There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure an arrangement of atoms in a crystal is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement.
The unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are possible crystal symmetries, called crystallographic space groups. These are grouped into 7 crystal systems, such as cubic crystal system where the crystals may form cubes or rectangular boxes, such as halite shown at right or hexagonal crystal system where the crystals may form hexagons, such as ordinary water ice.
Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see. Euhedral crystals are those with obvious, well-formed flat faces.
Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid. The flat faces also called facets of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index.
This occurs because some surface orientations are more stable than others lower surface energy. As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces.
Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. See diagram on right. One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying crystal symmetry.
This is determined by the crystal structure which restricts the possible facet orientations , the specific crystal chemistry and bonding which may favor some facet types over others , and the conditions under which the crystal formed. By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks typically range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are occasionally found.
Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock. The vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized; but many kinds of lava were poured out at the surface and cooled very rapidly, and in this latter group a small amount of amorphous or glassy matter is common.
Other crystalline rocks, the metamorphic rocks such as marbles, mica-schists and quartzites, are recrystallized. This means that they were at first fragmental rocks like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution, but the high temperature and pressure conditions of metamorphism have acted on them by erasing their original structures and inducing recrystallization in the solid state.
Other rock crystals have formed out of precipitation from fluids, commonly water, to form druses or quartz veins. The evaporites such as halite, gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in arid climates.
Water-based ice in the form of snow, sea ice and glaciers is a very common manifestation of crystalline or polycrystalline matter on Earth. A single snowflake is typically a single crystal, while an ice cube is a polycrystal. Many living organisms are able to produce crystals, for example calcite and aragonite in the case of most molluscs or hydroxylapatite in the case of vertebrates. Crystallization is the process of forming a crystalline structure from a fluid or from materials dissolved in a fluid.
More rarely, crystals may be deposited directly from gas; see thin-film deposition and epitaxy. Crystallization is a complex and extensively-studied field, because depending on the conditions, a single fluid can solidify into many different possible forms. It can form a single crystal, perhaps with various possible phases, stoichiometries, impurities, defects, and habits.
Or, it can form a polycrystal, with various possibilities for the size, arrangement, orientation, and phase of its grains. The final form of the solid is determined by the conditions under which the fluid is being solidified, such as the chemistry of the fluid, the ambient pressure, the temperature, and the speed with which all these parameters are changing. Specific industrial techniques to produce large single crystals called boules include the Czochralski process and the Bridgman technique.
Other less exotic methods of crystallization may be used, depending on the physical properties of the substance, including hydrothermal synthesis, sublimation, or simply solvent-based crystallization. Large single crystals can be created by geological processes. For example, selenite crystals in excess of 10 meters are found in the Cave of the Crystals in Naica, Mexico. Another way of obtaining a supersaturated solution is making use of the fact that many compounds are better soluble in hot solvents than in cold ones.
A hot solution that is almost saturated is likely to yield crystals at room temperature or, if appropriate, below. However, cystals that grow at higher temperature are frequently twinned or show static disorder. Another way to supersaturation, frequently the best way to grow quality crystals, is the use of binary solvent systems. You need two liquids that mix well, and your compound should be soluble in only one of them.
The liquid in which you compound is soluble is called the solvent, the other liquid the precipitant. As your compound is less soluble in a mixture of the two liquids, you can grow crystals by slowly mixing a not too concentrated solution of your compound with the precipitant. This can happen as liquid-liquid diffusion, gas-phase diffusion or via a membrane dialysis.
Crystallization is preceeded by nucleation, which happens either spontaneously or is incuced by vibration or particles. If nucleation sets in too quickly, too many too small crystals will grow. The figure below shows an equilibrium diagram of a crystallization from a solution. For a diffracton experiment you need no more than one good single crystal. The best way of growing a few nice crystals, when opposed to a lot of bad crystals, is to change the concentration slowly into the area of nucleation, without getting too deep into it.
The formation of nuclei not too many and the starting crystallization will reduce the concentration and bring the solution back into the regiom of oversaturation. That is where existing crystals grow, but no new nuclei form. You want to keep your system there. That means all changes of your system need to be slow. Diffraction quality crystals need to be relatively large. Maybe not quite on the engagement ring scale, but 0. In order to grow large crystals, it is important to avoid having to many nucleation sites see above.
Crystals that grow more slowly, tend to be larger. For crystals that were grown by slow cooling of the solvent: it usually improves the quality and size of the crystals, if the solution is slowly warmed up until alomst all crystals are dissolved again and than cooled dwon a second time very slowly. This can reduce the number of crystals obtained and usually improves quality and size.
A good crystal grows slowly. A good time frame for a crystalliztion experiment seems to be some two to seven days. As mentioned above this is the simplest method to grow crystals. Prepare a nearly saturated solution of your compound in a suitable solvent, transfer at least a couple of milliliters into a clean container, ideally with a large surface, and cover. Set the container aside and disturb the experiment as little as possible remember: vibration can cause nucleation.
Prepare a nearly saturated solution of your compound at or close to the boiling point of the solvent of your choice. Transfer the solution into a clean container and cover.
Place the container into a heat bath at about the same temperature and allow to cool slowly. A Dewar with hot water frequently does the trick. A variation of this method is to prepare a saturated solution at room temperature and place the container in a cold place.
This is simply a solution that cannot hold any more of the material. For example, if you are making a saturated solution of salt water, you would add salt to the water until no more could be dissolved. Eventually the salt will start collecting on the bottom of the container because the water cannot hold any more salt. Crystals grow when the solution becomes supersaturated , meaning that there is too much salt dissolved in the water.
The extra salt or other material takes the form of crystals. To get a supersaturated solution you can either cool down the solution or let some of the water evaporate. To begin, make some Epsom salt crystals.
These are easy to grow and you will begin to see crystals in a couple of hours. Start with one cup of warm distilled water not boiling. Start adding Epsom salts by the spoonful and stirring until they dissolve. Continue doing this until not more Epsom salts can be dissolved this will probably be about one cup.
Let the mixture sit for a couple of minutes until all of the undissolved salt is on the bottom of the container. Slowly pour off the solution into a shallow bowl, but stop pouring before you get to the undissolved salt. Put the bowl in the refrigerator for 3 hours. You should see some crystals beginning to grow. What do they look like? What shape and color are they? You have just made crystals using the cooling method. Next, make some alum crystals using the evaporation method.
Again, being with some hot distilled water. Start adding the alum by the spoonful and stirring until it dissolves. Follow the same method as above to make the supersaturated solution and pour off the solution, leaving the undissolved crystals behind. Use a small saucer or plate to grow the crystals. This will allow for maximum surface area for the volume of solution you have, increasing the evaporation rate. Let it sit not in the refrigerator for a couple of days. You will see a crystal garden begin to grow within a day.
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