Physical properties of minerals

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Physical properties of minerals

• Crystal Form
• External expression of the orderly internal
  arrangement of atoms
• Crystal growth is often interrupted because of
  competition for space and rapid loss of heat
• Note this isnt cleavage!

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Halite (NaCl) (Atom arrangement produces a cubic
crystal structure)
This mineral would have six crystal faces.
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The mineral garnet often exhibits good crystal
form
This is a dodecahedral crystal form. It has a
total of 12 crystal faces.
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Structure of minerals

• Polymorphs
• Two or more minerals with the same chemical
  composition but different crystalline structures
• Diamond and graphite are good examples of
  polymorphs

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Diamond and graphite polymorphs of carbon
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Physical properties of minerals

• Color
• Generally an unreliable diagnostic property to
  use for mineral identification
• Often highly variable for a given mineral due to
  slight changes in mineral chemistry
• But we use it anyway

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Quartz (SiO2) exhibits a variety of colors
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Physical properties of minerals
Streak

• Streak
• Color of a mineral in its powdered form
• Helpful in distinguishing different forms of the
  same mineral
• In this case hematite shows two different colors
  but one streak (red)

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Physical properties of minerals

• Luster
• Appearance of a mineral in reflected light
• Two basic categories
• Metallic
• Nonmetallic
• Other terms are used to further describe luster
  such as vitreous (like glass), silky, or earthy
  (like dirt)

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Metallic Luster
Non- metallic luster
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Galena is a lead sulfide that displays metallic
luster
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Hematite exhibits non-metallic luster
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Talc shows a pearly luster
Limonite shows an earthy luster
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Physical properties of minerals

• Cleavage
• Not how the crystal forms but rather how it
  breaks
• Tendency to break along planes of weak bonding
• Produces flat, shiny surfaces
• Described by resulting geometric shapes
• Number of planes
• Angles between adjacent planes

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Three examples of perfect cleavage fluorite,
halite, and calcite
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Micas have perfect cleavage in one direction
muscovite
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Physical properties of minerals

• Fracture
• Absence of cleavage when a mineral is broken

Conchoidal fracture
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Hardness

• Hardness
• Resistance of a mineral to abrasion or scratching
• All minerals are compared to a standard scale
  called the Mohs scale of hardness

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• Specific Gravity
• Ratio of the weight of a mineral to the weight of
  an equal volume of water
• Average value is approximately 2.7
• Simply hefting a mineral works too. Does it feel
  heavier than average.

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Physical properties of minerals

• Other properties
• Magnetism
• Reaction to hydrochloric acid
• Malleability
• Double refraction
• Taste
• Smell
• Flouresence

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Assignment

• Observe some tests being done on mineral samples.
• Finish handout questions
• Sign out a mineral set (be sure it is complete,
  check it before and after and inform me of any
  missing minerals)
• Fill out the attached chart and use your ID
  tables to help you try to identify each mineral.