Lecture17_DynEarth - EAS 1600 Introduction to Environmental...

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Unformatted text preview: EAS 1600 Introduction to Environmental Sciences ____________________________ Class19- Minerals and Rocks: Evidence of a Dynamic Earth ____________________________ Our discussion of the ocean brings up two Earth System Puzzles: Based on our understanding of oceanic processes, the ocean's biota should be running out of nutrients (which should be accumulating at the ocean bottom); and The ocean should have filled in with material from the eroding continents a long time ago. The answer to these puzzles is buried in the processes related to the solid Earth. We begin our discussion of the solid Earth with a review of 1. Mineral and rocks; and then 2. The structure of the solid Earth and methodology we use to probe the Earth's structure seismology. 3. And then, using what these tell us examine the theories which explain the dynamic Earth. So first let's look at mineral's and rocks. For many people, the minerals and rocks we find on the Earth hold a great fascination. Indeed, the mining, selling, and/or collection of precious gems, is a career. Today, we take a look at these objects to see what they can tell us about our planet ... NOTE: Most of the material covered in this lecture can not be found in the textbook, so please use these notes as your basic reference. Another useful source is: Conte, D.J., D.J. Thompson, and L.L. Moses, Earth Science: An Integrated Perspective, Wm.C. Brown Publishers, Chicago, IL, 432pp, 1997. This is the source of most of the graphics used here. FIRST: We need to know something about the Earth's interior... The major zones: crust mantle core Some Definitions: "A mineral is a naturally occurring, inorganic, element or compound, with a definite internal arrangement of ions (i.e., a solid) and a chemical composition that is fixed or varies within narrow limits" (Conte et al. 1997). Minerals: There are 92 naturally occurring elements, but most minerals contain 8 main elements: these are the elements that make up most of the Earth's crustal material. Element Average Weight Percent in Crust Oxygen 46.6 Silicon 27.7 Aluminum 8.1 Iron 5.0 Calcium 3.6 Sodium 2.8 Potassium 2.6 Magnesium 2.1 All others 1.5 Note: while O is most abundant element, it only appears in combination with other elements as oxides (e.g., silicates). "... rocks are aggregates of (one or more) minerals" (Conte et al. 1997). They are by definition found on the Earth's crust. Useful analogy from Conte et al. 1997 letter element word mineral sentence rock The minerals are typically divided into 10 groups or classes. Items of note: 1. Native elements = pure elements 2. Other categories comprise compounds where ions are combined with a common element or group (e.g., sulfides, sulfates, oxides, halides, etc). 3. Silicates are most abundant (> 90% of all minerals found are silicates). They are composed of varying numbers of silicate groups(SiO42- ). 4. Some minerals formed by ionic substitution (e.g. plagioclase, olivine). You don't need to memorize all these names. Within each group or class there can be many individual minerals Any given mineral will have a limited chemical composition. If the elements in that crystal are changed, it will no longer be the same mineral. sodium chloride is the mineral halite; potassium chloride is the mineral sylvite; calcium carbonate is the mineral calcite; calcium sulfate is the mineral gypsum. There is some minor flexibility in chemical make-up caused by ionic substitution. Some of the ions are similar in size and ionic charges (Ex.: Fe+2 and Mg+2). If the mineral is crystallizing out of a liquid that is rich in both elements, the crystal may incorporate some of the "substitute" ions instead of the more appropriate ion. This substitution is common in the silicate families. see next page and pages on silicates Ionic Substitution For example: The plagioclase series comprises minerals that range in chemical composition from pure NaAlSi2O8 (Albite) to pure CaAl2Si2O8 (Anorthite) Andesine by definition must contain 70-50% sodium to 3050% calcium in the sodium/calcium position of the crystal structure. The various plagioclase feldspars are identified from each other by gradations in index of refraction and density in the absence of chemical analysis and/or optical measurements. Mineral Identification 1. Hardness 2. Color, luster, streak 3. Density 4. Cleavage/fracture; i.e., the way in which minerals split 5. Magnetic properties (magnetite). 