Minerals Courtesy of Katryn Wiese
What is a mineral? (5 Requirements) Naturally occurring. Inorganic no O, H, and C bonded together. Solid Has an orderly internal structure with definite pattern. Has a definite chemical composition (varies within specific limits).
So What s a Rock? A rock is a naturally occurring solid aggregate of minerals and/or mineraloids. In general terms is composed of minerals.
There are over 4000 different minerals! (all with different chemical compositions) In this course you will need to: -- become familiar with only a handful. -- know the chemical formulas for two minerals (calcite and quartz). -- know the diagnostic properties and tests for identifying minerals.
The only formulas you need to memorize: SiO 2 Quartz CaCO 3 Calcite
Why study rocks and minerals??
Minerals is what most of what the Earth is made of. We build on and with minerals. Some are prized for beauty. Some are useful for the physical properties. We are dependent on minerals for health and nutrition---agriculture! Almost every aspect of modern technology is related to the understanding and availability of minerals. Understanding minerals helps in understanding many natural phenomena (earthquakes, meteor impacts, soil development, past climate...etc).
Minerals can form in melts (magma or lava) as they crystallize (freeze). Mineral crystals take time to grow: If a melt cools too fast for crystals to grow then a glass is formed and there will be no minerals present. Often the slower the cooling, the larger the crystals.
Minerals precipitate from solutions (water) that is carrying dissolved components (ions).
Biogenic (mineral created by organisms). In the photo are radiolarians which are small planktonic marine organisms that form shells out of silicon dioxide (glass). Our bones and teeth are composed of calcium phosphate--a common mineral called apatite.
ORGANIC is: O, H, C, bonded together (all three) (other things OK) INORGANIC is: NO O, H, C bonded (all three together) EXAMPLES: Which are inorganic? NaCl (halite/salt) C (graphite/diamond) C6H12O6 (sugar) CaCO3 (calcite)
Turn to Neighbor: Based on the definition of a mineral, determine if these substances are minerals or not. Why or why not? Gold Glass Sugar Salt Ice
Elemental Abundances in Continental Crust
Composition of Minerals (and everything else) Elements Basic building blocks of minerals Less than 100 are known (92 are naturally occurring) Atoms Smallest particles of matter Retains all the characteristics of an element
Basic structure and components of an atom.
Atomic Particles (amu = atomic mass unit) Name Symbol Charge Mass Location Electron e -1 0 Energy shells surrounding nucleus Proton p+ +1 1 amu Nucleus Neutron n 0 1 amu Nucleus
Valence electrons---the outer shell.
Atomic mass = total mass of atom = # of protons + # of neutrons Atomic number = # of protons = designation for element Energy shells: electrons reside in energy shells surrounding the nucleus of an atom. Shell 1 maximum = 2 electrons (no more allowed) Shell 2, 3, 4, 5 maximum = 8 electrons (no more allowed) Therefore, the first two electrons go into shell 1, the next eight into shell 2, and so on. Valence electrons = the number of electrons residing in the outermost unfilled energy shell
# of valence e- = 8 (A Full Energy Level so it does not readily react with other atoms).
Periodic Table of the Elements (Atoms) Avogadro's number = 6.0221415 10 23
Under STP conditions.
Isotopes and radioactive decay Isotopes are atoms that contain the same number of protons but a different number of neutrons (remember the # of proton dictates the element) The number of protons (the atomic number) is the same for each isotope, e.g. carbon-12, carbon-13 and carbon-14 each have 6 protons, but the number of neutrons in each isotope differs. Thus the mass is different. Some isotopes are unstable and spontaneously decay (this is radio activity). The rate of decay is constant but varies with the isotope type, but this decay rate is constant. We will look at this more closely when we cover geologic time.
Isotopes = atoms with same atomic number (same # protons), but different # of neutrons. Therefore, they are the same element, but with different masses. Atomic weight = the average atomic mass of a given sample of an element (averaging in all its naturally occurring isotopes).
