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Unit 2: Composition of Earth

Chapter 5: Igneous Rocks

Model Crystal Formation

The rate at which magma cools has an effect on the grain size of the resulting igneous rock. Observing the crystallization of magma is difficult because molten rock is very hot and the crystallization process is sometimes very slow. Other materials, however, crystallize at lower temperatures. These materials can be used to model crystal formation.

Problem: Model the crystallization of minerals from magma.

Materials

clean, plastic petri dishes
saturated alum solution
200-mL glass beaker
magnifying glass
piece of dark-colored construction paper
thermometer
paper towels
water
hot plate

Data Sources

• View Student Data

Objectives

In this GeoLab, you will:

• Determine the relationship between cooling rate and crystal size.
• Compare and contrast different crystal shapes.

Safety Precautions

The alum mixture can cause skin irritation and will be hot when it is first poured into the petri dishes. If splattering occurs, wash skin with cold water. Always wear safety goggles and an apron in the lab.

Procedure

1. Read and complete the lab safety form.
2. As a group, plan how you could change the cooling rate of a hot solution poured into a petri dish. For instance, you may want to put one sample in a freezer or refrigerator for a designated period of time. Assign each group member a petri dish to observe during the experiment. Make sure your teacher approves your plan before you begin.
3. Place a piece of dark-colored construction paper on a level surface where it won't be disturbed. Place the petri dishes on top of the paper.
4. Carefully pour a saturated alum solution that is about 95°C to 98°C, or just below boiling temperature, into each petri dish so that it is half-full. Use caution when pouring the hot liquid to avoid splatters and burns.
5. Observe the petri dishes. On the next page, draw a data table on which to record your observations. Below your data table, draw what you observe happening in the petri dish assigned to you.
6. Every 5 minutes for 30 minutes, record your observations of your petri dish. Make accurate, full-sized drawings of any crystals that begin to form.

Data and Observations

Trial	Initial Temperature	Final Temperature	Cooling Method	Crystal Formation
1
2
3
4
5
6
7

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*School
*Required field

Analyze and Conclude

1. Compare your methods of cooling with those of other groups. Did one method appear to work better than others? Eplain.
2. Examine your alum crystals. What do the crystals look like? Are they all the same size? Do all the crystals have the same shape?
3. Draw the most common crystal shape in your science journal. Compare your drawings with those of other groups. Describe any patterns you see.
4. Deduce what factors asffected the size of the crystals in the different Petri dishes. How do you know?
5. Infer why the crystals changed shape as they grew.
6. Compare and contrast this experiment with the magma crystallization.
7. Evaluate the relationship between cooling rate and crystal formation.