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A CTIVIT Y          5.5  Crystalline Textures of Igneous Rocks


                 Name: ______________________________________   Course/Section: ______________________   Date: ___________

               John and Sarah are doing an experiment to find out if crystal size in igneous rocks can be related to the speed of cooling a
               magma/lava. They did not have equipment to melt rock, so they used thymol to model pieces of rock. Thymol melts easily at low
               temperature on a hot plate, and it cools and recrystallizes quickly. Thymol is a transparent, crystalline organic substance derived
               from the herb, thyme, and is used in antiseptics and disinfectants. It is not toxic, but does give off a very strong pungent odor
               that can irritate skin and eyes and cause headaches. Therefore, John and Sarah used a spoon to handle the thymol and did all of
               their work under a fume hood with supervision from their teacher. Sarah placed some thymol in a small Pyrex beaker and melted
               it completely under a fume hood to model the formation of magma. John poured one half of the molten thymol into a cold Petri
               dish and the other half into a hot Petri dish of the same size.


                  A.   The results of John and Sarah’s experiment are shown below. Notice that the images are enlarged. Beside each image below,
                   measure and record the actual size range of the crystals (in mm) that formed.
                 Molten thymol in the cold Petri dish crystallized in 1 minute   Molten thymol in the hot Petri dish crystallized in 3 minutes
                 and looked like this enlarged view.               and looked like this enlarged view.

                                                5 mm                                              5 mm
                   Actual size of the                                Actual size of the
                   crystals in mm is:                                crystals in mm is:













                  B.   Igneous rocks that are made of crystals too small to see with your naked eyes or hand lens are said to have an  aphanitic
                   texture  (from the Greek word for invisible). Those made of visible crystals are said to have a  phaneritic texture
                   (crystals ~1–10 mm) or  pegmatitic texture  (crystals greater than 1 cm). Which of these three igneous rock textures
                   probably represents the most rapid cooling of magma/lava?



                  C.  REFLECT & DISCUSS   This rock has a “ porphyritic texture, ” which
                   means that it contains two sizes of crystals. The large white plagioclase
                   crystals are called phenocrysts and sit in a green-gray “groundmass” of
                   more abundant, smaller (aphanitic) crystals. Based on your work above,
                   explain how this texture may have formed (more than one answer is
                   possible).








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                  D.   In your collection of numbered igneous rock samples, record the sample numbers with these textures:


                Sample(s) with porphyritic texture:            Sample(s) with phaneritic texture:



                Sample(s) with pegmatitic texture:             Sample(s) with aphanitic texture:


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