Mass Spectrometry
Minerals are formed through the chemical bonding of atoms/elements. There are different types of bonds between atoms and often compounds can be formed with both ionic and covalent bonds. Ionic Bond: Atoms gain or lose outermost (valence) electrons to form ions. Ionic compounds consist of an orderly arrangement of oppositely charged ions. Covalent Bond: Atoms share electrons to achieve electrical neutrality. Generally stronger than ionic bonds. Metallic Bond: Valence electrons are free to migrate among atoms. Weaker and less common than other bonds.
There are other bonds! Hydrogen Bond: A weak bond but is very important. It gives water a number of special properties and it is important in waters interaction with other compounds. Van der Waals: A very weak bond that can be very important. It s common in gases and organic liquids and solids. Formed by residual charges from the other types of chemical bonds. The mineral graphite is a good example of this type of bond.
Relative strength Relative Example strength Covalent bonds Ionic bonds Hydrogen bonds Description Shared electrons to complete outer shell. Atoms exchange electrons to complete outer shell. Now atoms are ions that are oppositely charged and attracted to each other. Strongest Medium Weakest Water molecules (because of shape) have a slightly positive end and slightly negative end. These molecules are attracted to each other and to other ions. Example Diamond Quartz Water (between H and O atoms within the water molecule) Halite (salt) Water (between water molecules how they stick to each other)
A molecule with very special properties. Polar molecule it has a slight charge due to it geometry of the molecule.
Water molecules are asymmetrical. The positively-charged portions of one are attracted to the negatively-charged parts of another. It takes a lot of energy to pull them apart. Hence: Water melts and boils at unusually high temperatures for such a light molecule. Water has a high heat capacity. It takes a lot of energy to melt ice and vaporize water. Thus water is the principal heat reservoir on the Earth. The asymmetrical charge distribution on a water molecule makes it very effective in dissolving ionically-bonded materials. However, it is not an effective solvent of covalently bonded materials (oil and water don't mix). Hence: Water is very effective at weathering rocks and minerals. It is the closest thing to a universal solvent. Water is very effective at transporting ions and dissolved nutrients in the human body. Water is not an effective solvent of organic molecules. Thus we do not dissolve in our own cell fluids. Nifty feature. When water freezes, it assumes a very open structure and actually expands. Most materials shrink when they freeze and sink in their liquid phases. Implications: If ice sank like most frozen solids, it would accumulate at the bottoms of frozen lakes and seas. Most of the world's water would be ice. Expansion of ice in rocks is a powerful weathering agent.
What Type of Bond? Octet rule: All atoms want to completely fill their outermost energy shell: 2 electrons maximum in the first shell and 8 maximum in all other shells.
Periodic Table of the Elements (Atoms) Avogadro's number = 6.0221415 10 23
Halite (NaCl) Ionic Bonding
Evaporation pond for the production of salt.
The Covalent Bond Generally stronger than ionic bonds. A strong bond that is formed when atoms share an electron. Atoms share electrons achieving electrical neutrality.
What are the atoms bonded here? What type of bond?
Covalent Bonding as seen in the Silicon-Oxygen Tetrahedron---SiO4 4-
Metallic bonding The valence electrons are free to migrate among atoms. That s why metals are such great conductors. Metallic bonds are weaker and less common than other bonds. A. Outermost electrons wander freely through metal. Metal consists of cations held together by negatively-charged electron "glue." B. Free electrons can move rapidly in response to electric fields, hence metals are a good conductor of electricity. C. Free electrons can transmit kinetic energy rapidly, hence metals are good conductors of heat. D. The layers of atoms in metal are hard to pull apart because of the electrons holding them together, hence metals are tough. But individual atoms are not held to any other specific atoms, hence atoms slip easily past one another. Thus metals are ductile.
