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Demonstration goals:

Understand the effects of cooling price on crystal sizeUnderstand exactly how rapid cooling deserve to lead to decision fractionalizationIDEA:

When magma cools, crystals form because the systems is super-saturated v respect to part minerals. If the magma cools quickly, the crystals execute not have much time come form, so they are an extremely small. If the magma cools slowly, then the crystals have sufficient time to grow and also become large. Part granites contain minerals which are up to one meter (3 ft) across!


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Diorite with huge Ca-feldspar crystal


The dimension of crystals in an igneous rock is an important indicator that the conditions where the rock formed. An igneous absent with big crystals probably shows that the rock formed deep within the Earth, since it is generally warmer deep within the planet than near the surface. These room called intrusive rocks, and they have actually a phaneritic texture (from the Greek “phanerous” meaning visible). Similarly, a absent with little crystals probably created at or near the surface and also cooled quickly. These room called extrusive rocks and have an aphanitic texture (from the Greek “a-” meaning not, and also “phanerous”). And some magma cools so quickly that no crystals form; we say that these have a hyaline texture (from the Greek “hyalis” an interpretation glass).

You are watching: The faster an igneous rock cools, the its crystals will be.


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Granite with huge K-feldspar crystals


Sometimes, a rock will certainly contain both aphanitic and phaneritic crystals in it. This way that something truly odd happened to the magma prior to it was erupted. Due to the fact that we know that large crystals require time to grow, the magma must have actually spent part time deep underground. But the smaller sized crystals average that the remainder of the cooling happened really quickly. If a rock has both crystal types, it method that the mamga spent some time in a magma chamber, whereby the huge crystals grew, then was violently erupted onto the surface, whereby the tiny crystals to be formed. A great example of this is the Colbert Rhyolite in the Arbuckle mountains of Oklahoma.

We deserve to simulate the expansion of minerals using some common materials; just about everyone has actually grown salt and sugar crystals indigenous a supersaturated solution. However, those experiment take an ext time than is usually available in the classroom, so we have arisen a demonstration based on one offered in Jackson, J. H., and E. D. Evans, 1980, Spaceship Earth: planet Science, modification Edition, house turn Mifflin Company, p. 245-246.

To carry out this experiment, you will need:3 Mothballs (napthalene)3 Crayons3 check tubes (50 mL or larger) or 3 tiny beakers (50 mL or larger)1 huge beaker (250 mL or larger) with 100 ml of boiling water1 large beaker (250 mL or larger) with 100 ml of warm water1 huge beaker (250 mL or larger) through 100 ml of ice water1 hot platePair that pliers


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Melting the mothballs and also crayon mixture


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Cooling the mothballs and also crayon mixture


Before the demonstration:Fill one beaker through 100 mL that water and place it on the warm plate; bring to a boil.

Crush among the mothballs through the pliers and also place it into a test tube; to like a crayon and add it come the check tube. Shake the test pipe to mix the mothball and also the crayon. Repeat, placing the various other mothballs and also crayons into separate check tubes. Place all three test tubes right into the maker of boil water till the mixture melts completely.

Warning! Mothballs and also crayons both offer off flammable gasses as soon as heated. Do not place the molten mixture close to an open flame or spark!

1. Utilizing tongs, location one check tube into the warm water and also one into the cold water in ~ the start of class. Begin your great on crystal size and morphology.

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2. After around ten to fifteen minutes (depending on the temperature the the heat water), eliminate the test tubes indigenous the beakers. Compare the size of the crystals. You may need to usage a magnifying glass in bespeak to watch the crystals clearly; rotating the pipe (to catch specular reflection) may likewise help.

Do you an alert anything unusual about the crystals in the tube which was inserted into cold water? (HINT: Is there a color readjust from optimal to bottom that the check tube?)

For Discussion:

Why walk the crystals grow to different sizes? does crayon shade have any kind of effect top top the crystal size? What about the relative amount the mothballs? based on what you have actually discovered, have the right to you define why ice cream must be churned? (Try making ice cream there is no churning!)

This show was adapted from Jackson, J. H., and E. D. Evans, 1980, Spaceship Earth: planet Science, modification Edition, houghton Mifflin Company, p. 245-246