unit 3

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Note for those taking Geol 111. The material beginning on slide 37 will be useful for the mineral identification labs.

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For the next few units, we will look at the rocks and minerals that make up the earth’s crust. Some people get a kick out of identifying and classifying rocks - but there are reasons why “normal” people may also want to know something about rock properties. Suppose an acid tank leaks at you manufacturing plant. If the rock beneath your plant is limestone, you have time to consider the next step since the limestone will tend to neutralize the acid. If the rock is basalt , however, the acid plume will migrate and you will need to take quicker action. If you are in the business of making cleaning products, it is worth knowing that not all volcanic glass works the same – pumice makes a great abrasive cleaner, but obsidian does not! For example

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In order to understand rocks, we need to understand the minerals that make up the rocks. In order to understand the minerals, we need to understand a little about chemistry and atoms. Atoms are made from 1. protons , positive 2. neutrons , no charge 3. electrons , negative Orbit the nucleus and have insignificant mass. Protons and neutrons are in the nucleus of the atom, and have approximately the same mass. The total charge on an atom is neutral (zero) because it has the same number of electrons (-) and protons (+).

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Example Carbon has six protons in the nucleus and six electrons orbiting around the nucleus, so the net charge is neutral.

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An element refers to all atoms with the same number of protons. It is the number of protons that determines the physical and chemical properties of an atom. An atom with only 1 proton is hydrogen (H). Add another proton to the nucleus, and we have helium (He). Listing all the known elements yields the Periodic Table (shown in the next slide).

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(Note that the element symbols are all either a single capitalized letter, or a capitalized letter followed by a single lower case letter.)

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The difference a single proton can make is profound. Consider carbon (C) and nitrogen (N).

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Carbon has 6 protons. Nitrogen has 7 protons. The highest grade coal is nearly pure carbon. The air we breath is approximately 80% nitrogen. (hard to show a picture of air!) The difference is one proton!

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Elements are often shown using a special format: Atomic mass Element symbol C 12 6 number of protons + neutrons Atomic number number of protons

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Because the element changes if the atomic number changes (the number of protons), the atomic number is often left off  12 C Atoms of the same element can have different numbers of neutrons, however. These are called isotopes . Isotopes are atoms with the same number of protons (the same element) but different numbers of neutrons. For example, carbon is found in nature with 6, 7 and 8 neutrons: 12 C 6 protons, 6 neutrons (atomic mass = 12) 13 C 6 protons, 7 neutrons (atomic mass = 13) 14 C 6 protons, 8 neutrons (atomic mass = 14) If we combine 7 protons with 7 neutrons, we have an atomic mass of 14, but with 7 protons, this will be a nitrogen isotope: 14 N

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If an atom gains or loses an electron, the result will be a charged particle called an ion . An ion that is missing one or more electrons will have a positive charge. cation : ion with a positive charge Common examples: An ion with one or more extra electrons will have a negative charge. anion : ion with a negative charge Na + sodium 1 missing electron Ca 2+ calcium 2 missing electrons Cl – chloride 1 extra electron SO 4 2- sulfate 2 extra electrons The last example (sulfate) shows we can have a group of atoms (sulfur and oxygen) with a net charge that is also considered an ion.

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Bonds that form as a result of attraction between oppositely charged ions are called ionic bonds . To form a new chemical substance from ionic bonds, the final product must have a neutral charge. As an example, lets say we have a glass of water with dissolved ions: Na + , Ca 2+ , Cl – , and SO 4 2- . When we let the water evaporate, we will find a solid precipitate on the sides and bottom of the glass. There are several possible combinations we could end up with.

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Possible combinations (all neutral): NaCl Na 2 SO 4 2- CaSO 4 2- CaCl 2 (Subscripted numbers refer to the number of atoms in the molecule. For example, the “2” on CaCl 2 means there are 2 Cl atoms in this molecule.) Impossible combinations (negative or positive final arrangement): NaCa (+3) ClSO 4 (-3) CaCl (+1) NaSO 4 (-1)

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If we put atoms together into a structured arrangement, we can make a mineral. Not all possible combinations make minerals, however. To be considered a mineral, it must meet 4 criteria: Mineral Naturally occurring Crystalline solid Inorganic Specific chemical composition We will consider each of these criteria individually.

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The first criteria may actually have an impact on your emotional well being! Consider the diamond. Diamonds can now be made using ultra-high presses at high temperature. The result is chemically identical to mined diamonds. Only mined diamonds are considered minerals, however. So-called “synthetic” diamonds are very real and genuinely diamonds, but they are not considered minerals. When I was dating my wife, synthetic diamonds were not available, but I can envision how a hypothetical conversation may have gone. Naturally Occurring

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Me: “Think of how much we can save on an engagement ring!” Her: “Do you want the symbol of our marriage to be based on something real or something artificial ?” Me: (with feeling) “But a synthetic diamond is not artificial. It’s a real diamond, just not dug up from the ground!” Her: (no verbal reply, just “the look”) Conversation ends with trip to the jeweler to get a stone that meets the definition of a mineral – naturally occurring!

