Minerals

Lecture Notes Minerals GLG 101 – Physical Geology Minerals‐ The Building Blocks of Rocks Rocks = solid aggregates of crystals (minerals) with some exceptions (e.g. volcanic glass and coal) What are minerals? Mineral Definition: • Naturally Occurring = formed in nature (synthetic diamonds are NOT minerals) • Inorganic (Amber‐fossil tree sap is NOT a mineral, Sugar is NOT a mineral, Salt is a mineral) • Crystalline (Glass is NOT a mineral‐NO definite internal structure) • Solid (Water is a mineral because it is a solid a normal Earth temperatures when it’s frozen) • With characteristic physical properties Crystalline: definite internal structure & composition How can we tell one mineral from another ? Through a characteristic set of physical properties of minerals ‐ Composition and structure dictate physical properties Basic Physical Properties: Crystal Form (shape) – How minerals grow. External expression of internal structure. Geometrically symmetrical shape that mineral will attain if it has the space & time to grow. (e.g. blocky, platey, fibrous, etc.) Cleavage/Fracture ‐ The way a mineral breaks. This is dependent on internal structure. There is a tendency to break along planes of weak bonding. Produces flat, shiny surfaces. Cleavage is described by resulting geometric shapes, the number of planes and angles between adjacent Hardness: Resistance to abrasion “scratchability”. The Mohs Scale is used to describe mineral hardness. Mohs Scale: (1 =Talc‐Gypsum‐Calcite‐Fluorite‐Apatite‐Feldspar‐Quartz‐Topaz‐Corundum‐Diamond = 10) Memory tool you may want to learn to help remember the scale. The Geology Class Failed, Apparently From Quizzes That their Cool instructor Delivered Specific Gravity: The “heaviness” of a mineral. This is similar to density but specific to Earth. S.G. = weight in air/difference between weight in air and weight in water. S.G.=2 to 3 for most minerals. S.G. for gold=20. Luster: How it Reflects Light (e.g. metallic or non‐metallic‐glassy, dull, silky etc.). Diaphaneity: How it transmits Light (e.g. opaque, translucent or transparent). Streak: Color of powdered/pulverized mineral on unglazed ceramic plate. (good test for metallic minerals). Color: Color in Calcite‐ may not be diagnostic. This can be misleading as impurities may change the mineral colors. Example ‐ Mineral Color in Azurite‐ diagnostic Double Refraction: The ability of a mineral to split light. Example – calcite will show a double image when placed over printed material. Magnetism: Ability of a mineral to act as a magnet (to attract other magnets). Mineral Striations: Diagnostic lines observed on some minerals(e.g. plagioclase feldspar). Smell: How a mineral smells on its own or when scratched (Sulfur is a good example of a mineral than smells) Taste: How a mineral tastes. (Salt can be identified by taste). Reaction to Acid: Reaction of a mineral exposed to weak hydrochloric acid (carbonates “fizz/bubble” sd carbon dioxide gas is released) Fluorescence: Mineral glows or emits different colors in ultraviolet Light Piezoelectricity: Minerals that emits electric current when stressed Radioactivity: Minerals that emit radiation (e.g. Uranium minerals) Melting point: Temperature at which a mineral will melt. Minerals are made up of much smaller particles. We need to understand the basic concepts governing mineral formation and properties and understand why minerals behave the way they do. So…….What are these smaller particles? What is an Element ? • A substance that cannot be broken down further by ordinary chemical or physical means. These are the basic building blocks of minerals. Less than 100 are known (92 are naturally occurring). What are Atoms? • The smallest particle of an element that retains the properties of that element. Molecules/Elements/Compounds • Molecules = two or more atoms bonded together to form a