Rocks and Minerals Part 2 - September 18

Rocks and Minerals – Part 2

Goals:  Students should 1) be able to define the properties of a mineral; 2) describe how different minerals are identified; and 3) list common uses for minerals.


  1.  Rocks and Minerals are similar because they are both inorganic (non-living) compounds; can be classified by their chemical composition; are naturally formed solids; and found around the world in many of the same places on Earth.
  2. Rocks are most commonly classified by how they form – sedimentary, metamorphic or igneous.  They are composed of more than one mineral.  They have no definite crystal structure or chemical composition.
  3. Minerals may have a definite chemical composition and a definite crystal structure (unique arrangement of atoms/molecules).
  4. We identify minerals by the following tests:
    • Color
    • Streak (color of the streak across a streak plate)
    • Luster (metallic or non-metallic)
    • Hardness (Mohs Scale)
    • Density (specific gravity)
    • Breakage pattern (cleavage and fracture)
  5.  Color – color should never be used as the only test to identify a mineral
  6. Streak – the color a mineral displays in a finely powdered form; might be completely different from the color of the actual mineral.
  7. Luster is the way a mineral’s surface reflects light.  There are two types of luster:  metallic which is a shiny reflection and nonmetallic – examples are listed below:
    • Glassy – quartz
    • Pearly – talc
    • Greasy – graphite
    • Silky – gypsum
    • Resinous – sulfur
    • Adamantine – diamond
  8.  One of the most reliable ways to identify minerals is by Mohs Hardness Scale.  This scale compares the resistance of a mineral to being scratched by 10 reference minerals.  This scale has been in used since 1812.
  9. Density is defined as the amount of matter per unit volume (Density = Mass/Volume).
  10. Cleavage refers to the way some minerals break along certain lines of weakness in structure.  Mica is a good example because it flakes in sheets very easily.
  11. Fracture describes the way a mineral tends to break.  There are different types of fracturing including a smooth curve (conchoidal), sharp jagged edges (hackly), uneven, and fibrous.
  12. Gemstones are minerals that have been cut to specific guidelines.  Do you know your birthday’s gemstone?

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