Minerals of Earth's Crust

Chapter 5, Section 1 - What Is a Mineral?                                                                                                                                  

Mineral - A natural, usually inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties.
  • To determine if a substance is a mineral, ask yourself these four questions...
    • Is it inorganic?  (Not made of living things or their remains.)
    • Does it occur naturally?  (Not man-made.)
    • Is it a crystalline solid?  (Its atoms are arranged in repeating patterns.)
    • Does it have a consistent chemical composition?  (A pure substance, not a mixture.)
Types of Minerals
  • Silicate Minerals
    • Contain a combination of silicon and oxygen.
    • May also contain one or more metals.
    • Make up 96% of Earth's crust.  (50% of the crust is quartz and feldspars).
    • Examples
      • Quartz (Si & O)
      • Feldspar (Si & O + a metal)
        • Orthoclase Feldspar (K)
        • Plagioclase Feldspar (Na, Ca or both)
      • Ferromagnesian Minerals (Si & O + Fe & Mg)
        • Olivines
        • Pyroxenes
        • Amphiboles
        • Biotite
  • Nonsilicate Minerals
    • Does not contain compounds of silicon and oxygen.
    • Six groups...
      • Carbonates (Contain a carbonate group: CO3)
      • Halides (Contain Cl or F combined with Na, K or Ca)
      • Native Elements (pure elements)
      • Oxides (Contain O and an element other than Si)
      • Sulfates (Contain a sulfate groups: SO4)
      • Sulfides (One or more elements combined with S)
Crystal - A solid whose atoms/molecules are arranged in a regular, repeating pattern.

Silicate Mineral Crystalline Structures
  • All silicate minerals are made up of silicon-oxygen tetrahedra.
    • Si-O tetrahedron = 1 silicon atom surrounded by 4 oxygen atoms
  • Tetrahedra can bond to each other OR to other atoms.
    • How they bond form the various silicate minerals.
  1. Isolated Tetrahedra Silicates
    • Do not link with other Si or O atoms.
    • ex. - Olivine (tetrahedra bond to Mg and Fe atoms)
  2. Ring Silicates
    • Tetrahedra share O atoms to form 3-, 4-, or 6-sided rings.
    • If rings align, they can form tubes that contain atoms, molecules, and/or ions.
    • ex. - Beryl, Tourmaline
  3. Single-Chain Silicates
    • Each tetrahedron shares 2 corner O atoms to form a chain.
    • ex. - Pyroxenes
  4. Double-Chain Silicates
    • Two single chains share O atoms to link up.
    • ex. - Amphiboles
  5. Sheet Silicates
    • Each tetrahedron shares 3 O atoms to form a sheet.  The 4th O atom bonds with Al or Mg atoms.  (This holds the sheets together.)
    • ex. - Mica minerals (biotite, muscovite)
  6. Framework Silicates
    • Each tetrahedron shares all 4 O atoms to form a 3-D network.
    • ex. - Quartz (Contain only Si-O tetrahedra.)
    • ex. - Feldspars  (Substitute Al atoms for some of the Si.)
Nonsilicate Mineral Crystal Structures
  • Nonsilicates show a wide variety of crystal structures, including cubes, hexagonal prisms, and irregular masses.
  • The crystal structure depends on the composition.
    • Some may form tetrahedrons around atoms/ions other than Si.
      • Smaller groups can be classified by the central atom/ion.
  • Native Elements
    • Very dense; atoms are as close together as possible.  (Closest Packing)
      • Each metal atoms is surrounded by 12 others.


Chapter 5, Section 2 - Identifying Minerals                                                                                                                                  

Mineralogist - A person who  examines, analyzes and classifies minerals.

Mineral Properties
    Mineral properties are a result of chemical composition (what the mineral is made of) and crystal structure.
  • Color
    • Some minerals have distinct colors.
    • Problems:
      • Many minerals are similar in color.
      • Small amounts of certain elements can change a mineral's color (corundum = ruby/sapphire)
      • Weathering can change the surface color.
        • Always check color on a newly broken surface.
  • Streak
    • The color of a mineral in powdered form.  (The color left when the mineral is scratched on a streak plate.)
      • Metals tend to have dark streaks.
      • Nonmetal streaks tend to be colorless OR very light versions of the mineral's color.
      • Minerals harder than the streak plate will not leave a mark.
  • Luster
    • The way that light reflects off a mineral.
      • Metallic - Looks like polished metal.
      • Nonmetallic - can be...
        • Glassy 
        • Waxy 
        • Pearly
        • Brilliant
        • Dull or Earthy
  • Cleavage and Fracture
    • The way that the mineral breaks.
    • Cleavage - When a mineral breaks along specific planes to form flat, smooth surfaces.
    • Fracture - When a mineral breaks along curved or irregular surfaces.
      • Uneven / Irregular fracture gives a rough appearance.
      • Splintery / Fibrous fracture gives an appearance like a piece of broken wood.
      • Conchoidal fracture gives curved surfaces.
  • Hardness
    • The ability of a mineral to resist scratching.  (Not about breaking!)
    • Measured on a scale of 1 to 10 (Moh's Hardness Scale).
      • To use it, find the hardest mineral on the scale that your unknown mineral can scratch.
    • Hardness is determined by bonds between the mineral's atoms.
      • Diamond (10) - atoms in strong crystalline structure.
      • Graphite (1-2) - atoms in sheets.
  • Crystal Shape
    • Minerals form in six basic shapes.
    • Caused by patterns in how the atoms combine.
      • Can be affected by environmental conditions (i.e. - heat and pressure).
  • Density
    • The ratio of a substance's mass to its volume.  (g/cm3 for solids and liquids, g/L for gases)
    • Most common minerals are between 2 and 3 g/cm3.
    • Minerals containing heavy elements (i.e. - Pb, U, Au, Ag, etc.) are between 7 and 20 g/cm3.
  • Special Properties
    • Properties that are specific to certain minerals.
    • Fluorescence - The mineral glows under a UV light.
    • Phosphorescence - The mineral continues glowing after the UV light has been turned off.
    • Chatoyancy - "Cat's-Eye Effect" - The mineral has a silky appearance when reflecting light.
    • Asterism - The mineral shows a six-sided star shape when reflecting light.
    • Double Refraction - When a transparent mineral shows a double image as light is refracted through it.
    • Magnetism - Usually nonsilicate minerals that contain iron.
    • Radioactivity - When unstable atomic nuclei decay into stable ones by giving off particles and/or energy.

Resources

Chapter Assignment:
  • Read Chapter 5.
  • Pg. 128-129 #7-22, 24-25, 36-38.