substance • Elements – when atoms are the same • Compounds – when atoms are different types Composition of Minerals Atomic structure • Central region is called the nucleus. Consists of protons and neutrons. Protons and neutrons form the nucleus of the atom and give the atom its mass. • Electrons ‐ Negatively charged particles that surround the nucleus. These are located in discrete energy levels called shells. Electrons surround the nucleus at predictable distances and give the atom its size. The Periodic Table – The Elements Columns = Elements with similar properties Determining Subatomic Particles from Periodic Table Why is this important? Using atomic weight or atomic mass number of an element we can predict some of the physical properties of minerals . Example: compounds containing lead (Pb) such as galena will be heavier, or have a higher specific gravity, than compounds containing carbon (C) such as graphite (pencil “lead”). • Atomic Weight – Sum of the # protons + AVERAGE # neutrons • Atomic Mass Number – Sum of the # protons + EXACT # neutrons Determining the number protons, electrons and neutrons of a neutral atom from Periodic Table • Protons = the Atomic # • Electrons = # of protons = the Atomic # • Neutrons = Atomic Mass # (minus)‐ Atomic #(protons) • The Atomic Mass # = Atomic Weight rounded to the nearest whole # EXAMPLE : For K (Potassium) Protons = Atomic # = 19 Electrons = Atomic # = 19 Neutrons = Atomic Wt. rounded to nearest whole number (minus)‐ Atomic# = 39 – 19 = 20 Isotopes ‐ atoms of the same element that contain a different number of neutrons….and they have different Atomic Mass Numbers. Isotope Example How many neutrons are there in this Isotope? 40 K where 40 = the Mass Number 40 (Mass#) – 19 (Atomic Wt. or protons) = 21 neutrons Why Isotopes are Important • Many isotopes are naturally radioactive • They allow us to "tell geologic time" by their rate of radioactive decay into isotopes of other elements. • Decay is a function of time. • Example: Uranium 238 (238U) decays into Lead 206 (206Pb). • By measuring the amount of U and Pb in a mineral we can tell how old it is. • The process = radiometric dating/isotopic dating. Let’s look at how atoms behave…. Atom = a nucleus surrounded by a "cloud" of electrons moving around at predictable distances (energy levels) from the nucleus. Knowing the electron configuration helps us understand why certain elements combine with others to form minerals and give them the properties we observe. Electrons are in energy shells around the nucleus • An electrically neutral atom has the same number of electrons as protons. Atoms want to: (1) minimize their "internal" energy‐ generally minimized when electron shells are full (outer shell has 8 electrons) and the atom has the same electron configuration as one of the noble gases (e.g. Argon, Krypton) (2) neutralize their net electrical charge Structure of Minerals • Minerals consist of an orderly array of atoms chemically bonded to form a particular crystalline structure. • The internal atomic arrangement in ionic compounds is determined by ionic size. Chemical bonding ‐ formation of a compound by combining two or more elements Ionic bonding ‐ Atoms gain or lose outermost (valence) electrons to form ions. Ionic compounds consist of an orderly arrangement of oppositely charged ions. The Prime Example of Ionic Bondinig….Salt. NaCl (Sodium Chloride) – Salt = a compound and a primary example of how atoms behave and what that can tell us about mineral and crystal formation. Na (Sodium) and Cl (Chlorine) • Both Na and Cl want to lose/gain electrons. • This process creates ions out of the original atoms of sodium and chlorine • Na (positive charge = Cation) • Cl (negative charge = Anion). • Each now has stable noble gas electron configurations but has a charge. • To neutralize the charge‐ the two ions must “stick” together ….an IONIC BOND Ionic Bonding & Geologically Iimportant Mineral Groups • Chlorides: ZxClx Example: NaCl (Halite) • Oxides: ZxOx • Sulfides: ZxSx • Fluorides: ZxFx Example: LiF • Where Z = represents any element Example: Fe2O3 Example: PbS Covalent Bonding ‐ Adjacent atoms share outer electrons to achieve electrical neutrality. These are Very strong bonds – stronger than ionic Both ionic and covalent bonds typically occur in the same compound. Isolated purely covalent bonds are rare in nature but include some important minerals. An example of minerals with one type of bond = Carbon will bond covalently with other carbon atoms to produce the minerals diamond and graphite‐ native elements. Polymorphs ‐ Minerals with the same composition but different crystalline structures. Examples include diamond and graphite. There is a phase change is when one polymorph changes into another. Diamond and Graphite—Polymorphs of Carbon Diamonds are formed under high temp./pressure whereas graphite forms at low temp./pressure Van der Waals Bond ‐ weak bond between atoms with left over nuclear forces. Example: Graphite (formed at low temp/pressure). Carbon atoms bonded covalently in sheets. The sheets held together with Van der Waals bonds. The weakness contributes to the ability of graphite to cleave along planes between sheets giving graphite a cleavage allowing it to easily to write words with a pencil. • Metallic Bonding ‐ Nuclei of atoms pack together closely and clusters of nuclei share the net cloud of outer electrons. Valence electrons are free to migrate between atoms. This usually occurs in heavier metals near the center of the periodic table. • Tight packing accounts for "heaviness" metals and their ability to conduct electricity as well as their malleability, and ductility. • Elements that can form metallic bonds often occur naturally as native elements. Examples: Gold, silver, copper. Mixed Bonds • Ionic bonds and covalent bonds can occur in the same compound • Mixed bonds are common in some of the fundamental mineral groups • Examples: the silicates, carbonates and sulfates Mixed Bonds‐ Silicates: ZxSiOx The Silicon Oxygen Tetrahedron Silicon and Oxygen can bond together to form silica (SiO4). Geometrically it looks like a tetrahedron. These tetrahedra have a (‐ 4) charge deficiency and want to find positive ions (cations) to bond with to achieve electrical neutrality. The addition of these cations (e.g. Fe, Mg, Al, Na, Ca & K) and the bonds that are formed between the tetrahedral create a class of minerals called silicates, which make up about 95% of the minerals in Earth’s crust. Of these, we only need to learn about a few (~ 10 or so) to help us identify rocks that will help us interpret the history of our planet. Key points about the Silicon Oxygen Tetrahedra • Sizes of ions allows tetrahedral fit/form • Silicon ion = +4 Oxygen ions = ‐ 8 (4 x ‐2) • There is an Excess negative charge ( ‐ 4 ) • Silica tetrahedra bond with other tetrahedra and with cations to balance the excess negative charge • Tetrahedra bond together and form the “silicates” Mixed Bonds‐ Carbonates: ZxCOx Mixed Bonds‐ Sulfates: ZxSOx Factors that influence Mineral Formation… 1. Composition: the elements present at the time of crystallization 2. Pressure and temperature conditions (diamond versus graphite). 