Mineralogy

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Mineralogy

• Carleton College
• Winter 2003

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Why study minerals?
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Why study minerals?

• All field so earth science use mineral knowledge
  one way or the another way. For example
• Igneous or metamorphic petrologists
• Sedimentologists
• Geophysicists
• Material science

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Mineralogy requires an understanding of

• Chemistry - elements, electronic structure,
  bonding, chemical analyses forms of crystals

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Mineralogy requires an understanding of

• Physics - physical properties (density,
  electrical properties, cleavage, hardness,
  magnetic properties, etc.)

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Mineralogy requires an understanding of

• Crystallography - the structure of minerals -
  internal external geometry and trigonometry
  symmetry

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Mineralogy requires an understanding of

• Mineral genesis - environments of formation
  (e.g., ocean floor vs. subduction zone)

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Mineralogy requires an understanding of

• Determinitive mineralogy - physical and chemical
  testing analytical equipment
• Classification schemes
• Interrelationships between humans and their
  natural environment

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Definition of a crystal
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Definition of a crystal

• Usually formed by inorganic processes (i.e., coal
  is not a mineral)
• Definite chemical composition (some atomic
  substitution allowed)
• "ordered atomic arrangement" - eliminates
  liquids and gases also eliminates amorphous
  solids such as glass and obsidian

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Composition of Earth

• Earth's crust
• composition - 95 igneous rock (or it's
  metamorphic equivalent) - e.g., basalt, granite,
  granitic gneiss, schist...
• 8 most abundant elements in crust (99 weight
  and approx. volume) O, Si, Al, Fe, Ca, Na, K, Mg

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Composition of Earth

• Most common crustal minerals are
• Silicates - e.g., quartz SiO2
• aluminosilicates - e.g., K-feldspar (orthoclase)
  KAlSi3O8
• oxides - e.g., hematite Fe2O3

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Composition of Earth

• Total earth
• composition - 99 Fe, O, Si, Mg, Ni, S, Ca, Al
• - difference due to core and mantle
• Fe, Mg-silicates mantle
• S, Ni, Fe core

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Definition of a crystal

• Crystal A crystal is an anisotropic, homogenous
  body consisting of a 3-D periodic ordering of
  atoms, ions or molecules.

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Few properties that are characteristic of
crystals.

• Regular geometric shapes
• Cleavage
• Pleochroism
• Hardness
• Optical properties
• Electrical Conductivity
• Anisotropy/isotropy

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Few properties that are characteristic of
crystals.

• Regular geometric shapes

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Few properties that are characteristic of
crystals.

• Cleavage
• When crystals of NaCl, Calcite, and Biotite are
  split, the resulting fragments have similar
  shapes with smooth faces. This phenomenon is
  called cleavage.

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Few properties that are characteristic of
crystals. Hornblende and Pyroxene
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Few properties that are characteristic of
crystals.

• Pleochroism
• Color that appears to an observer depends on the
  optical absorption of the crystal in that
  particular direction. For example if the crystal
  absorbs all spectral colors of the white light
  except blue then the crystal appears blue.

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Few properties that are characteristic of
crystals.

• Pleochroism
• If you were to examine a mineral of biotite under
  the microscope in its two different sections, as
  seen in the following figure you will see two
  different colors, as you rotate the stage of the
  microscope.

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Few properties that are characteristic of
crystals.

• Pleochroism
• When as in the above case the absorption differs
  in two or three directions the crystal is said to
  exhibit pleochroism.

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Few properties that are characteristic of
crystals.
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Few properties that are characteristic of
crystals.
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Few properties that are characteristic of
crystals.

• Hardness
• When you took introduction to geology you learned
  the hardness of minerals, most minerals can be
  identified by their hardness. It is controlled
  by the inter atomic forces or what kind of
  bonding they have. As you may recall there is
  hardness scale of 1-10.

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Few properties that are characteristic of
crystals.

• Hardness
• If you look at a crystal of kyanite Al2SiO5, This
  mineral if scratched parallel to its length by a
  steel needle, a deep indentation will be made in
  it, while a scratch perpendicular to the crystal
  length will leave no mark. Thus the hardness of
  the crystal in different in two different
  directions. (4.5 to 6.5)

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Few properties that are characteristic of
crystals.

• Optical properties
• In this slide, we have a dot, and a clip for a
  scale.

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Few properties that are characteristic of
crystals.

• Optical properties
• Take a crystal of calcite and glass and see what
  happens.

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Few properties that are characteristic of
crystals.

• Optical properties
• With the glass and we see only one dot.

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Few properties that are characteristic of
crystals.

• Optical properties
• With calcite we see two dots. What is happening?

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Few properties that are characteristic of
crystals.
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Few properties that are characteristic of
crystals.

• Electrical conductivity
• If one face of a gypsum in covered with a thin
  layer of wax and heated with a metal tip, the
  melting front in the wax layer will be
  ellipsoidal rather than circular showing that the
  thermal conductivity is greater in direction II
  than in direction I.

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Few properties that are characteristic of
crystals.

• Anisotropy/isotropy
• Such behaviors-different values of physical
  properties in different direction-is called
  anisotropy.
• As in the glass where the melting front is
  circular, which imply that the thermal
  conductivity is the same in all directions, such
  behavior-the same value of physical property in
  all directions-is called isotropy.

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Few properties that are characteristic of
crystals.

• Anisotropy/isotropy
• So all of the above properties that we discussed
  tell us something about the crystal. The origin
  of all of the phenomena listed 1-6 lies in the
  internal structure of the crystal. So in order
  to understand this better let us consider the
  various aspects of a crystal.