Activity 5.2 Crystalline Textures of Igneous Rock Two students are doing an espe

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Activity 5.2 Crystalline Textures of Igneous Rock Two students are doing an esperiment so fid out if crystal magma. They did not have mel emperature on a hot plase, and it cools and quickly from the herbs thyme and is uned in antiseptics and d sise in igneous rocks cam be related to the speed of cooling a aekw equipment to mek rock, so they used thymol to model pieces of rock. Thym? melts and recrystalliees quickly. Thymol is a transparens, crystalline organic sabstance derived disinfectants. Thymol gives off a very strong pungent odor thas can irritase the srudents used a spocn to handle the thysol and did all of their work under r placed sose drysmol in a small Pynrex beaker and mclhed f magma. The other student poured one-balf of the molies dhymol with supervision from their teacher One of the students a fume hood it completely under a fume hood to model the foermation of inso a cold peeni dish and the other half into a hot petri dish of the same sine. A The results of the sden's experiment are shown in Fig A5.2.1. Nosice that t the images are enlarged, Beside each brlow, measure and record the actual sine range of the crptals (in mm) thas formed Molten thymol in the cold petri dish crystallized in 1 minute and looked like this enlarged view Molten thymol in the hot petri dish crystallized in 3 minutes and looked lke this enlarged view Actual size of the crystals in mm is Actual sine of the crystals in mm is Figure A5.21 lgneous rocks that are made of crystals too small to see with your naked eyes or hand lens are said to have an sextare (from the Greek word for unseen). Those made of visible crystals are said to have a (crystals-1-10 mm) or pegmatitic sextare (crystals greater than I cm). Which of these three igncous rock textures probably represents the most rapid cooling of magma/lava phaneritic testure C REFLECT &DISCUSS; The rock shown in Fig. A5.2.2 has a porphyritie texture, which means that it contains teo sizes of crystals. The large white plagioclase crystals are called phenocryses and sit in a green gray groundmass of more albundant, smaller (aphanitic) crystals. Based on your work in part A, explain how this testure may have formed. More than one potentialy correct answer is possilble Figure A5.22 D In your collection of numbered igneous rock samples, record the sample numbers with these tessures Samplels) with porphyritic Sexture: Sampleljsi with phanentic beture Samplels) with pegmabtic texture: Sampleij with aphanitic texture 140

Explanation / Answer

A. In the first figure, the size of each crystal is approximately 0.5 mm. In the second figure, the size of each crystal is approximately 1.5 mm.

B. The first rock in the figure A5.2.1 represent an aphanatic texture which is the result of rapid cooling of magma/lava.

C. Porphyritic texture is formed when magma cools and crystallizes in two stages. Phenocrysts are formed in the first stage when the magma is cooled rapidly in the deeper parts of the earth and large sized crystals (diameter > 5 mm) are formed. The second stage of cooling occurs at shallower levels where magma is cooled rapidly and small crystals are formed which are unrecognizable with the unaided eye.These small crystals are the groundmass.

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