IDEA: When magma cools, crystals
form because the solution is super-saturated with respect to some
minerals. If the magma cools quickly, the crystals do not have much
time to form, so they are very small. If the magma cools slowly, then
the crystals have enough time to grow and become large. Some granites
contain minerals which are up to one meter (3 ft) across!
|Granite with large
|Diorite with large
The size of crystals in an igneous rock is an important indicator of the conditions where the rock formed. An igneous rock with large crystals probably indicates that the rock formed deep within the Earth, since it is typically warmer deep inside the Earth than near the surface. These are called intrusive rocks, and they have a phaneritic texture (from the Greek "phanerous" meaning visible). Similarly, a rock with small crystals probably formed at or near the surface and cooled quickly. These are called extrusive rocks and have an aphanitic texture (from the Greek "a-" meaning not, and "phanerous"). And some magma cools so quickly that no crystals form; we say that these have a hyaline texture (from the Greek "hyalis" meaning glass).
Sometimes, a rock will contain both aphanitic and phaneritic crystals in it. This means that something truly odd happened to the magma before it was erupted. Since we know that large crystals need time to grow, the magma must have spent some time deep underground. But the smaller crystals mean that the rest of the cooling happened very quickly. If a rock has both crystal types, it means that the mamga spent some time in a magma chamber, where the large crystals grew, then was violently erupted onto the surface, where the small crystals were formed. A good example of this is the Colbert Rhyolite in the Arbuckle Mountains of Oklahoma.
We can simulate the growth of minerals using some common materials; just about everyone has grown salt and sugar crystals from a supersaturated solution. However, those experiments take more time than is usually available in the classroom, so we have developed a demonstration based on one given in Jackson, J. H., and E. D. Evans, 1980, Spaceship Earth: Earth Science, Revised Edition, Houghton Mifflin Company, p. 245-246
To do this experiment, you will need:
|Melting the mothballs and crayon mixture||Cooling the mothballs and crayon mixture|
Before the demonstration
Fill one beaker with 100 mL of water and place it on the hot plate; bring to a boil.
Crush one of the mothballs with the pliers and place it into a test tube; crush a crayon and add it to the test tube. Shake the test tube to mix the mothball and the crayon. Repeat, placing the other mothballs and crayons into separate test tubes. Place all three test tubes into the beaker of boiling water until the mixture melts completely.
Mothballs and crayons both give off flammable gasses when heated. Do not place the molten mixture near an open flame or spark!
1. Using tongs, place one test tube into the warm water and one into the cold water at the start of class. Begin your lecture on crystal size and morphology.
2. After about ten to fifteen minutes (depending on the temperature of the warm water), remove the test tubes from the beakers. Compare the size of the crystals. You may need to use a magnifying glass in order to see the crystals clearly; rotating the tube (to catch specular reflection) may also help.
Do you notice anything unusual about the crystals in the tube which was placed into cold water? (HINT: Is there a color change from top to bottom of the test tube?)
Why did the crystals grow to different sizes? Does crayon color have any effect on the crystal size? What about the relative amount of mothballs? Based on what you have discovered, can you explain why ice cream must be churned? (Try making ice cream without churning!)
This page designed by John DeLaughter firstname.lastname@example.org Update: Dec 16 1997