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What's Inside?: Why are Table Salt Crystals Cube-Shaped?

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Author: Janice VanCleave

Problem

Why are table salt crystals cube-shaped?

Materials

  • Scissors
  • Ruler
  • 3 different-colored drinking straws
  • Index card
  • Modeling clay

Procedure

  1. Cut two 1-inch (2.5-cm) pieces from each of the 3 straws, which will be referred to as straws A, B, and C.
  2. Lay the index card on a table so that one short end is facing you.
  3. Roll a ball of clay about the size of a marble and press it into the center of the index card. This piece of clay will be called the support stand.
  4. Roll a second marble-size piece of clay. This piece of clay will be called the holder.
  5. Insert about one-fourth of each piece of straw A into opposite sides of the holder.
  6. Insert the free end of one of the A straws into the support stand so that both straws are perpendicular to the index card. Perpendicular means that the straws are at a 90-degree (90°) angle to the index card.
  7. Insert the two pieces of straw B into opposite sides of the holder so that they are parallel to the short edges of the index card and perpendicular to the A straws.
  8. Insert the two pieces of straw C into opposite sides of the holder so that they are parallel to the long edges of the index card and perpendicular to the B straws.

What's Inside?

Results

You have made a molecular model of halite, the mineral called common salt or table salt.

Why?

All minerals and many rocks are made up of crystals. Crystals, like all solids, are three-dimensional (having three measurements— height, width, and length). In the halite model, the different-colored straws represent these three measurements: A—height, B—width, C—Iength. The model represents the length of each measurement and the angle between them.

Halite is one example of a cubic crystal. Crystals are cubic when the combined atoms inside the crystal form a cube, or box-shaped solid having six equal, square sides that are at right angles (angles that measure 90°) to each other.

Let's Explore

What would the crystal look like if one of the three pairs of sides is longer? Repeat the experiment, making the length of each of the A straws 2 inches (5 cm). This is a model of a tetragonal crystal. Crystals are tetragonal when their atoms form a solid shaped like a rectangular shoe box or like a cube, but with a pyramid at the top and bottom. Science Fair Hint: Display the models of the cubic and tetragonal crystals along with mineral samples of each, such as halite for a cubic crystal and zircon or rutile for a tetragonal crystal.

Show Time!

  1. Build a paper model of a cubic halite crystal by carefully tracing the cubic pattern diagram on a sheet of typing paper. Cut the pattern out of the paper. Fold the paper along the dashed lines, making all folds in the same direction. Fold the tabs over their corresponding Sides—tab A over side A, tab B over side B, and so on. Use tape to secure the tabs to the sides.
  2. The mineral zircon has tetragonal-shaped crystals. Make a model of a tetragonal zircon crystal by using the previous instructions and the tetragonal pattern diagram. Create stand-up signs for this crystal model and the previous cubic model by folding an index card in half lengthwise. Label one sign Cubic (Halite) and the other Tetragonal (Zircon). Display the signs with the paper models.

What's Inside?

Check it Out!

Cubic and tetragonal crystals are examples of two of the six common crystal systems. What are the names of all six systems? Use a rock and mineral field guide to find out more about crystal systems and mineral examples of each.

What's Inside?

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