Polymorphs --minerals which have the identical chemical composition, but different internal structure. A great example are the carbon polymorphs, diamond and graphite, which are both composed of pure carbon but have substantial differences in their atomic packing and bonding. Diamond: Composition= C Structure=Framework Graphite: Composition= C Structure=sheets
Ionic Substitution (also called solid solution) Ionic substitution occurs because some elements (ions) have the same size and charge, and can thus substitute for one another in a crystal structure. Color of the mineral is a common result of Ionic substitution.
X-ray diffraction shows the internal structure of a crystal. This method is often used to identify minerals.
Most minerals are silicates! (silicate) (silicate) (silicate) (silicate) (silicate) (silicate) (silicate)
SILICATES Amphibole family: Hornblende [Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 ] Feldspar family Plagioclase Feldspar: [CaAl 2 Si 2 O 8 ] to [NaAlSi 3 O 8 ] Potassium Feldspar: [KAlSi 3 O 8 ] Garnet Fe,Mg,Ca, Al Silicate Mica family: Biotite [Silicate with K, Mg, Fe, Al, Ti, OH, F] Muscovite [Silicate with K, Al, OH, F] Olivine (Mg,Fe) 2 SiO 4 Pyroxene family: Augite [Silicate with Fe, Mg] Quartz SiO 2 Serpentine Mg 6 Si 4 O 10 (OH) 8 CARBONATES Calcite CaCO 3 SALTS Halite NaCl SULFATES Gypsum CaSO 4 *2(H 2 0) SULFIDES Galena PbS Pyrite FeS OXIDES Hematite Fe 2 O 3 Magnetite Fe 3 O 4 NATIVE ELEMENTS Graphite (C) Talc Mg 3 Si 4 O 10 (OH) 2
Physical Properties of Minerals Minerals have physical properties that are dependent on their composition and crystal structure. Diagnostic properties: are determined by observation or performing simple tests. are used to identify hand samples of minerals.
Here is a list of diagnostic properties used to identify hand samples: Color--- Least useful since color can vary. Luster How light plays of the surface (metallic, dull, etc) Streak The color of the mineral in powder form. Hardness What it can scratch or what scratches it. Crystal Form The shape of a crystal (crystal structure). Cleavage/Fracture How the crystal breaks. Specific Gravity The density related to water (heft). Transparent, Translucent, or Opaque.
There are also other tests that help ID certain minerals: -Double Refraction (in Calcite) -Salty Taste (Halite) -Effervescence (Calcite) -Phosphorescence (Fluorite) -Magnetic (Magnetite) -Twinning Striations (Plagioclase Feldspar) -Exsolution Lamellae (Potassium Feldspar)
Gold has a specific gravity (density) of 20. Specific gravity is measured in grams per cubic centimeter OR is just a multiple of how much more something weighs than an equal volume of freshwater. If a pail of water weighs 2 kg, what does the same size pail of gold weigh? 2 kg X 20 = 40 kg
Color these are all quartz! Smoky Quartz comes from natural radiation damaging the crystal structure. Milky white quartz comes from water-rich fluid inclusions. Amethyst (purple quartz) comes from Fe 4+ ionic substitution; Citrine (yellow quartz) comes from Fe 2+ ionic subsitution.
Titanium impurities lead to blue Sapphire a form of Corundum: (Al 2 O 3 )
Chromium impurities lead to red RUBY a form of Corundum: (Al 2 O 3 )
Hardness
Streak
Cleavage Planes
Luster: metallic on the right and submetallic on the left. Cleavage? Opaque all metallic luster minerals are opaque.
Luster Color Crystal form
Crystal Form (6 sided prism) Luster (vitreous) Conchoidal fracture Transparent (mostly)
Translucent Some Crystal Forms Examples
Note the difference between cleavage/fracture and crystal form. Crystal form shows the underlying pattern of atomic bonding and represents the way the crystal grows. Cleavage and fracture are indicators of the strength of the underlying bonding, and shows how a crystal breaks.
