Crystals-Sabiha+and+Beth

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=Examples of Crystals=

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 * Crystal Structure**

All crystals have one uniform structure called a crystal lattice or a Bravais lattice, a structure where the atoms, ions, and/or molecules are arranged in an orderly, three-dimensional, repeating pattern called unit cells. There are seven different crystal systems/structures that specifically reflect the arrangement of the particles. You can also find the melting point of crystals based on the seven different types of bonding. Crystals are made up of sides (faces). These faces intersect at certain angles which are the same for any substance and therefore become a characteristic of that substance. The seven different structures are characterized by the angles between the faces and the number of edges of the length of the face. (Willbraham 280-81).

(trigonal trapezohedron) || ||
 * **The 7 lattice systems** |||||||| **The 14 Bravais Lattices** ||
 * triclinic (parallelepiped) || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/1/17/Triclinic.svg/80px-Triclinic.svg.png width="80" height="96" caption="Triclinic" link="@http://en.wikipedia.org/wiki/Crystal_families"]] ||
 * monoclinic (right prism with parallelogram base; here seen from above) || simple || centered ||
 * ^  || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Monoclinic.svg/80px-Monoclinic.svg.png width="80" height="125" caption="Monoclinic, simple"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Monoclinic-base-centered.svg/80px-Monoclinic-base-centered.svg.png width="80" height="125" caption="Monoclinic, centered"]] ||
 * orthorhombic (cuboid) || simple || base-centered || body-centered || face-centered ||
 * ^  || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Orthorhombic.svg/80px-Orthorhombic.svg.png width="80" height="116" caption="Orthohombic, simple"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Orthorhombic-base-centered.svg/80px-Orthorhombic-base-centered.svg.png width="80" height="116" caption="Orthohombic, base-centered"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Orthorhombic-body-centered.svg/80px-Orthorhombic-body-centered.svg.png width="80" height="116" caption="Orthohombic, body-centered"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Orthorhombic-face-centered.svg/80px-Orthorhombic-face-centered.svg.png width="80" height="116" caption="Orthohombic, face-centered"]] ||
 * tetragonal (square cuboid) || simple || body-centered ||
 * ^  || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Tetragonal.svg/80px-Tetragonal.svg.png width="80" height="133" caption="Tetragonal, simple"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Tetragonal-body-centered.svg/80px-Tetragonal-body-centered.svg.png width="80" height="133" caption="Tetragonal, body-centered"]] ||
 * rhombohedral
 * hexagonal || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Hexagonal_lattice.svg/80px-Hexagonal_lattice.svg.png width="80" height="107" caption="Hexagonal"]] ||
 * cubic || simple || body-centered || face-centered ||
 * ^  || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Cubic.svg/80px-Cubic.svg.png width="80" height="93" caption="Cubic, simple"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Cubic-body-centered.svg/80px-Cubic-body-centered.svg.png width="80" height="93" caption="Cubic, body-centered"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Cubic-face-centered.svg/80px-Cubic-face-centered.svg.png width="80" height="93" caption="Cubic, face-centered"]] ||

=The Seven Different Crystal Systems=

There are seven different crystal systems. The first is a Triclinic System. This system is not symmetrical and therefore creates fairly strange shapes. The next is a Monoclinic System which forms prisms and double pyramids. There is also the Orthorhombic Systems which are made up of Tetragonal crystals but form rhombic prisms or dipyramids (which are two attached pyramids). The Tetragonal Systems are similar to the cubic crystals but are longer on one axis. They also form double pyramids as well as prisms. The Trigonal Systems one single 3-fold axis of rotation. The Hexagonal Systems are 6-sided prisms that have a hexagonal cross section. And finally, the Cubic Systems are not always cute shaped but can also be made up of octahedrons or dodecahedrons. 2

= = =Crystallization=

Crystallization is how solid crystals form from a homogeneous solution. It is referred to as a separation between a solid and a liquid. But in order for crystallization to occur, the solution has to be supersaturated. For a solution to be supersaturated, the solvent has to have more solutes than at normal temperature. 1 = =

=Physical Properties=

The color of crystals greatly depends on the presence of metals within the crystal. Metals such as chrome, iron, cobalt, copper, manganese, nickel, and vanadium all contribute to the absorption of certain wavelengths, which cause the crystal to reflect a specific color. 6
 * //Color//**

The velocity of light is inversely related to its index of refraction, which is the ratio of the speed of light in a vacuum to the speed of light in a substance. This velocity ranges from 1.4 to 3.2, 1.4 being low index of refraction and 3.2 being high. Basically this means that the higher the index of refraction, the higher the velocity of light, making the crystal more "sparkly." 6
 * //Refraction of Light//**

