# Crystallography

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Crystallography is the study of crystals. When matter passes into the solid state from liquid or gas, it may take a crystalline form , a crystal being a solid of regular geometric shape, possessing flat faces and sharp edges. The formation of crystals appears to be favored by a gradual transition to the solid state. The molecules then tend to arrange themselves so as to form polyhedra. Most substances can be prepared in a crystalline form; metals such as cast iron and zinc solidify in a crystalline form; the most perfect natural examples result from gradual cooling of the Earth's crust.

Study of the external form of the crystal is known as morphological or geometrical crystallography; the internal structure is studied in physical crystallography.

The angles between the crystal faces in crystals of the same substance are invariably the same. This outer symmetry is the key to the internal arrangement of the molecules or ions in the crystal.

A crystal has a plane of symmetry when it can be divided by an imaginary plane into two parts each a mirror image of the other. An axis of symmetry is a line about which the crystal may be rotated so as to present the same appearance to a stationary observer more than once in one revolution. If the aspect is repeated every half-revolution (180°) the axis is one of two-fold, or diad, symmetry. While a crystal may have several planes and axes of symmetry, it may, in addition, possess a unique center of symmetry, such that any line drawn through that center intersects the surface at equal distances on either side.

## Physical Crystallography

By cleavage is meant the tendency of a crystal to split along certain planes parallel to a face. When a crystal is struck by a sharp point, percussion lines are produced which agree in number and direction with the symmetry of the face. A crystal immersed in a solvent shows different degrees of resistance according to the symmetrical arrangement of the particles. The hardness of a face depends on its direction.

X-ray crystallography, first practiced in 1912 by von Laue, gives direct evidence about the internal arrangement of molecules. The regular array reflects the X-rays in a characteristic pattern. From photographs of these diffraction patterns, the internal structure can be determined. X-ray crystallography has provided information about the strength of metals and alloys, and structural data on complex organic molecules such as DNA.

## Physical Crystallography

By cleavage is meant the tendency of a crystal to split along certain planes parallel to a face. When a crystal is struck by a sharp point, percussion lines are produced which agree in number and direction with the symmetry of the face. A crystal immersed in a solvent shows different degrees of resistance according to the symmetrical arrangement of the particles. The hardness of a face depends on its direction.

X-ray crystallography, first practiced in 1912 by von Laue, gives direct evidence about the internal arrangement of molecules. The regular array reflects the X-rays in a characteristic pattern. From photographs of these diffraction patterns, the internal structure can be determined. X-ray crystallography has provided information about the strength of metals and alloys, and structural data on complex organic molecules such as DNA.

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