3. Presence or absence of volatiles such as Oxygen, Water, Carbon Dioxide, and others 4. Time (for crystals to form) More on Pressure/Temperature Changes • Minerals that form under one set of T/P conditions may not be stable at other T/P. • Changes brought about (e.g. by plate tectonics) may force minerals into new conditons. • Minerals may be “metastable” Some ways that minerals form… Crystallize from a cooling melt (IGNEOUS) depends on…. • Elements in the melt • Temperature & pressure in the melt • Other volatiles in the melt • How fast it cools Crystallization from evaporation of fluid (often SEDIMENTARY) again…. mineral that forms will depend on • Elements in the fluid • Temperature/pressure • etc. Some ways that minerals form… Crystallization from chemical or biochemical reaction • Chemical reactions can occur in any rock type • Biochemical reactions are typically SEDIMENTARY Rock Forming Minerals Nearly 4000 minerals have been named Rock‐forming minerals ‐ Common minerals that make up most of the rocks of Earth’s crust. There are only a few dozen members. And, there are only eight elements that make up 97% of the continental crust: Oxygen – 60.5%; Silicon‐20.5%; Aluminum‐6.2%; Iron‐1.9%; Calcium‐1.9%;Sodium‐2.5%; Potassium‐ 1.8%; Magnesium‐1.4%. Some Mineral Categories • Rock forming minerals – common rocks (e.g. for building or landscaping) • Ore‐forming minerals – mined for chemical constituents • Industrial minerals – minerals used for their properties (e.g. drywall – “gypsum”) • Accessory minerals – small percentage of a rock’s composition (poss. clues to origin of a rock) • Gemstones – valued for beauty Note: some minerals can be in more than one category Mineral Groups • Common minerals fall into distinct chemical groups on the basis of their composition. • Because of the relative abundance of elements in the Earth’s crust, there are only about 7 common mineral groups: the silicates (ZxSiOx), the carbonates (ZxCO3), the sulfates (ZxSO4), the sulfides (ZxSx), the oxides (ZxOx), the chlorides (ZxClx), and the native elements (Z) themselves. Silicates (ZxSiOx)– The most important mineral group. These comprise most rock‐forming minerals and are very abundant due to large percentage of silicon and oxygen in Earth’s crust. The Silicon–oxygen tetrahedron = Fundamental building block of silicates (Four oxygen ions surrounding a much smaller silicon ion) Joining silicate structures ‐ Single tetrahedra are linked together to form various structures including Isolated tetrahedral, Ring structures, Single‐ and double‐chain structures, Sheet or layered structures, and Complex three‐dimensional structures Isolated Tetrahedral Structure (Nesosilicates) • Isolated tetrahedra held together by ionic bonds with other elements (e.g. Fe, Mg, Al, or Ca) • Typically=high temp. minerals, not stable at lower temperature Isolated Silicates: Olivine Group ‐ High Temperature (>1300°C);Fe‐Mg silicate ‐ Prob. a major component of upper Mantle; Tetrahedra linked together by Fe and Mg ions; No Cleavage; Hardness = 7;Green to Dark Green; Igneous Rock Mineral; No commercial uses; Gem: Peridot Isolated silicates: Garnet Group ‐ High Temperatures & high pressure; Hardness = 7;Fe‐Mg, Ca, Na, Al silicate; No Cleavage; Pea Green to Dark Red; 12 sided crystals‐typical; Metamorphic Mineral; Commercial abrasive; Gem: Red variety Chain Tetrahedral Structures(Inosilicates) • Silica tetrahedra are chained together • Chains held together by ionic bonds with Fe, Mg, Al, Ca, etc. • Moderate to high temperature minerals • Chain structures not stable at lower temperatures. Single Chain silicates: Pyroxene Group ‐ Mod. High Temperature (1200‐1500 °C); Hardness = 6; Fe‐Mg silicates; 2 Cleavages at ~ 90 degrees;“Blocky” crystals; Dark Green to Black; Igneous Rock Mineral; No commercial uses; Gem: None; Most common mineral = Augite (pr. Aw‐jite) Double Chain silicates: Amphibole Group – Intermediate High Temperature (1000‐1200 °C); Fe‐Mg silicates; Hardness = 6; 2 Cleavages at 124 & 56 degrees; “Elongated” crystals; Dark Green to Black; Igneous/Meta Rock Mineral; No commercial uses; Gem: None; Most common = Hornblende Sheet silicates: Micas & Clays (Phyllosilicates) • Silica chains are bonded together in sheets • Sheets held together by ionic or van der waals bonds with Fe. Mg, Al, Ca, Na, K, etc. • Typically these are low temperature minerals Sheet silicates: Mica Group ‐ Biotite: