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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.