Birefringence is when one ray of light entering a crystal leaves the crystal as two rays of light. These two rays are called ordinary and extraordinary. The ordinary ray stays still, while the extraordinary ray traces a circle around the ordinary ray. A popular example of this is calcite. 6 The term pleochroism implies that a crystal seems to have different colors when it is seen from different angles. Amorphous and cubic materials never exhibit pleochroism. Dichroism is when the crystal reflects two colors, and trichroism is when it reflects three. Iolite is an example of dichroism; it reflects both dark blue and a colorless crystal. Color change can also result from the presence of artificial versus natural light. 6 9
 * //Birefringence//**
 * //Color Change//**
 * //Polarization of Light//**
 * In some crystals, such as tourmaline, the polarization of light occurs. The polarization of light is when more waves of light vibrate horizontally, and more of the waves vibrate in the same direction. Essentially, the direction of the light is influenced so that the light is reflecting in the same direction through the crystal. 6 **

Cleavage of a crystal is essentially how a crystal breaks. Certain crystals break along certain planes because the attraction between the molecules on that plane are less than the attraction of molecules on another. Perfect cleavage is when a crystal breaks along a specific plane, or cleavage plane. Fluorite, calcite, and diamond are all crystals that have perfect cleavage. Imperfect cleavage is when a crystal does not break along any plane. Quartz or beryl are examples of crystals with imperfect cleavage. They can break in any direction. 6, 5
 * //Cleavage//**

The Piezoelectric Effect is when compressed crystals produce an electric charge. Also, when an electric charge is applied to the crystal, the crystal shrinks or expands. This occurs in crystals that have no center of symmetry. In crystals, each molecule is polarized. In a crystal that is symmetric, the polarized molecules face the same direction, whereas in crystals without symmetry, the polarized molecules face different directions. Heating up the crystal in an electric field causes the dipoles of each polarized molecule within the crystal to line up. The result is the stretching or compressing of the crystal. 6, 7 =How are Crystals Formed?=
 * //The Piezoelectric Effect//**

Crystals are formed by a process known as "nucleation." There are two kinds of nucleation: unassisted and assisted. Unassisted nucleation occurs when there is an attraction among the particles of the solute in a solution. What often occurs is that the molecules are attracted to each other and are broken away from each other very quickly because of other forces. Sometimes, these molecules stay together long enough in order to attract other molecules and create a larger combination of molecules. As more molecules accumulate, the attraction for other molecules increases. Soon there are enough molecules joined together by their attraction to form a combination of molecules of "critical size;" the combined attractive forces are able to defy the forces that once pulled them apart. The site of nucleation is where the molecules reach this point of critical size. The system continues to attract molecules and grow until it "falls out" of the solution and is no longer dissolved. It continues to grow until it reaches equilibrium between the molecules in the crystal and the molecules in the solvent. Alas, a crystal is formed. As for the other formation of crystal, assisted nucleation, the process is essentially the same, but instead of being formed in a solvent, the crystal is formed on a solid surface. The molecules combine the same way to form the combination of molecules that form the crystal. Crystals form more quickly in solutions where the concentration of the solute is closer to saturation. When there are more molecules, there is more of a chance that molecules will be attracted to one another and stay together. Heating the solution while the crystal is forming destroys the formation of the crystal because the molecules gain energy and move more quickly within the solution. The molecules therefore don't have enough attractive forces between each other to stay together because of the great amount of energy that they have. Crystals form in different shapes and sizes based on the rate of growth in different directions on the crystal. For example, if the crystal grows quickly in only one direction and slowly in the other directions, the crystal will be flatter.4

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=**Sources**=

1. "Crystallization." //Chemical Engineering, The Chemical Engineers' Resource Page, Distillation, Heat Transer, Design, Spreadsheet Solutions, Departments, Chemistry.// The Chemical Engineers' Resource Page, 2008. Web. 05 June 2010. . 2. "Crystal Types." //Chemistry and New Zealand.// 2008. Web. 05 June 2010. . 3. "Crystal Uses." //Aethereal Minx//. Web. 05 June 2010. . 4. "How Crystals Form." //Chemistry and New Zealand//. 2008. Web. . 5. Jessey, David, and Don Tarman. "Cleavage." //Geological Sciences | Cal Poly Pomona//. Web. . 6. "Physical Properties." //Web Physics//. Davidson College. Web. . 7. "The Piezoelectric Effect." PZT Application Manual, 14 Mar. 2000. Web. 8. Willbraham, Antony C. "States of Matter." //Chemistry.// Needham, Mass.: Addison-Wesley, 2002. 280-81. Print. 9. Willis, Bill. "Polarized Light." //Worsley School OnLine... the Website for Worsley School//. 2001. Web. .