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Crystals Science Fair Projects - Distinguishing Physical Characteristics

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Author: Janice VanCleave

Minerals have a particular crystalline structure. The shape of the mineral crystal is the same no matter how large the crystal is.

In this project, you will study the six basic crystal shapes, called crystal systems, and show their axes' length and orientation. You will build paper models of the six crystal systems. You will also grow crystals in a solution.

Getting Started

Purpose:   To model the axis orientation of a cubic crystal.

Materials

• sheet of typing paper
• scissors
• ruler
• three drinking straw of different colors
• lemon-size piece of modeling clay

Procedure

1. Cut three 1-inch (2.5-cm) pieces from each of the three colored straws.
2. Divide the clay to form two balls of clay, one to form a walnut-size holder and the other to form a support stand.
3. Insert six straw pieces, two of each color, into the clay holder at 90° to each other, as shown in Figure 12.1. Letters A, B, and C represent the three axes: A, x-axis; B, y-axis; and C, z-axis. Use the remaining three straw pieces to form a legend on the sheet of paper. (An axis is a line about which a three-dimensional structure is symmetrical.)
4. Set the clay support stand on the paper.

Results

A model showing the axis orientation (the directional position of the axis of an object) of a cubic crystal is made.

Why?

The Earth's crust contains many kinds of minerals that are identified as one of six crystal systems based on the orientation of their axes. A crystal is a solid made up of atoms arranged in an orderly, regular pattern, forming flat faces (sides). It has a recognizable shape that results from the repetition of the same combination of atomic particles. The shapes of the crystals in minerals are classified into six different groups called crystal systems. There are six common crystal systems: cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic. All crystals are three-dimensional, meaning they have length, width, and depth. In this experiment, a model of a cubic crystal is made. Cubic crystals have three axes that are the same length and at 90° to each other.

Try New Approaches

1.
1. A tetragonal crystal system has two axes of equal length and one unequal axis. All three of the axes are at 90° to each other. Use the steps of the previous experiment to make a model for a tetragonal crystal. Make the vertical y-axis longer or shorter than the other two axes.
2. Make models for the four remaining crystal systems. For information about the length and orientation of the axes of these systems, see the National Audubon Society's Familiar Rocks and Minerals of North America (New York: Knopf, 1995), pp. 11–15.

A solution is a mixture of two or more substances whose makeup is the same throughout. The substance in a solution that dissolves another substance is called the solvent. The substance in a solution that is dissolved is called the solute. A saturated solution is one in which the solvent has dissolved the maximum amount of solute at a given temperature. As the solvent evaporates from a saturated solution, the molecules of excess solute will precipitate (separate in solid form from a solution). Crystals can be grown by preparing a surface for the precipitate to stick to. Do this by mixing 11/2 cups (375 ml) of epsom salt and 3 cups (750 ml) of water in a 1-quart (1-liter) jar. Coil a 12-inch (30-cm) pipe cleaner around a pencil. Slip the pipe cleaner off the pencil and wrap one end around the middle of the pencil. Suspend the coiled pipe cleaner into the epsom salt solution, as shown in Figure 12.2, so that the pipe cleaner is about 1/2 inch (1.25 cm) shorter than the depth of the jar.

When the pipe cleaner is soaked with the solution, remove it, lay it on a piece of waxed paper, and let it dry for two or three days. (Keep the jar sealed with a lid during this time.) As the water evaporates from the pipe cleaner, small crystals of epsom salt will cover the surface of the pipe cleaner. Lower the prepared pipe cleaner into the solution and place the uncovered jar where it will be undisturbed at room temperature. Observe the surface of the pipe cleaner periodically for six or more weeks. The tiny crystals on the pipe cleaner provide a surface for the epsom salt precipitates (solids separated from a solution) to stick to, and thus grow into large crystals. If crystals form on the bottom of the jar, carefully lift the pipe cleaner out of the jar and stir to redissolve the precipitated crystals. For more information about crystal growth, see Janice VanCleave's A+ Projects in Chemistry (New York: Wiley, 1993), pp. 117–122.

Get the Facts

1. A diamond is classed as a gemstone, but graphite isn't. Find out more about the formation of mineral crystals. What are the characteristics of a gemstone? What are crystal twins? See Martyn Bramwell, Understanding and Collecting Rocks and Fossils (London: Usbome, 1983), pp. 20–21.
2. Silica is made of silicon and oxygen. It is the most common chemical found in the Earth's crust. The mineral quartz is made of pure silica. Silicates, the largest class of minerals, are composed of metals combined with silica. Find out about silicates. What is the composition of ore minerals? Are silicates considered ore minerals? For information about silicates and other types of minerals, see Dougal Dixon, The Practical Geologist (New York: Fireside, 1992), p. 20.
3. A mineral's habit refers to its preferred mode of growth, which means the arrangement and proportion of the faces on a single crystal. Find out more about mineral habits in The Audubon Society Field Guide to North American Rocks and Minerals (New York: Knopf, 1979), pp. 37–39.