All three are translucent. Crystal Form vs Cleavage Planes
Cleavage (one perfect cleavage plane)
Conchoidal Fracture Vitreous Luster
Effervescence reaction to HCl (hydrochloric acid) demonstrates that this is calcium carbonate (the mineral calcite).
Plagioclase Feldspar Twinning Striations (cannot feel)
Sub-parallel Exsolution Lamellae Color Cleavage (2 at 90 degrees)
Dull or earthy luster and red color----cinnabar
The Naica Mine of Chihuahua, Mexico, is a working lead, zinc and silver mine in which large voids have been found, containing crystals of selenite (gypsum) as large as 4 feet in diameter and 50 feet long. The chamber holding these crystals is known as the Crystal Cave of Giants, and is approximately 1000 feet down in the limestone host rock of the mine. The crystals were formed by hydrothermal fluids emanating from the magma chambers below. The cavern was discovered while the miners were drilling through the Naica fault, which they were worried would flood the mine. The Naica mine was first discovered by early prospectors in 1794 south of Chihuahua City. Fluorescent Calcite Sample unknown locality
METALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity Mineral H S G Streak Color Form Cleavage/Fracture Distinctive properties Pyrite FeS 2 6-6.5 5 Dark grey Brass yellow; tarnishes brown. Cubes or octahedrons Brittle. No cleavage. Cubic form, brassy color, and SG=5. Magnetite 6 5.2 Dark grey Silvery grey to black. Fe 3 O 4 Tarnishes grey. Opaque. Octahedrons No cleavage. Attracted to a magnet. SG=5.2. No cleavage. Hematite 1.5- Fe 2 O 3 6 2.1-2.6 Red to redbrown Silvery grey, black, or brick red. Luster can also be nonmetallic. Thin tabular crystals or shapeless masses. No cleavage. Red streak. Metallic + nonmetallic. Earthy red. Galena PbS 2.5 7.6 Grey to dark grey Silvery grey. Tarnishes dull grey. Cubes and octahedrons Brittle. 3 good cleavage planes (cubes). SG=8. Dense! Silver cubes (form and cleavage). Graphite C 1 2.1-2.3 Dark grey Silvery grey to black. Flakes, short hexagonal prisms, and masses. 1 excellent cleavage plane. Dark grey. H=1. Greasy. Dark grey streak.
NONMETALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity Mineral H S G Strea k Color (and/or luster) Form Cleavage/Fracture Distinctive properties Garnet 7 3.5 (Ca 3,Mg 3,Fe 3 Al 2 )n(sio 4 ) 3-4.3 White Red, black, or brown; can be yellow, green, pink. Glassy. Translucent. Dodecahedrons (12-sided polygons) No cleavage. Brittle. Conchoidal fracture. Dodecahedron form, red, glassy, conchoidal fracture, H=7. Olivine (Mg,Fe) 2 SiO 4 7 3.3-3.4 White Pale or dark olive green to yellow or brown. Glassy. Transparent. Short prisms (usually too small to see). Conchoidal fracture. Brittle. Green, conchoidal fracture, glassy, H=7. Usually granular. Quartz SiO 2 7 2.7 White Colorless, white, or gray; can occur in all colors. Glassy and/or greasy. Massive; or hexagonal prisms that end in a point. Conchoidal fracture. Glassy, conchoidal fracture, H=7. Hex. prism with point end. Plagiociase Feldspar family: Anorthite and Labradorite CaAl 2 Si 2 O 8 to Oligoclase and Albite NaAlSi 3 O 8 6 2.6-2.8 White Colorless, white, gray, or black; can have iridescent play of color from within. Translucent to opaque. Tabular crystals or thin needles 2 good cleavage planes at nearly right angles. Twinning. 2 cleavages at 90. Potassium Feldspar family: Orthoclase and Microcline KAlSi 3 O 8 6 2.5-2.6 White Pink. Or white, orange, brown, gray, green. Translucent to opaque. Tabular crystals 2 good cleavage planes at nearly right angles. Subparallel exsolution lamellae. 2 cleavages at 90. Pink color. Pyroxene family: Augite 5.5 Ca(Mg,Fe,Al)(Al,Si)O 6-6 3.2-3.5 White, pale grey Green to black; opaque. Short, 8-sided prisms (if visible). 2 good cleavage planes at nearly right angles. H=5.5. Dark green or black. 2 cleavages at 90. (Looks like HB.) Amphibole family: Hornblende (Ca,Na) 2-3 (Fe,Mg,Al) 5 Si 6 (Si,Al) 2 O 22 (OH) 2 5.5 3-3.3 Greygreen, white Dark green to black. Opaque. Long, perfect prisms. 2 cleavages planes. Angles: 60 and 120. Brittle. Splintery fracture. H=5.5. Dark green or black. 2 cleavages at 60 & 120. Splintery fracture. Long prisms.
