Demonstration goals:

  • Understand the effects of cooling rate on crystal size
  • Empathize how rapid cooling can pb to crystal fractionalization

IDEA:

When magma cools, crystals grade because the solution is super-saturated with respect to some minerals. If the magma cools quickly, the crystals do not accept much time to form, so they are very small-scale. If the magma cools slowly, and then the crystals take plenty time to grow and get large. Some granites contain minerals which are up to ane meter (3 ft) across!

Diorite with large Ca-feldspar crystal

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 stone formed deep within the Globe, since it is typically warmer deep within the Earth than nigh the surface. These are calledintrusive rocks, and they have aphaneritic texture (from the Greek "phanerous" pregnant visible). Similarly, a rock with small crystals probably formed at or nearly the surface and cooled chop-chop. These are calledextrusive rocks and have anaphanitic texture (from the Greek "a-" meaning non, and "phanerous"). And some magma cools and so quickly that no crystals grade; nosotros say that these have a hyaline texture (from the Greek "hyalis" pregnant glass).

Granite with large K-feldspar crystals

Sometimes, a rock volition incorporate both aphanitic and phaneritic crystals in it. This means that something truly odd happened to the magma before information technology was erupted. Since we know that large crystals need fourth dimension to grow, the magma must have spent some time deep underground. But the smaller crystals mean that the residue of the cooling happened very quickly. If a rock has both crystal types, it means that the mamga spent some fourth dimension in a magma chamber, where the large crystals grew, then was violently erupted onto the surface, where the small crystals were formed. A adept example of this is the Colbert Rhyolite in the Arbuckle Mountains of Oklahoma.

We tin can simulate the growth of minerals using some common materials; just about anybody has grown salt and sugar crystals from a supersaturated solution. However, those experiments have more than time than is usually available in the classroom, so nosotros have developed a demonstration based on one given in Jackson, J. H., and E. D. Evans, 1980,Spaceship Earth: Earth Scientific discipline, Revised Edition, Houghton Mifflin Visitor, p. 245-246.

To do this experiment, y'all will need:
  • 3 Mothballs (napthalene)
  • 3 Crayons
  • 3 Examination tubes (l mL or larger) or 3 small beakers (50 mL or larger)
  • 1 Big beaker (250 mL or larger) with 100 ml of boiling water
  • 1 Large beaker (250 mL or larger) with 100 ml of warm water
  • 1 Big chalice (250 mL or larger) with 100 ml of ice water
  • 1 Hot plate
  • Pair of pliers

Melting the mothballs and crayon mixture

Cooling the mothballs and crayon mixture


Before the demonstration:

Fill 1 beaker with 100 mL of water and identify it on the hot plate; bring to a eddy.

Vanquish one of the mothballs with the pliers and place it into a test tube; vanquish a crayon and add together it to the test tube. Milkshake the test tube to mix the mothball and the crayon. Repeat, placing the other mothballs and crayons into separate test tubes. Identify all iii examination tubes into the beaker of boiling water until the mixture melts completely.

Warning! Mothballs and crayons both requite off combustible gasses when heated. Do not place the molten mixture almost an open flame or spark!

one. Using tongs, identify one exam tube into the warm water and one into the cold water at the start of class. Begin your lecture on crystal size and morphology.

ii. Subsequently about 10 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 apply a magnifying glass in order to see the crystals clearly; rotating the tube (to catchspecular reflection) may as well aid.

Exercise yous find annihilation unusual about the crystals in the tube which was placed into cold water? (HINT: Is there a colour alter from top to bottom of the test tube?)

For Discussion:

Why did the crystals grow to different sizes? Does crayon color have whatever effect on the crystal size? What nearly the relative amount of mothballs? Based on what yous have discovered, can y'all explain why ice cream must be churned? (Effort making water ice cream without churning!)


This sit-in was adapted from Jackson, J. H., and Due east. D. Evans, 1980,Spaceship Earth: Earth Scientific discipline, Revised Edition, Houghton Mifflin Company, p. 245-246

Related pages:

An experiment with Ice Cream and Liquid Nitrogen!

The science of water ice cream