Matthew A. Beebe
Department of Geology
College of William and Mary
Williamsburg, VA 23185
Level: Upper elementary to junior high
(part one may be suitable for lower levels)
Anticipated Learning Outcomes
Minerals are crystalline. Crystalline structures are characterized by an ordered internal arrangement of atoms or molecules. The external shape of a mineral may or may not reflect this ordered structure. If a mineral is in the form of a crystal its external form reflects its internal structure. Quartz is commonly found as crystals and as fractured pieces that do not have a crystal form. It is not necessary for students to understand the concept of what atoms and molecules are, but they should understand the concept of ordered structure. An analogy of stacking blocks (or some other object) in a regular pattern is useful to explain ordered structure.
Rocks are composed of mineral grains. Some sedimentary rocks contain, or are composed completely of, minerals that crystallize from concentrated seawater. These include rock gypsum and rock salt (halite). Igneous rocks are those that solidify from molten material. Grain (crystal) size in igneous rocks is controlled by rate of cooling. If a magma (molten material) cools slowly crystals will grow to a visible size resulting in the coarse-grained texture characteristic of such rocks as granite. If a magma cools quickly (at or near the surface of the earth) crystals will be quite small, resulting in the fine-grained texture of rocks such as basalt.
Part One -- Comparing Crystal Shapes
In this part of the activity you will grow crystals of two different substances: alum and salt. Have the students observe that both the materials are similar in color. The alum will probably be granulated and finer than the salt. You may wish to have a student taste a small bit of the alum and the salt to determine that these are indeed different substances. Encourage the students to make predictions of whether the crystals formed from these substances will be similar or different (color, size, shape) from each other. Heat two cups (approximately 500 ml) of water and dissolve about four oz. of alum in the water. When the alum is dissolved pour about one inch of the solution into a clean jar, set this jar aside, uncovered, in a place where it will not be disturbed. Small crystals about 1/4 to 1/2 inch in diameter should begin to form at the bottom of the jar within an hour. Pour the remainder of the solution into a clean jar and cover it. Heat one cup of water and dissolve as much salt as possible in the water (about three or four tablespoons). There will be some salt that does not dissolve. Pour about one inch of the salt solution into a clean jar and set this jar aside uncovered. Very small crystals of salt should form within an hour or so. When crystals have formed in both solutions have the students examine them and compare the shapes, color, and size of the crystals to test their predictions. A magnifying glass will be useful to see the salt crystals.
Part Two -- The Effect of Cooling on Growth of Crystals
In this part of the exercise you will grow crystals of alum under different conditions. One jar will be cooled while the crystals grow, the other jar will cool under room temperature conditions. Encourage students to make predictions about what effect cooling will have on size or shape of crystals. Remove two of the crystals of alum that you have grown in part one of the exercise to use as seed crystals. Tie the seed crystals to separate pieces of thread, each about a foot long. This is a little tricky. It may be useful to notch the crystals with a pen-knife on opposite sides, cast an overhand knot in the thread, insert the crystal in the loop and tighten so the thread lies in the notches. Another overhand knot will secure the crystal. These crystals will be suspended in clean jars by tying the free end of the thread around a pencil and propping the pencil across the top of the jar (see Figure 1). Use the alum solution you have reserved from part one of the exercise and divide the solution equally between the jars. If crystals have formed in the reserved solution they will have to be dissolved by heating (allow to cool to near room temperature). Do not pour hot solution over the seed crystals or they may dissolve. Place one of the jars in a container of ice-water (a small cooler or sink would work). Place the other jar someplace where it will not be disturbed. Several hours are required for the crystals to form completely. When the crystals have grown sufficiently, have students observe and compare the size and shape of the crystals grown under different conditions and compare the results with their predictions.
Results and Discussion
Students should observe that the shapes of crystals of salt and alum are quite different. The salt crystals will be cubical and most of the alum crystals will be variants of octahedrons (see Figure 2). The students will also notice that the salt crystals are considerably smaller than the alum crystals. This probably is a function of the kinetics of crystallization. This is probably a good point to display mineral crystals, if they are available, to demonstrate the variety of crystal shapes possible in minerals.
This part of the exercise has met with variable results. Ideally the crystals grown under room temperature conditions will be larger than the crystals grown while cooling with ice water. However, if there are many nucleation points in the room temperature jar there may be many crystals that are rather small; this may result from dirt in the jar, or the jar being disturbed. Larger crystals result from greater time for crystallization to occur, as should be the case if the solution cools slowly. On the other hand, if the solution cools rapidly, less time is available for the ions to organize into large crystals, and the crystals tend to be smaller. At this point you might discuss how such evaporite minerals as halite and gypsum form from restricted bodies of seawater that evaporate, thus concentrating the salts to the point that the minerals crystallize. Tremendous, thick deposits of halite have formed this way during the geological past at various places on Earth. Significant salt deposits in the United States include those in Michigan, New Mexico, and Utah. Display samples of basalt and granite and explain that both are igneous rocks that formed by cooling of molten material (magma), the crystals in the granite are larger because they cooled very slowly, deep below the surface of the earth, whereas the crystals in the basalt are small because they cooled rapidly, at or near the surface of the earth.
It is suggested that you experiment with the exercise before attempting it in a class. The alum crystals are quite easy to grow but a little experience will prevent possible fumbling. The exercise is probably best not done in a single day, but over two days. Considerable time may be saved if the seed crystals for part two of the exercise are prepared (tied on thread) beforehand. The exercise may also be modified to meet the needs of your students. You may choose simply to grow alum crystals and observe the shapes. You may modify part two of the experiment by placing a jar in a cooler of warm water to slow cooling further. You may try mixing solutions of salt and alum to see what sorts of crystals are formed and whether the crystals will segregate or grow together. It is also suggested that you construct a student lab sheet where students may list the purpose of the experiment, materials involved, the procedure, the expected outcome (a prediction), the results, and any conclusions that may be drawn.
|Fig. 1. Apparatus for growing crystals. Seed crystal is suspended by thread from a pencil placed across the top of a jar or beaker.||Fig. 2. Approximate shapes of alum crystals.|
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