Tulane university crystallography notes

  • How are crystallographic axes determined in each of the 6 crystal classes?

    Each of the six crystal systems have a set of unique crystallographic axes that are defined by their unit length and the angles at which the axes intersect.
    Now instead of using symmetry operations to define the position of crystal faces in space, we want to be able to describe the positions of atoms in space..

  • What is 111 in the triclinic system?

    In the monoclinic, triclinic, and orthorhombic crystal systems, the form {111} is a general form because in these systems faces of this form will intersect the a, b, and c axes at different lengths because the unit lengths are different on each axis..

  • What is the crystal form notation?

    To designate a crystal form (which could imply many faces) we use the Miller Index, or Miller-Bravais Index notation enclosing the indices in curly braces, i.e.
    Such notation is called a form symbol.
    An important point to note is that a form refers to a face or set of faces that have the same arrangement of atoms..

  • Why is the angle at which crystal faces meet important?

    The symmetry of the lattice will determine the angular relationships between crystal faces.
    Thus, in imperfect crystals or distorted crystals where the lengths of the edges or faces of symmetry related faces are not equal, the symmetry can still be determined by the angles between the faces..

  • In crystallography, the three edges of the lattice are labelled a, b, and c (or x, y, and z), and they define both the edges of crystals in the seven crystal systems and their crystallographic axes. 2 In remote sensing, a boundary between area of different tones.
  • In this class there is no symmetry, so all crystal faces are unique and are not related to each other by symmetry.
    Such faces are called Pedions, thus this is the Pedial Class.
    Only a few rare minerals are in this class.

How can a unit cell be oriented on a crystallographic axes?

Once we have chosen a unit cell for the crystal, then it can be oriented on the crystallographic axes to define the angles between the axes and to define the axial lengths

This will allow us to define directions within the crystal that become important when we realize that many properties of crystals depend on direction in the crystal


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