Crystal symmetry (G)

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Crystal symmetry (G)
2012-10-03 22:51:37

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  1. What are the Miller indices of all the faces of a simple cube? Use a well-labelled
    drawing in your answer.
  2. • What is Steno’s Law? Use a
    drawing in your answer.
    • Steno’s Law – “Law of constancy of interfacial angles” states that the angles between corresponding faces are constant. E,g. Quartz has interfacial
    • angles of 120 degrees.
  3. How many plane lattices are there in two dimensions
    and how are they defined (axes and angles)?
    In 2D dimensions there are two dimensions a and b and an angle γ. There are 5 plane lattices in 2D, which are square, rectangle, diamond, hexagonal and oblique
  4. • What is the difference between a unit mesh and a unit cell?
    A unit mesh: shape that can be repeated by translation to fill space (square, hexagonal etc..)

    A unit cell: volume outlines by lattice modes; simplest shape that can be translated in 3D to fill space
  5. • What is the relationship between plane lattices and Bravais lattices?
    The 5 plane lattices can be repeated in 3Dto produce 14 different space lattices known as the Bravais Lattices, these 14 bravais lattice are divided in to 6 groups based on the shape of the unit cell
  6. What are the differences between point groups and space groups?
    • Point groups represent the number of different combinations of symmetry around a point in three
    • dimensions (reflection, rotation, inversion. They may be grouped into the 6 different crystal sytems based on unit cell geometry generated from the bravais
    • lattices

    • Space groups represent
    • all the combinations of point symmetry with translational symmetry. This requires two additional compound symmetry operations – glides and
  7. • What is a glide plane? Use a diagram in your answer.
    Translation followed by areflection in the mirror plane
  8. • What are quasi crystals and how is their internal structure different from regular crystals?
    Quasi crystals present 3D symmetry that is strictly forbidden for crystals (e.g. 5-fold and 10-fold rotational axes). Internal structure is ordered but not periodic as it lacks symmetry.

    - only known naturally occuring camefrom a meteorite - Icosahedrite
  9. Why are planes with low lattice-node density unlikely to be major faces in a crystal?
    Crystal faces parallelto  Q are less likely to develop or will be minor.
  10. • What is the meaning of using the following different notations when describing crystals: (001), [001], {001}
    A cleavage (shown with thin lines)parallel to the (001) crystal face is identified as {001} cleavage. Use braces{ } to indicate cleavage or twin planes.
  11. • What is the Miller index when a crystal face is parallel to a crystal axis?
    • The crystal faces (110), (1-10), (-110), and (-1-10)
    • constitute a [001] zone because they
    • intersect in edges parallel to
    • [001 which is the zone axis

  12. What is the Law of COnstancy of interfacial angles?
    The angles between corresponding faces are constant e.g. quartz has angles of 120 degrees between each face
  13. What is the study of crystallography?
    Thestudy of crystalline solids and the principals that govern their growth, external shape and internal structure.
  14. Primitive vs centered unit mesh?
    • Primitive = lattice nodes only at the corners.
    • Centered unit mesh = lattice nodes also occur in the centre
  15. P =

    how many known crystals are there
    • P= primitive
    • C= face-centered
    • I = body-centered
    • F= Fully face-centered

    = >4500
  16. What is an entantiomorphic crystal?
    • Frorms lack of centre symmetry and mirrors so it is possibl to have left handed and right handed versions of the crystal
    • - also means they have a screw axis 
    • -Common in QUARTZZZZ
  17. WHat is the law of Huay vs Law of Bravais?
    Huay : Crystal faces make simple rational intercepts on crystal

    Bravais: common crystal faces are parallel to the lattice planes that have high-lattice node density
  18. PLANE "T" VS "Q"
    • T = high lattice-node density = likely to be a crystal face
    • Q= low Lattice-node density , likely not a common crystal face

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