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Mineral Crystallography

Calcite Crystals in Amethyst Matrix - Collection of Marlene

When we hear about "Crystal" think of a set of glasses or a stylish design animal placed in a glass case. In terms of a crystal is mineral a homogeneous portion of matter with an ordered atomic structure and externally defined and limited by flat and uniform surfaces symmetrically arranged. The crystals are produced when a liquid forms a solid slowly , this training may result from the freezing of a liquid, the reservoir of dissolved or direct condensation of a gas in a solid. The angles between corresponding faces of two crystals of the same substance are always identical, regardless of size or shape surface difference.

Most solid matter shows an ordered arrangement of atoms and has a crystalline structure. The solid without crystalline structure, such as glass, are called amorphous. Due to its structure, are more like a liquid to a solid. They are known as supercooled liquids.

To understand the formation of crystals, we can establish a sequence following an atom to join with others will give way to a molecule, in turn, a set of molecules will give way to the formation of a cell and an ordered set of cells will lead to the formation of a crystal.

Inside the Earth's mantle, at great depth, temperature and pressure conditions are such that the rocks are melted to form a fluid material called magma. The magma may cool down as it approaches the surface and solidify, forming igneous rock type. If cooling is very fast, as in the case of the lava comes out in volcanic processes is not enough time in the solidification process, the atoms of minerals forming crystals are organized, but if the cooling is slower and takes place within the cortex, the atoms have time to form ordered structures is what we call crystals. This happens, for example, in the case of granite, formed by slow cooling of certain types of magma resulting in a rock on which we can see the different individual components and crystallized minerals.

The same liquid, which solidify slowly in the depths of the earth to form granite are sometimes ejected as lava at the surface where they cool quickly to form a glassy rock called obsidian. If cooling is somewhat slower, is a rock called felsite, its crystalline structure, but with crystals too small to be seen without a microscope. Such a structure is called aphanitic or cryptocrystalline. An even slower cooling produces a porphyritic rock structure, in which some crystals are large enough to be visible, this rock, which can have the same composition as obsidian, felsite and granite, is called rhyolite.

granite, rhyolite and felsite are not homogenous and therefore can not be a single crystal are, however, crystalline rocks. Each of the constituent minerals of these rocks is present in the form of crystals which, although small, are homogeneous. The first solidified substances during the cooling of molten rock show the usual arrangement of the faces of the crystals. Which solidified later, have a lower freezing temperature, were forced to occupy the interstices free and, therefore, its appearance is warped.

trend leading to the formation of homogeneous crystals from liquid mixtures, can be exploited to purify many crystalline substances. Chemists often use this method. In particular, organic compounds, usually purified by recrystallization.

In some mineral groups, certain ions of an element can be replaced by ions of another, leaving the same crystal structure but forming something that can be considered as a series of solid solutions. These groups, in which there is a range of chemical compositions between the pure extremes of different material, are called isomorphic. An example is the variety of feldspar called plagioclase, which contains a complete series, with compositions ranging from pure sodium aluminosilicate (albite) to pure calcium aluminosilicate (anorthite). Other mineral groups that form isomorphic series include apatite, barite, calcite and spinel.

crystal growth begins when a tiny crystal that has formed extracted from its material environment of their own constitution. Sometimes, in the absence of the first glass, or seed crystallization does not occur, and the solution is supersaturated, the same way as a liquid under its freezing point is supercooled. When a new organic substance, it is often difficult to form the first glass unless a substance is isomorphic. The tendency to crystallize decreases with increasing viscosity of the liquid, if a solution is highly supersaturated or supercooled becomes very viscous, and the crystallization becomes almost impossible. An additional cooling or solvent evaporation produces a syrup first and then a glass.

Some

substances have a strong tendency to form crystal seeds. If a solution of these substances is cooled slowly, a few seeds grow into large crystals, but if cooled rapidly, forming numerous seeds and grow only small crystals. Table salt, industrial purified by recrystallization, is composed of many perfect cubic crystals barely visible, the rock salt, formed by slow geological processes, has huge windows with the same cubic crystalline form.

The study of growth, shape and geometry of crystals is called crystallography. When conditions are favorable, each element or compound chemical tends to crystallize into a definite form and feature. Thus, salt tends to form cubic crystals, but the garnet , occasionally also form cubes, is found more frequently in dodecahedra (12-sided solid) or triaquisoctaedros (solid with 24 faces). Despite their different habits (ways of crystallization), salt and garnet always crystallize in the same class and system. Theoretically any thirty-two crystal classes, but only a dozen class includes almost all common minerals, and some classes have never been observed. The thirty-two classes are grouped into six crystal systems, characterized by the length and position its axis (imaginary line passing through the center of the crystal and intersect the faces defining relations of symmetry in the crystal). The minerals in each system share some characteristics of symmetry and crystalline form as well as many important optical properties.

The six crystal systems are of great importance to mineralogists and gemologists, the specification of the system is necessary in the description of any mineral.

"Cristal," Microsoft ® Encarta ® Online Encyclopedia 2008
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Graphic and review of crystal formation: the author.