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Minerals and Rocks

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Minerals and Rocks

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  1. Minerals and Rocks

  2. Lecture Outline • What are minerals? • Common rock-forming minerals • Physical properties of minerals • Basic rock types • The rock cycle

  3. Minerals A mineral is a naturally occurring, solid crystalline substance, generally inorganic, with a specific chemical composition • Natural • Solid • Atoms arranged in orderly repeating 3D array: crystalline • Not part of the tissue of an organism • Composition fixed or varies within defined limits Minerals are the “building blocks” of rock

  4. Large individual crystals (rare) Mass of small grains: each is a crystal, but grown up against each other

  5. Atomic Structure of Minerals • NaCl - sodium chloride Halite

  6. Chemical Bonds: Ionic • Electrical attraction between ions of opposite charge • Bond strength increases with the electrical charges of the ions • Bond strength decreases as the distance between the ions increases • Most minerals are this kind of compound

  7. Ionic Bonding example: halite Cation Anion Na+ Cl-

  8. Covalent Bonds: • Electron sharing • Generally stronger than ionic bonds (e.g., diamond)

  9. Crystallization of Minerals • Need starting material with atoms that can come together in the proper proportions • Growth from a liquid or a gas • Time and space for crystallization • Appropriate temperature and pressure • Examples • Magma that has cooled below its melting point • Supersaturated solution --> precipitation

  10. Crystallization of Minerals • Crystals begin as an initial “seed” - a microscopic crystal • Atoms keep being added in a 3D array, repeating the basic arrangement • Crystal faces are based on the array structure

  11. Cations and Anions • Anions are typically large • Cations are relatively small • Crystal structure is determined largely by the arrangement of the anions

  12. Common cations and anions Radii given in angstroms; 10-8 cm

  13. Ions can be compound • So far, we’ve talked about individual atomic ions • Many common minerals are silicates SiO44- Complex ions act as a single ion in forming crystal structure

  14. Cation Substitution • Crystal structure determined by those large anions • Various cations can substitute for each other in many minerals • Same crystal structure • Different chemical composition

  15. Polymorphs Minerals with the same composition, but different crystal structure.

  16. Common Rock-Forming Minerals Minerals fall into a small number of related “families” based mainly on the anion in them

  17. Silicates • Quartz (SiO2), K-feldspar (KAlSi3O8), olivine ((Mg, Fe)2SiO4), kaolinite (Al2Si2O5(OH)4) • Most abundant minerals in the Earth's crust • Silicate ion (tetrahedron), SiO44-

  18. Quartz (SiO2)

  19. Silicate structure • Most of the most common rocks in the crust are silicates • Silicate tetrahedra can combine in several ways to form many common minerals • Typical cations: K+, Ca+, Na+, Mg2+, Al3+, Fe2+

  20. Different numbers of oxygen ions are shared among tetrahedra

  21. Carbonates • Cations with carbonate ion (CO32-) • Calcite (CaCO3), dolomite (CaMg(CO3)2), siderite (FeCO3), smithsonite (ZnCO3) • Make up many common rocks including limestone andmarble • Very important for CCS!

  22. Calcite (CaCO3)

  23. CaCO3 + 2H+ = Ca2+ + CO2 + H2O

  24. Smithsonite (ZnCO3)

  25. Oxides • Compounds of metallic cations and oxygen • Important for many metal ores needed to make things (e.g., iron, chromium, titanium) • Ores are economically useful (i.e., possible to mine) mineral deposits

  26. Hematite (Fe2O3)

  27. Sulfides • Metallic cations with sulfide (S2-) ion • Important for ores of copper, zinc, nickel, lead, iron • Pyrite (FeS2), galena (PbS)

  28. Galena (PbS)

  29. Sulfates • Minerals with sulfate ion (SO42-) • Gypsum (CaSO4.H2O), anhydrite (CaSO4)

  30. Gypsum

  31. Gypsum • Cave of the Crystals • 1,000 feet depth in the silver and lead Naica Mine • 150 degrees, with 100 % humidity • 4-ft diameter columns 50 ft length

  32. Identification of Minerals • Chemical composition (microprobes and wet chemical methods) • Crystal structure (X-ray diffraction) • Physical properties

  33. Physical properties • Hardness

  34. Physical properties • Hardness • Cleavage: tendency of minerals to break along flat planar surfaces into geometries that are determined by their crystal structure

  35. Cleavage in mica

  36. Cleavage in calcite

  37. Halite (NaCl)

  38. Physical properties • Hardness • Cleavage • Fracture: tendency to break along other surfaces (not cleavage planes)

  39. Conchoidal fractures

  40. Physical properties • Hardness • Cleavage • Fracture • Luster (metallic, vitreous, resinous, earthy, etc.) • Color (often a poor indicator; streak color is better) • Specific gravity • Crystal habit (shape)

  41. Rocks An aggregate of one or more minerals; or a body of undifferentiated mineral matter (e.g., obsidian); or of solid organic matter (e.g., coal) • More than one crystal • Volcanic glass • Solidified organic matter • Appearance controlled by composition and size and arrangement of aggregate grains (texture)

  42. Rock Types • Igneous • Form by solidification of molten rock (magma) • Sedimentary • Form by lithification of sediment (sand, silt, clay, shells) • Metamorphic • Form by transformations of preexisting rocks (in the solid state)

  43. Igneous Rocks Intrusive Extrusive

  44. Intrusive (plutonic) • Form within the Earth • Slow cooling • Interlocking large crystals • Example = granite

  45. Extrusive (volcanic) • Form on the surface of the Earth as a result of volcanic eruption • Rapid cooling • Glassy and/or fine-grained texture • Example = basalt

  46. Basalt: igneous extrusive