Ideas for Discussing the Crystalllization Demonstration in Class
What is Happening in the Demonstration?
The solid salicylate is melted and the resulting liquid then cooled, so that it freezes into a solid. Seeds (small crystals) of salicylate enhance the crystallization process, forming nucleation points for crystal growth. A liquid/solid interface forms along the edge of each growing crystal. For a short while, crystals float in the liquid, but as they continue to grow their edges abut one another. Remaining melt is squeezed between crystals, and in some cases even squeezed from crystals along indentations. Liquid is pushed along the solid/liquid interface until all remaining melt freezes and crystallizes.
Linking the Demonstration to Geologic Processes
A mineral is a naturally formed, inorganic, crystalline solid with a specific chemical composition or range of compositions. Minerals form when atoms bond into orderly, three-dimensional crystal structures in the process of crystallization. During crystallization, atoms become arranged in regular patterns that are repeated throughout a given mineral. When crystalline substances grow freely in open cavities and spaces, their external shapes develop unhindered and express the internal geometric order of the component atoms. Crystals have regular, planar (that is, flat) faces. Usually, however, minerals that grow during crystallization are confined by other crystallizing substances that surround them, so their size and external regularity are inhibited. Nevertheless, the orderly framework of atoms within the minerals can be seen from a microscopic view of its internal atomic structure.
The crystals in the hand specimen of granite (photo on left) are large enough to be visible even to the naked eye because the magma cooled relatively slowly. A thin section shows the crystals even better under a microscope (photo on right). Most of the brightly colored minerals are mica; the large white crystals, and some of the gray ones, are quartz. The gray crystals with bands, slightly above and to the right of center, are feldspar minerals.
The process of crystallization occurs in two ways-through cooling and through precipitation from a solution. Crystallization occurs through cooling when liquids or gases lose enough thermal energy for their atoms to slow down and bond strongly to one another so that they freeze in place, barely able to move, thereby forming solids. Snowflakes, for example, are crystals of ice that solidify from water vapor in the atmosphere when the air temperature falls below the freezing point of water. Similarly, minerals crystallize in magma as it cools on its way up from the hot mantle. The slower the rate of cooling, the more time the atoms will have to form extended frameworks of repeating patterns, and the larger and more perfectly formed the crystals will be, whether they are snowflakes or crystals of quartz.
In contrast, instantaneous cooling freezes atoms in place before they can become arranged into orderly frameworks. The resulting solid is not a mineral because it is not crystalline. Glass formed in volcanoes, such as obsidian, is not a mineral because its atoms have no internal symmetrical order and do not form a crystalline array. Lacking the planar faces of crystalline solids, obsidian does not break into regularly shaped pieces. Instead, it breaks into flakes along curved, irregular surfaces.
Crystallization through precipitation occurs when atoms or ions left behind in an evaporating solution bond with one another to form solids. A common example is the formation of salt crystals along the rim of a bowl from which salt water has evaporated. Similarly, the mineral used as common table salt, halite (NaCl), forms by crystallization of sodium and chloride ions from evaporating seawater, as along the margins of a shallow bay. As water evaporates, the concentration of oppositely charged ions of sodium and chloride increases and they become closer to one another. Because ions of opposite charge attract each other, they arrange themselves so that each ion is surrounded by ions of opposite charge.
Demonstration of Crystallization from Solution by Evaporation
For comparison, a second demonstration could be done to show crystallization of halite from solution. Add salt to warm water and stir until dissolved, adding as much salt as possible. Pour the solution into a clear glass bowl that is shallow and broad (e.g., shaped like a pasta bowl). Set the bowl and solution somewhere in the classroom where students can see it each time they come to class. Over a period of days, the solution will evaporate and leave behind a rind of salt. Students can examine the salt crystals with a hand lense.
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