance can be found. The tendency to crystallize decreases with increasing viscosity of the liquid; if a solution becomes considerably supersaturated or supercooled it becomes very viscous, and crystallization becomes almost impossible. Further cooling or evaporation of the solvent produces first a syrup and then a glass.

Some substances have a strong tendency to form seed crystals. If a solution of such a substance is cooled slowly, a few seeds grow into large crystals; but if it is cooled rapidly, numerous seeds form and grow only into tiny crystals. Table salt, purified at a factory by recrystallization, is composed of numerous perfect cubic crystals, which are barely visible to the naked eye; rock salt, formed by the slow processes of geology, contains enormous crystals of the same cubic form.

III

Crystallography

The study of the growth, shape, and geometric character of crystals is called crystallography. When conditions are favorable, each chemical element and compound tends to crystallize in a definite and characteristic form. Thus, salt tends to form cubic crystals; but garnet, which also occasionally forms cubes, more commonly occurs in dodecahedrons (solids with 12 faces) or trisoctahedrons (solids with 24 faces). Despite their differences in habit (shape of crystallization), salt and garnet always crystallize in the same class and system. Thirty-two classes of crystal are theoretically possible; almost all common minerals fall into one of about twelve classes, and some classes have never been observed. The thirty-two classes are grouped into six crystal systems, based on the length and position of the crystal axes, imaginary lines passing through the center of the crystal, intersecting the faces, and bearing definite relations to the symmetry of the crystal. Minerals in each system share certain details of symmetry and crystal form and many important optical properties.

The six crystal systems are of great importance to mineralogists and gemologists; specification of the system is necessary in the description of any mineral (see Mineralogy).

A

Isometric

This system comprises crystals with three axes, all perpendicular to one another and all equal in length.