Lower High Temperature (800‐1000 °C); Fe‐Mg silicate; Hardness = 2‐3; One (1) Cleavage direction; Dark Brown to Black; Igneous/Meta Rock Mineral; Sometimes used as electrical insulator; Gem: None Sheet silicates: Mica Group ‐ Muscovite: Lower. High Temperature (600‐800 °C); Hardness = 2‐3; Potassium (K) silicate; One (1) Cleavage direction; Light brown to silvery clear; Igneous/Meta Rock Mineral; Used as electrical insulator; Gem: None Sheet silicates: Clay Group: Low Temperature (<600 °C); Hardness = 1‐2; One (1) Cleavage direction; Various colors‐ white to dark; Sedimentary Rock Mineral; Mostly as product of chemical weathering; Uses: ceramics, construction material, gastrointestinal disorders (Kaopectate), making coated paper etc.; Gem: None Framework Silicates (Tectosilicates) • Silica tetrahedra bond together 3‐dimensionally (each oxygen is shared) • Can accommodate other elements in structure (e.g. Ca, Na and K) • These are the most common minerals The light silicates: feldspar group = the most common mineral group • Exhibit two directions of perfect cleavage at 90 degrees • Orthoclase (potassium feldspar) and plagioclase (sodium and calcium feldspar) are the two most common members. Framework silicates (Tectosilicates)‐ Plagioclase Feldspars: Mod. To High Temperature (1000‐1500 °C); Hardness = 6; Two (2) Cleavage directions; White to bluish or grayish black; Igneous/Meta Rock Mineral; No commercial uses (light ceramics uses); Gem: None Framework silicates ‐ Orthoclase or Potassium Feldspars: Lower to Mod Temperature (800‐1000 °C); Hardness = 6; Two (2) Cleavage directions; White to Pink sometimes blue‐green; Igneous/Meta Rock Mineral; No commercial uses; Gem: some crystals can be gems. Framework silicates – Quartz (The only silicate composed entirely of silicon and oxygen): Low to Moderate Temperature (0‐800 °C); Hardness = 7; No Cleavages (concoidal fracture); Typically white or clear but many color varieties; Igneous/Sedimentary/Meta Rock Mineral; 2nd most common mineral in Continental crust; Uses: Sand & Glass, ore of silican for circuits, radio & watches; Gem: in crystal form. Bowen’s Reaction Series – KNOW THIS CHART and the concepts behind it. Bowen demonstrated that minerals crystallize systematically as a basaltic magma cools and becomes rock, based on the mineral melting points. There is a continuous series and a discontinuous series. At the top of the chart there are Fe, Mg and Ca rich minerals. As the magma cools and these elements are depleted, the magma is enriched in Na, K and Silica, and minerals rich in those elements crystallize out. The chart depicts the temperature of formation and stability of the silicate mineral family. We will also use this chart as we learn about igneous rocks. Important nonsilicate minerals • Typically divided into classes based on anions • Comprise only 8% of Earth’s crust • Often occur as constituents in sedimentary rocks The Carbonates: ZxCOx Carbonates: Calcite (CaCO3): Stable at range of temperatures; SOLUBLE in water; Hardness = 3; Rhombohedral Cleavages; Clear/Whitish to dark; Sedimentary/Meta Rock Mineral; Major ingredient for cement; Gem: in crystal form Carbonates: Dolomite (Ca,Mg)CO3 Stable at range of temperatures; SOLUBLE in water; Hardness = 3; Rhombohedral Cleavages; Clear/Whitish to dark; Sedimentary/Meta Rock Mineral; Major ingredient for cement; Gem: in crystal form The Sulfates: ZxSOx Sulfates: Gypsum (CaSO4): Stable at range of temperatures; SOLUBLE in water; Hardness = 2; One (1) main Cleavage direction; Whitish/clear; Sedimentary Rock Mineral; Uses: Drywall; Plaster of Paris; soil conditioner; Gem: None The Chlorides (ZxClx) & Fluorides (ZxFx) Chlorides: Halite (NaCl): Stable at range of temperatures; SOLUBLE in water; Hardness = 3; Three [3] (cubic) Cleavages; Clear or whitish (other varieties); Sedimentary Rock Mineral; Uses: Salt for