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Demonstration goals:

Understand the effects of cooling rate on crystal sizeUnderstand just how rapid cooling can lead to decision fractionalizationIDEA:

When magma cools, crystals form because the systems is super-saturated with respect to part minerals. If the magma cools quickly, the crystals perform not have actually much time to form, therefore they are very small. If the magma cools slowly, climate the crystals have sufficient time come grow and become large. Part granites contain mineral which are up to one meter (3 ft) across!


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Diorite with huge Ca-feldspar crystal


The size of crystals in one igneous absent is an important indicator that the problems where the rock formed. One igneous rock with big crystals probably suggests that the rock created deep within the Earth, since it is typically warmer deep inside the earth than near the surface. These space called intrusive rocks, and they have a phaneritic texture (from the Greek “phanerous” definition visible). Similarly, a rock with little crystals probably formed at or close to the surface and cooled quickly. These are called extrusive rocks and have an aphanitic texture (from the Greek “a-” definition not, and also “phanerous”). And also some magma cools so quickly that no crystals form; us say the these have a hyaline texture (from the Greek “hyalis” definition glass).

You are watching: Why does rhyolite have smaller crystals than granite


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Granite with big K-feldspar crystals


Sometimes, a rock will certainly contain both aphanitic and also phaneritic crystals in it. This means that something truly odd occurred to the magma prior to it was erupted. Because we know that huge crystals need time to grow, the magma must have actually spent some time deep underground. Yet the smaller sized crystals median that the remainder of the cooling happened very quickly. If a rock has both decision types, it means that the mamga invested some time in a magma chamber, wherein the huge crystals grew, climate was violent erupted top top the surface, where the tiny crystals to be formed. A great example of this is the Colbert Rhyolite in the Arbuckle hills of Oklahoma.

We have the right to simulate the expansion of minerals utilizing some usual materials; just around everyone has grown salt and sugar crystals from a supersaturated solution. However, those experiment take an ext time 보다 is usually accessible in the classroom, so we have occurred a demonstration based upon one given in Jackson, J. H., and E. D. Evans, 1980, Spaceship Earth: earth Science, revised Edition, houghton Mifflin Company, p. 245-246.

To execute this experiment, you will certainly need:3 Mothballs (napthalene)3 Crayons3 test tubes (50 mL or larger) or 3 small beakers (50 mL or larger)1 large beaker (250 mL or larger) with 100 ml of boiling water1 huge beaker (250 mL or larger) v 100 ml of warm water1 large beaker (250 mL or larger) through 100 ml of ice water1 hot platePair of pliers


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Melting the mothballs and crayon mixture


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Cooling the mothballs and also crayon mixture


Before the demonstration:Fill one beaker with 100 mL the water and also place that on the warm plate; carry to a boil.

Crush one of the mothballs through the pliers and also place it right into a check tube; like a crayon and include it to the test tube. Shower the test tube to mix the mothball and the crayon. Repeat, put the various other mothballs and crayons right into separate test tubes. Location all 3 test tubes into the manufacturer of boiling water till the mixture melts completely.

Warning! Mothballs and crayons both provide off flammable gasses when heated. Do not place the molten mixture near an open up flame or spark!

1. Using tongs, location one test tube into the warmth water and one right into the cold water at the start of class. Start your class on crystal size and morphology.

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2. After about 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 stimulate to see the crystals clearly; rotating the pipe (to catch specular reflection) may likewise help.

Do you notice anything unusual around the crystals in the pipe which was inserted into cold water? (HINT: Is over there a color change from height to bottom of the test tube?)

For Discussion:

Why did the crystals thrive to different sizes? go crayon shade have any kind of effect ~ above the crystal size? What around the family member amount of mothballs? based upon what you have discovered, deserve to you describe why ice cream must it is in churned? (Try making ice cream cream without churning!)

This demonstrate was adjusted from Jackson, J. H., and E. D. Evans, 1980, Spaceship Earth: planet Science, modification Edition, house turn Mifflin Company, p. 245-246