6. Etc. Silicates: The basic building block of the silicates is the silica tetrahedron. Each silicon atom is attached to four oxygen atoms by tetahedral bonds. This results in a 4charge on the SiO4 (or silicate) group. Examples of silicate minerals: Olivines and Garnets: Isolated tetrahedra balanced by the cations magnesium (Mg), iron (Fe),calcium (Ca) There are many ways in which the SiO4 tetrahedra can be assembled to build neutral silicate mineral structures. These structures are the major rock-forming minerals. Olivine: (Mg, Fe)2SiO4 , Magnesium iron silicate Garnet: A3B2(SiO4)3 . The A represents divalent metals such as calcium, iron, magnesium and manganese. The B represents a trivalent metal such as aluminum, chromium, iron, and other elements found in rarer members of the group. Pyroxenes: Single chains of tetrahedra balanced by similar metal cations and sodium (Na) Amphiboles: Double chains of tetrahedra balanced by similar cations. Silicates - continued Micas and Clay Minerals: Sheets of tetrahedra are the building blocks. Aluminum is also involved in these sheet structures which are chargebalanced by the cations Mg, Na and K. Feldspars: A second group of alumino-silicates, the tetrahedra form three-dimensional frameworks with Ca, Na and K as the balancing cations. The very abundant feldspars are subdivided in the K-Na bearing alkali feldspars and the Ca-Na solid-solution series called the plagioclase feldspars. (see above) Quartz: Silica tetrahedra alone can form a neutral three-dimensional framework structure with no need for other cations. This arrangement forms a very stable structure. Want to know more about minerals? Visit: http://mineral.galleries.com/ Plagioclase Amethyst Diamond Element Ruby Sapphire Rhodochrosite Carbonate Oxides both Corundum Turquoise Phospate Opal Amber Mineraloids (not minerals) Silicates Minerals are formed by process of: crystallization Within the Earth's interior high temperatures and pressures cause rocks to melt forming magma. As the magma cools, various elements and ions come together to form solids with a specific structure; i.e., a crystal. Different minerals form at different temperatures Emerald Garnet Jade Peridot Topaz Zircon Rocks are mineral aggregates... The variety of rocks that are found is determined by the processes that lead to the formation of rocks and their mineral constituents. For example, the minerals that form in a given rock will tend to crystallize at similar temperatures and pressures. Seven mineral categories account for most rocks: Feldspars (silicate) Quartz (silicate) Micas (silicate) Pyroxenes (silicate) Amphiboles (silicate) Clays (silicates) Calcite and dolomite (carbonate) (Because of the dominance of silicates, these minerals are referred to as the "rock-former" class.) Rocks fall into 3 broad categories based on how they were formed: Igneous rock is formed from the crystallization of magma Extrusive (or volcanic) igneous rock - rapid cooling Usually derived from upper mantle and so relatively rich in iron and magnesium (i.e., mafic) Intrusive (or plutonic) igneous rock - slowly cooling Usually derived from crust and so relatively rich in silicates and aluminum (i.e., sialic) Sedimentary rock is formed by the sedimentation and compaction of material (called lithification) Tends to form in layers Metamorphic rock is rock that has been chemically altered while in the solid state from exposure to high temperatures and pressures. The process is re-crystallization Rocks of the Crust Igneous Rocks Extrusive: Basalts (mafic) Olivine, Pyroxine, Plagioclase Intrusive: Granites (sialic) Feldspar, Quartz, Mica Sedimentary Rocks Detrital: Shale (from mud), Sandstone (from sand) Biological: Limestone (from shells, coral), Chert (diatoms) Evaporites: Gypsum(CaSO42H2O), Anhydrite (CaSO4), Halite (NaCl), Calcite (CaCO3) Metamorphic Rocks Slate: Dominant Material clay, chlorite, mica, quartz (shale derivative) Marble: Dominant Material calcite (limestone derivative) Gneiss: Dominant Material mica, quartz, amphibole, feldspar (slate or granite derivative) A schematic representation of metamorphism of a sedimentary rock All three rock-types are linked by the rock cycle... Here is another illustration of the rock cycle Note: The schematic on the previous page describes how the rock-types are linked by process, but it does not indicate: how the rocks are spatially linked; i.e., how does sedimentary material get transported to the deep interior where it can be melted and crystallized into new igneous rock, and vice versa To some extent, weathering along with river runoff, etc can provide an explanation; but not entirely. where the energy comes from to drive the cycle A complete explanation requires an examination of plate tectonics. ...
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This note was uploaded on 04/18/2008 for the course EAS 1600 taught by Professor Jimstjohn during the Spring '08 term at Georgia Tech.

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