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Natural Quasicrystals

centuries been known that solid can be crystalline or amorphous. In a crystalline solid or crystal, the atoms or molecules are arranged symmetrically in cells that are repeated periodically in space, while in an amorphous solid there is no such symmetry.

The symmetries of a crystal are of two types of translation and rotation. Translational symmetry means that the crystal structure is periodic, so it's the same around all elementary cells. Rotational symmetry implies that the crystal structure remains invariant if we apply a rotation of an angle. In our three dimensional world, these angles of rotation are limited to a few values, namely, 180 º, 120 º, 90 º and 60 º, or in other words, the symmetries of rotation of the crystals can only be of order 2, 3, 4 or 6. (A rotational symmetry of order 6, for example, means that if the rotation is applied 6 times, then return to the starting position this is the rotation of 60 º.) This limitation is imposed by the way they should have the basic cells for, like a puzzle, fit together and fill all space. From the shape of these cells, all crystalline solids are classified into seven systems:

• Cubic (formed by cubes)
• Tetragonal (prisms formed by square)
• Hexagonal (consisting of the basic hexagonal prisms)
• Orthorhombic (consisting rhombic prisms base)
• monoclinic (formed by base oblique rhombic prisms)
• rhombohedral (formed by parallelepipeds whose faces are rhombuses)
• Triclinic (formed by parallelepipeds any )

However, in 1982, a group of researchers from Israel, France and the U.S. discovered an alloy of aluminum and magnesium artificial structure which had a rotation symmetry of order 5. The material was not amorphous, as had a symmetrical structure, or crystal, since the rotational symmetry of order 5 is incompatible with translational symmetry. To describe the new material, coined the term "quasicrystal". A quasicrystal is defined as a solid which has an ordered structure but not regular.

The structure of quasicrystals, though still not well understood, has been associated with aperiodic tilings, finite sets of geometric figures that can cover the plane in a non-periodic. Although aperiodic tilings formally began to study in the twentieth century, some properties have been found in medieval Islamic decorative motifs.

So far, all known quasicrystals had been produced artificially, and it was thought that such a complex structure could not exist in nature. But a group of scientists from Italy and USA just discovered in the Koryak Mountains in the far east of Russia, a mineral consisting of aluminum, iron and copper, the structure is quasicrystalline.

The quasicrystals have not only theoretical interest, and are used in manufacturing bearing and non-stick surfaces for frying pans, for example. Are good thermal and electrical insulation, and resistant to rubbing.

Quasicrystals by Youtube

Source: http://elneutrino.blogspot.com/2009/06/cuasicristales-naturales.html

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