The crystal systems They are a set of geometric characteristics and symmetry elements that allow classifying different crystal conglomerates. Thus, depending on the relative lengths of its sides, the angle between its faces, its internal axes, and other geometric aspects, the shape of a crystal ends up differentiating itself from another..
Although crystalline systems are directly linked to the crystalline structure of minerals, metals, inorganic or organic compounds, these refer more to the qualities of their external form, and not to the internal arrangement of their atoms, ions or molecules..
The six crystal systems are cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. From the hexagonal system derives the trigonal or rhombohedral. Any crystal in its pure state, after being characterized, becomes one of these six systems..
In nature, sometimes it is enough to look at the crystals to know which system they belong to; provided you have a clear command of crystallography. On many occasions, however, this is an arduous task, because the crystals are "scrambled" or "deformed", a product of the conditions of their environment during their growth..
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Crystalline systems can at first seem like an abstract and difficult subject to understand. In nature you are not looking for crystals that have the exact shape of a cube; but share with it all its geometric and isometric qualities. Even with this in mind, it may still be visually impossible to figure out which crystal system a specimen belongs to..
For this, there are instrumental characterization techniques, which among their results show the values for certain parameters that reveal which crystalline system is under study; and also points out the chemical properties of the crystal.
The technique of choice for characterizing crystals is thus X-ray crystallography; specifically, X-ray powder diffraction.
In short: an X-ray beam interacts with the crystal and a diffraction pattern is obtained: a series of concentric points, the shape of which depends on the internal arrangement of the particles. Processing the data, it ends up calculating the parameters of the unit cell; and with this, the crystalline system is determined.
However, each crystalline system is in turn composed of crystalline classes, which add up to a total of 32. Likewise, other different additional forms derive from these. That is why the crystals are very varied.
The cubic or isometric system corresponds to highly symmetric crystals. The cube, for example, presents a series of symmetry operations that characterize it. In the center of the cube, let's imagine that a cross is drawn that touches the top, bottom, and side faces. Distances are equal and intersect at right angles.
If a crystal complies with the symmetry of the cube, even if it does not have precisely this shape, it will belong to this crystalline system.
This is where the five crystalline classes that make up the cubic system come to light: the cube, the octahedron, the rhombic dodecahedron, the icositetrahedron and the hexacisohedron. Each class has its own variants, which may or may not be truncated (with flat vertices).
The tetragonal system can be visualized as if it were a rectangle that has been given volume. Unlike the cube, its axis c is longer or shorter than their axes to. It could also look like a cube stretched up or compressed.
The crystal classes that make up the tetragonal system are the prime and four-sided pyramids, the double eight-sided pyramids, the trapezohedrons, and again the icositetrahedron and the hexacisohedron. Unless you have paper shapes on hand, it will be difficult to recognize these shapes without the help of years of experience..
Any crystalline form whose base corresponds to that of a hexagon will belong to the hexagonal crystal system. Some of its crystalline classes are: twelve sided pyramids and double pyramids.
The base a crystal that belongs to the trigonal system is also hexagonal; but instead of having six sides, they have three. Its crystalline classes come to be: prisms or three-sided pyramids, the rhombohedron and the scalenohedron.
In the orthorhombic system, its crystals have a rhombohedral base, giving rise to shapes whose three axes have different lengths. Its crystalline classes are: bipyramidal, bisphenoidal and pinacoid.
This time, in the monoclinic system the base is a parallelogram and not a rhombus. Its crystalline classes are: sphenoid and three-sided prisms.
The crystals that belong to the triclinic system are the most asymmetric. To begin with, all its axes have different lengths, as well as the angles of its faces, which are inclined..
This is where its name comes from: three inclined, triclinic angles. These crystals are often confused with orthorhombic, hexagonal, and also adopt pseudocubic shapes.
Among its crystalline classes are the pinacoids, pedions, and shapes with even numbers of faces..
Some corresponding examples for each of the crystal systems will be cited below..
Halite, also known as common salt or sodium chloride, is the most representative example of the cubic or isometric system. Among other minerals or elements that belong to this system are:
-Fluorite
-Magnetite
-Diamond
-Spinel
-Galena
-Bismuth
-Silver
-Gold
-Pyrite
-Garnet
In the case of the tetragonal system, the mineral wulfenite is the most representative example. Among other minerals in this system we have:
-Cassiterite
-Zircon
-Chalcopyrite
-Rutile
-Anatase
-Scheelite
-Apophyllite
Among the minerals that crystallize in the orthorhombic system we have:
-Tanzanite
-Baryta
-Olivine
-Sulfur
-Topaz
-Alexandrite
-Anhydrite
-Potassium permanganate
-Ammonium perchlorate
-Chrysoberyl
-Zoisite
-Andalusite
Among the minerals of the monoclinic system we have:
-Azurite
-Cast
-Pyroxene
-Mica
-Spodumene
-Serpentine
-Moonstone
-Vivianita
-Petalite
-Chrysocolla
-Lazulite
Among the minerals of the triclinic system we have:
-Amazonite
-Feldspar
-Calcanthite
-Rhodonite
-Turquoise
In the image above we have an example of when natural forms instantly reveal the crystalline system of the mineral. Among some minerals that crystallize in the hexagonal system we have:
-Emerald
-Calcite
-Dolomite
-Tourmaline
-Quartz
-Apatite
-Zincite
-Morganite
And finally, among some minerals belonging to the trigonal system we have:
-Axinite
-Pyrargyrite
-Nitratin
-Jarosite
-Agate
-Ruby
-Tiger's Eye
-Amethyst
-Jasper
-Sapphire
-Smoky quartz
-Hematite
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