Lecture 2 Notes (Geoscience 331)
Lecture 2 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
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This 4 page Class Notes was uploaded by Hannah James on Thursday September 10, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 112 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 09/10/15
Geo 331 Lecture 1 Chemistry 1 The Atom 0 An element cannot be broken down into simpler thing 0 An atom is the smallest particle that retains all the properties of an element 0 Atoms combine together to form compounds 0 Most minerals gems are chemical compounds 0 The nucleus is the center of an atom and is where most of its mass lies 0 Protons and neutrons live within the nucleus and each has a mass of l I Protons have a charge of 1 while neutrons have no charge 0 A cloud of electrons surrounds the nucleus I Electrons have very little mass but have a charge of 1 O In most atoms the number of protons and electrons are the same so the net electrical charge of most atoms is zero 2 Elements 0 Elements are defined by the number of protons they have in their nucleus each element has a different number Hydrogen has one Helium has two etc 0 There are 92 natural elements and 25 artificial elements on the periodic table 0 In this class you will need to know ten of them 39 Oxygen 0 Silicon Si Aluminum Al Iron Fe Calcium Ca Sodium Na Potassium K Magnesium Mg Titanium Ti Hydrogen H and Phosphor P 3 Ions 0 When an atom has a higher or lower number of electrons than its number of neutrons the atom is called an ion 0 Strength of the atom s charge is directly related to the number of protons and electrons 0 If an ion has a positive charge it is called a cation 0 If it has a negative charge it s an anion 0 Anions are attracted to cations and vice versa opposites attract 4 Oxidation States 0 When an element has lost electrons it is in a state of oxidation 0 Elements can have more than one oxidation state they don t need to lose a certain number of electrons 5 Isotopes 0 An isotope is an atom with a different number of neutrons than what is normal 0 A normal number of neutrons often means that the atom has the same number of neutrons as protons O Isotopes tend to be radioactive 6 Chemical Bonds 0 Ionic bonds are when two atoms combine with a net charge of zero 0 These are formed when the extra electron of one atom is attracted to another atom that needs an electron ex Sodium and Chloride Na1 CLl 0 Covalent bonds are when atoms share electrons to gain stability neither one of the atoms has a bigger pull on the electrons O The carbon atoms in diamonds are bonded by covalent bonds II Geologic Principles 1 The Earth 0 Most of the earth is made of rock nickel and iron 0 The outermost layer is called the crust 0 Very thin the crust is the only layer we observe O The vast majority of gemstones are found within the crust 0 Oxygen and silicon are the two most common elements in the crust so it s not surprising that most gems are silicates having silicon and oxygen 0 The mantle lies below the crust and the crust sits atop the mantle in the form of plates 0 The mantle is not brittle like the crust it will ow and bend under stress 0 The core is mostly nickel and iron 0 The outermost layer of the core is molten but the inside has enough pressure to keep the iron and nickel solid 2 Minerals and rocks 0 A mineral is a naturally occurring crystalline substance that is solid and inorganic with a specific chemical composition 0 Most gems are minerals although minerals can have different gem names associated with them same mineral different colors 0 Rocks are combined groups of minerals although some of them only contain one type of mineral 0 Some rocks are known to contain certain minerals ex igneous always contains quartz feldspar and mica 0 Specific areas of earth produce specific types of rock 3 Igneous rocks and volcanic processes 0 Igneous rocks form from molten material and form most of the earth s crust O The source material for igneous rock comes from the mantle I Partial melting causes it to rise to the surface where it cools 0 The speed at which the rock cools determine the size of crystals found in it longerbigger crystals I Volcanic rocks exposed at the surface cool quickly while molten material in magma chambers will cool slowly I These chambers where most large crystals are found are called plutons 0 After cooling a small amount of material remains 0 Water vapor carbon dioxide and sulfur dioxide are common gases found in magma and are confined by the immense pressure of the toprocks I The gases called volatiles remain part of the magma until it is almost completely crystallized I These often play an important role in contact metamorphism 0 Pegmatites are exceptionally coarsegrained igneous rocks 0 0 They contain crystals larger than a centimeter in diameter They form in the last stages of crystallization then volatiles constitute a high percentage of the melt I Ions easily move through this uid allowing the growth of large crystals Pegmatites often contain significant quantities of valuable materials Pegmatite crystals are often large and the volatiles that are left will determine what types of gems form 4 Sedimentary rocks I Sedimentary rocks occur when water washes away mineral material and it settles into layers 0 O The lower layers are eventually lithified into rocks through compaction and cementation I The rocks are broken down by weathering which also exposes and concentrates gems Gems are not often formed by weathering but it is often by weathering that they are found Being more dense gems are often concentrated by weathering in stream beds or along beaches I These are called alluvial or placer deposits 0 Gems retrieved from these deposits are often rounded from weathering I Gems formed by sedimentary processes occur as a result of precipitation of dissolved minerals because the minerals from the water pile up in large deposits 0 When water collects nears magma chambers at a high temperature and pressure it contains minerals that might not normally be soluble in water 0 Hydrothermal deposits occur when a mineralrich solution encounters open spaces such as cracks and the gems crystallize precipitate out of the solution I As a result veins of minerals fill the cracks 5 Metamorphism 0 Metamorphic rocks are changed by heat pressure and interaction with high temperature uids 0 Can be considered as intermediary between igneous and sedimentary rocks I Some metamorphic rocks barely change others look nothing like their parent rocks the rocks or source they come from O The type of parent rock affects the type of metamorphic rock produced I Contact metamorphism occurs when rocks touching molten magmas are altered O Involves high temperatures but low pressure ex garnet 0 When a body of magma intrudes into an area of limestone rare and brightly colored minerals can form these are skarn deposits 0 When the magma also introduces new elements into the metamorphosed rock it makes new minerals this is metasomatism 0 Regional metamorphism is when deeply buried parent rocks are altered by increased pressure and temperature 0 This often results in attened layers of minerals which look banded or laminated this texture is called foliation O The type of rock that forms is a result of the temperature and pressure while the types of minerals that form are result of the parent rock 0 Shales clayrich sedimentary rocks react strikingly to changes in pressure and temperature 0 As temperature and pressure increase the amount of foliation and crystallization also increases I Lower grade metamorphic shales produce slate blackboard roofing etc I Higher grade produces schists and neocrystallization new crystals that form from parent rock I Highest grade produces gneiss 6 Formed vs Found surface processes 0 There s often a difference between where a gem is formed and where it is found 0 Most are formed deep underground where it is too hot to collect them 0 Most gems are found by mining I Natural processes like faulting and volcanism can carry gems from deep within the earth 0 EX rubies are often formed in igneous rock but as they weather out of the rock they are often found in alluvial deposits