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Times may be changing though … (For any interested, my wife and I have been happily married for more than 23 years, and looking forward to the next quarter century!) With all the concern about “blood diamonds” and the environmental problems associated with mining, there may be a cultural shift in the works. (For those unfamiliar with “blood diamonds,” there are many places in the world, particularly in Africa, where people are enslaved to mine diamonds to fund bloody civil wars.) I asked my wife a while back if she would be interested in replacing the little-diamond-that-was-all-we-could-afford-when-I-was-in-grad-school with a bigger “synthetic” diamond. I was sure she wouldn’t go for it, but after being assured that the only difference was not being mined (or meeting the definition of a mineral), she actually said “Yes!”

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Crystals are the result of a unique arrangement of atoms. Crystalline Solid Consider ice and salt as examples. The difference in the appearance is caused by differences in the way the atoms are arranged.

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Take table salt first, known by the mineral name halite (NaCl). Halite is formed by joining Na and Cl ions in a cubical pattern. This shape is why it is often possible to balance a salt shaker on a very small pile of salt. Na -------- Cl Cl -------- Na Cl -------- Na Cl The result is a cube-shaped crystal.

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Sugar looks awfully similar to salt, but it’s a lot harder to do this with sugar because …

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 sugar salt  These are electron microscope images of sugar and salt

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Now consider solid water: ice (H 2 O). Water molecules cannot be arranged in a cubical pattern. To get water molecules to line up, they form the pattern below. The result is a six-sided, flat crystal. O O O O O O H H H H H H H H H H H H individual molecule

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To understand “inorganic”, its easier to first understand organic. Inorganic Organic : contains carbon and hydrogen (and other optional elements) and is usually associated with life forms or processes. Inorganic : generally lacking carbon, usually associated with non-living processes Inorganic substances include: 1. any substance with no carbon 2. some substances with carbon including a. pure carbon (diamond, graphite) b. carbon dioxide and carbon monoxide gas (CO 2 , CO) c. materials with CO 3 (found in water and in shells)

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Examples Organic CH 4 C 6 H 5 Cl Inorganic C CO 2 CaCl 2 NaHCO 3

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A mineral is defined by its specific chemical composition. Specific chemical composition 1. In some cases, it is exact: Halite is always NaCl Calcite is always CaCO 3 2. In other cases, it fits within a well defined range: Olivine has the general formula (Fe,Mg) 2 SiO 4 What this means is that each of the following is a possible form of olivine: Fe 2 SiO 4 Mg 2 SiO 4 FeMgSiO 4 In other words, olivine will always have 1 silica atom and 4 oxygen atoms, but can vary in Fe and Mg so long as it has a total of 2 of these (2 Fe, 2 Mg, or one of each).

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So, is an icicle hanging from a ledge a mineral? Is it naturally occurring? Is it a crystalline solid? Is it inorganic? Does it have a specific chemical formula? Since the answer is “yes” to each question, ice on the pond is indeed considered a mineral! So what about ice in your freezer? Not naturally occurring – not a mineral.

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If we take a mineral and keep the same number and type of atoms, but put them together in a different arrangement, we will create a different mineral. Polymorphs A polymorph is a mineral with the same chemical formula as another mineral, but with a different structural arrangment resulting in different physical and chemical properties (including crystal form). Examples 1. CaCO 3 : calcite and aragonite In this case, the differences are subtle. Aragonite and calcite look similar, but aragonite is slightly more soluble in water than calcite at room temperature. aragonite calcite

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2. C: diamond and graphite In this case, the differences are profound. Diamond requires immense pressures to force the atoms into a tighter arrangement than found in graphite. I don’t need to tell you which is which! Graphite: silvery grey color, very soft (can you say “pencil lead”?) Diamond: hardest mineral known, transparent

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates: calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 There are thousands of different minerals, but the vast majority fit into 6 groups:

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Need ideas for remembering which group an element fits into? The following slides point out things to look for in a chemical formula to recognize what group it belongs to.

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Mineral Groups Examples native elements : copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates: calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 Native elements always appear as a single element.

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Mineral Groups Examples native elements: copper, Cu oxides / hydroxides : hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates: calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 Oxides are usually a metal with oxygens attached. Hydroxides always have a hydrogen (hydr) and an oxgyen (oxide) together, often inside parentheses.

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides : halite, NaCl carbonates: calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 Halides will always end with one of the elements in the periodic table known as the halides. These include F, Cl, Br, and I.

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates : calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 Carbonates have carbon (end with CO 3 ).

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates: calcite, CaCO 3 sulfates / sulfites : anhydrite, CaSO 4 pyrite, FeS 2 silicates: olivine, Mg 2 SiO 4 Sulfates have sulfur (S) and oxygen Sulfides have sulfur with no oxygen.

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbonates: calcite, CaCO 3 sulfates/sulfites: anhydrite, CaSO 4 pyrite, FeS 2 silicates : olivine, Mg 2 SiO 4 Silicates have silica (Si) and oxygen.