NONMETALLIC MINERALS (listed in decreasing hardness) Review mineral formula to connect to family! H=Hardness; SG = specific gravity Mineral H S G Strea k Color (and/or luster) Form Cleavage/Fracture Distinctive properties Serpentine 2-5 2.2 Mg 6 Si 4 O 10 (OH) 8-2.6 White Pale or dark green, yellow, grey. Opaque. Dull or silky. Smooth, rounded masses. No cleavage. Mottled green color. Smooth, curved surfaces. Fluorite CaF 2 4 3-3.3 White Colorless, purple, blue, grey, green, or yellow. Glassy. Opaque to transparent. Usually cubes or octahedrons. 4 excellent cleavage directions. Brittle. Cubic or octahedral form. 4 directions of cleavage. Calcite CaCO 3 3 2.7 White Usually colorless, white, or yellow, can be green, brown, or pink. Glassy. Opaque to transparent. Rhombohedrons. 3 excellent cleavage planes. Angles: < 90 and > 90. Bubbles in HCL. Double refraction (2 images visible through clear sample). Rhombs, 3 cleavage planes (not 90 ), H=3. Mica family: Biotite 2.5 K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2-3 2.7-3.1 Greybrow n Black, green-black, brown-black. Transparent to opaque. Short tablets. Like a tablet of paper. 1 excellent cleavage splits easily into thin, flexible sheets. 1 flexible cleavage plane (sheet), dark colored; brown streak. Mica family: Muscovite 2- KAl 3 Si 3 O 10 (OH) 2 2.5 2.7-3 White Colorless, yellow, brown, or red-brown. Transparent to opaque. Short tablets. Like a tablet of paper. 1 excellent cleavage splits easily into thin, flexible sheets. 1 flexible cleavage plane (sheet), light colored; white streak. Halite NaCl 2.5 2.1-2.6 White Colorless, white, yellow, blue, brown, or red. Glassy. Cubes. Brittle. 3 excellent cleavage planes: cubes. Salty taste. H=2.5. Cubic form and cleavage. Gypsum CaSO 4 *2(H 2 0) 2 2.3 White Colorless, white, or grey. Translucent to transparent. Tabular, prisms, blades, or needles. 1 good cleavage plane. H=2. 1 cleavage plane. Translucent. Talc 1 2.7 Mg 3 Si 4 O 10 (OH) 2-2.8 White White, grey, pale green, or brown. Opaque. Greasy or silky luster. Shapeless masses (if no cleavage visible) or tabular. 1 poor cleavage plane (may not be visible). Feels greasy or soapy. H=1. Opaque.
Sulphur Mining A sulfur miner stands inside the crater of the Kawah Ijen volcano at night, holding a torch, looking towards a flow of liquid sulfur which has caught fire and burns with an eerie blue flame. ( Olivier Grunewald)