flavor/preservatives, roads; Gem: None Halite & Salt Domes – Salt can “flow” and often forms domes. Around salt domes the surrounding rock is displaced upward and oil and gas can be trapped in these rocks next to the domes. Oil and gas in the Gulf Coast is often found in association with salt domes. Fluorides: Fluorite (CaF2): Stable at range of temperatures; SOLUBLE in water; Hardness = 4; Four (4) (octahedral) Cleavages; Commonly purplish or greenish; Hydrothermal Rock Mineral (associated with hot mineral rich water); Uses: Source of fluorine, fluxes; Gem: in crystal or cleaved form The Oxides ZxOx Oxides: Magnetite (Fe3O4): Stable at range of temperatures; SOLUBLE in water; Hardness = 5‐6; No Cleavages; Black; Igneous/Sed/Meta Rock Mineral; Ore of iron. Magnetic properties useful in paleomagnetism; Gem: None Oxides: Hematite (Fe2O3): Stable at range of temperatures; SOLUBLE in water; Hardness = 5‐6; No Cleavages; Gray/Silvery or Rust color; Igneous/Sed/Meta Rock Mineral; Ore of iron. Weak Magnetic properties useful in paleomagnetism; Gem: Specular variety used in jewelry Oxides: Limonite (Fe2O3)(H2O): Stable at range of temperatures; SOLUBLE in water; Hardness = 5‐6; No Cleavages; Black or Yellow brown; Igneous/Sed/Meta Rock Mineral; Ore of iron; Gem: None The Sulfides (ZxSx) Sulfides: Galena (PbS): Stable at range of temperatures ‐ Unstable with Oxygen; Hardness = 3‐4; Three [3] (cubic) Cleavages; Silvery gray; Hydrothermal Rock Mineral; Uses: Principle ore of lead for batteries, bullets…. etc.; Gem: None Sulfides: Sphalerite (ZnS): Stable at range of temperatures ‐ Unstable with Oxygen; Hardness = 3‐4; Six (6) Cleavages; Dark brown; Hydrothermal Rock Mineral; Uses: Principle ore of zinc for batteries steel etc..; Gem: None Sulfides: Chalcopyrite (CuFeS2): Stable at range of temperatures ‐ Unstable with Oxygen; Hardness = 3‐ 4; No Cleavages; “Fool’s Gold” color; Hydrothermal Rock Mineral; Uses: Principle ore of copper for Pipes, wires, ….etc.; Gem: None Sulfides: Pyrite (FeS) “Fools Gold”; Stable at range of temperatures ‐ Unstable with Oxygen; Hardness = 5‐6; No Cleavages; “Fool’s Gold” color; Hydrothermal Rock Mineral; Uses: None really but is associated with other minerals; cause of acid mine drainage; Gem: None Pyrite and Acid Mine Drainage – Pyrite associated with ore deposits has been a major contributor to acidic water drainage from mining operations and a cause of pollution from those sources. Pyrite (iron sulfide) breaks down to sulfuric acid and iron oxides. The Native Elements (Z) Native Elements: Diamond (C): Stable at range of temperatures but forms at high pressure; Hardness = 10; Four (4) Cleavages; Clear to yellowish in color; Igneous/Metamorphic Rock Mineral; Uses: Cutting devices; Gem: popular when cleaved Native Elements: Graphite (C): Stable at range of temperatures but forms at lower pressure; Hardness = 1; No Cleavages; Gray “pencil lead” color; Metamorphic Mineral; Uses: industrial lubricants, pencils, high‐strength light frames; Gem: None Native Elements: Copper (Cu): Stable at range of temperatures; Hardness = 3‐4; No Cleavages; copper color; Hydrothermal Rock Mineral; Uses: Principle ore of copper for electronics, pipes etc.; Gem: none Native Elements: Gold (Au): Stable at range of temperatures; Hardness = 3‐4; No Cleavages; Yellow gold color; Hydrothermal Rock Mineral; Uses: Principle ore of gold for electronics, currency base; Gem: jewelry Native Elements: Silver (Ag): Stable at range of temperatures; Hardness = 3‐4; No Cleavages; Silver color; Hydrothermal Rock Mineral; Uses: Principle ore of silver for electronics, currency base; Gem: jewelry Southwest Minerals: Azurite (blue) and Malachite (green) are carbonates of copper found in many mines including those at Jerome, Bisbee, and Morenci. Rhodochrosite is a manganese carbonate associated with gold deposits in southwestern Colorado.