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111lab-2 - GY 111 Lecture Note Series Lab 2 Introduction to...

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1 GY 111 Lecture Note Series Lab 2: Introduction to Igneous Rocks Lecture Goals: A) Definitions and advice B) Compositions C) Textures D) Nomenclature (AKA rock names) E) More advice References: GY 111; Earth Materials Lab Manual; Chapter 2. A) Definitions There are a couple of critical definitions that pertain to igneous rocks that you need to know before we get to the rocks themselves. If you can understand these and know how they apply to the igneous rocks, you will succeed. Failure to do this will result in… well, failure. Here they go: Composition: The composition of igneous rocks deals with two components. Their geochemistry (this is their bulk chemistry) and their mineralogy (the composition and percentages of the minerals that comprise them). In general, the mineralogy of the igneous rocks is much easier to determine at this level than is their geochemistry. All you have to do is identify the major minerals that comprise the igneous rocks and estimate their proportions. Of course, in order to do this, you need to be able to identify the minerals. This was what we did during the first phase of this course. By now, all of you should be able to pick up an unknown mineral and determine what it is based upon its properties. In order to identify igneous rocks, you will have to be able to pick up an unknown rock and identify the minerals that it contains . Here is your first piece of advice. There aren’t that many minerals that make up igneous rocks because there aren’t that many elements that make up the magma/lava that will ultimately form igneous rocks. The only major minerals that you have to be able to identify in igneous rocks are: quartz, orthoclase, Ca-plagioclase, Na-plagioclase, olivine amphibole, pyroxene, biotite, muscovite Now 9 minerals (all of which comprise the Bowen’s Reaction Series) doesn’t sound that bad does it? The problem is that these minerals are going to be much smaller than in the specimens you saw in the minerals component of the course. You are absolutely going to need a hand lens for the igneous part of this class. And you are also going to have to be able to estimate the proportions of each of the major minerals in the rocks. This will take a bit of practice. The other term that you must understand is texture . The texture of an igneous rock is more or less related to the size, shape and density of the crystals that comprise the rocks. It is related to the cooling speed of the magma and is best explained in a series of cartoons and pictures. B) Composition of igneous rocks In order to understand igneous compositions, we need to reconsider the Bowen’s Reaction Series. As a magma/lava cools, minerals will precipitate in a specific suite. The ferromagnesium minerals precipitate in the following order (from very hot to moderately hot temperatures): Olivine, Pyroxene, Amphibole, Biotite. Feldspars precipitate in the following order (from moderately hot to cool temperatures): Ca-plagioclase, Na-plagioclase, Orthoclase. At the bottom
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2 of the Bowen’s Reaction Series is Muscovite and Quartz.
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