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Mineral Groups Examples native elements: copper, Cu oxides/hydroxides: hematite, Fe 2 O 3 brucite, Mg(OH) 2 halides: halite, NaCl carbon ates : calcite, CaC O 3 sulf ates /sulfites: anhydrite, CaS O 4 pyrite, FeS 2 silic ates : olivine, Mg 2 Si O 4 A little extra help on the sulfate vs sulfide: Note that all the mineral groups ending with “ ate ” have oxygen. Sulf ide has no oxygen.

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Minerals have unique characteristics as a result of their chemical and physical structure that enable us to identify them. Mineral Identification The list of properties we will consider in the following slides includes Luster Color Streak Hardness Diaphaneity Tenacity Cleavage Fracture Specific Gravity Crystal Form Unique properties

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Luster : appearance of the surface Luster is divided into two principle categories: Metalic and Non-metalic Metalic luster is somewhat self defining – the surface looks like it is metal. This generally means the surface looks like copper, silver or gold.

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Non-metalic luster is further divided into several sub-categories. Most of these relate the surface appearance to other common substances. Examples Vitreous (glassy) Dull or Earthy Pearly

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Examples (cont.) Silky (literally looks like silk) Resinous (like resin, such as used on a violin bow) (these are difficult to show in photographs)

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Color The color of some minerals can be diagnostic. For example, pyrite always has a gold color to it. Other minerals, however, come in lots of different colors due to impurities in the crystal structure. Quartz is a good example of a mineral where color is not a very useful tool for identification since it comes in lots of colors.

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Streak : the color of a mineral’s powder when scraped on an abrasive surface In many cases, a mineral may come in different colors in bulk form, but all will have the same color streak. This can be very useful in identifying the mineral. Hematite (an iron mineral) is a good example. All four of the mineral samples in this photo are hematite. They are not only different colors, but some have metalic luster and others non-metalic, yet they all have the same streak.

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Hardness : resistance to abrasion A guy named Moh came up with a list of minerals with increasing hardness with which to compare the hardness of other minerals. The list is thus known as Moh’s harness scale. There are 10 minerals on the list, numbered 1 through 10. Diamond is the hardest known mineral and is number 10.

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Common items are listed along with these minerals. Fluorite will scratch a copper penny, but gypsum will not. A fingernail will scratch talc.

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Corundum and diamond are both used to make rock and concrete saws. Very hard substances require a diamond blade, but its more expensive!

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Diaphaneity : the ability of a thin slice of a mineral to transmit light Transparent : light passes through unscattered (clear image) (like clear or tinted glass) Translucent : light passes through, but is scattered (results in cloudy image) (like etched glass on a shower door) Opaque : does not transmit light

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x x x transparent translucent opaque

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Tenacity : resistance to being broken or bent Brittle : breaks rather than bend (like a stick of chalk) Elastic : bends and returns to its original shape (like a diving board) Flexible : bends and stays bent (like a metal wire)

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Cleavage and Fracture are related. Cleavage : tendency to break along well defined planes of weakness that are not crystal faces. Some minerals have multiple cleavage planes, some have none. Fracture : appearance of a break that is not along a cleavage plane Cleavage planes in calcite observed after breaking. Quartz has no cleavage, so striking quartz results in lots of irregular shards.

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The appearance of a Fracture surface can often help identify a mineral. Varieties of Fracture include: Conchoidal : a circular, concave break (typical of glass) Uneven : irregular and rough Earthy : smooth, dull appearing surface

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Specific Gravity : the weight of an object relative to the weight of an equivalent volume of water. Minerals with a high specific gravity (often containing metal ions) feel heavy compared to other minerals of similar size. A one inch cube of galena (a lead mineral) feels heavy, whereas a one inch cube of talc feels very light. This is another way of describing density. Density is same as water: specific gravity = 1 Density is greater than water: specific gravity > 1 (will sink in water) Density is less than water: specific gravity < 1 (will float in water)

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Crystal Form (habit) : the shape of a well formed crystal Minerals have characteristic crystal forms, also known as crystal habit, that aid in identification. (Note that many minerals in rocks form in a confined space so they do not show their characteristic crystal form.) Examples on the following slide show how specific minerals form crystals in a precise arrangement of atoms that results in crystal faces with characteristics angles that can be useful in identification.

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halite quartz

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Special properties : many minerals have unique properties that can be used to identify the mineral Examples Magnetism magnetite: magnets will stick to magnetite; compass needles point directly toward or away from sample

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Double refraction : an image viewed through a thin slice appears in double calcite: note how the secondary image appears shifted

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Chemical reaction calcite: reacts with acid to bubble off carbon dioxide

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Taste If you see someone walk up to a rock, whack off a piece and lick it, they are not necessarily crazy. Some minerals have a characteristic taste. For example, halite is salty; selenite is bitter. Odor Minerals like kaolinite have a musty odor. Feel Minerals like talc have a soapy feel.

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Mineral Formation Minerals form by one of three ways: Solidification : crystallize from a magma (melted rock) Precipitation : crystallize from dissolved ions in water Alteration : change some of the atoms in an existing mineral, or change the way existing atoms are put together

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End of Minerals

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