Crystal Morphology: External Form

1
Crystal Morphology External Form

• Klein, pages 189-192
• Geol 3055
• Prof. Merle

2
Review of crystal growth

• Crystals are formed by the repetition of a 3D
  unit structure.
• Faces of the crystal depend in part, on the shape
  of the unit and the conditions of which the
  crystal grows.
• The conditions are temperature, pressure, nature
  of solution, and availability of open space for
  free growth.

3
About crystal morphology

• Crystal morphology form of a crystal
• Aspects related to crystal morphology are
  angular relationships, size, and shape of faces.
• Crystal form seems smooth to the eye because unit
  cell dimensions are in the Angstrom level (10-8cm)

4
Morphology crystal faces

• The type of crystal faces in a crystal is
  controlled by the internal lattice
• Lattice imaginary pattern of points in which
  every point has an environment that is identical
  to that of any other point in the lattice
  (regular arrangement of atoms of a crystal). It
  has no specific origin and can be shifted
  parallel to itself.
• Faces are most likely to form on crystal parallel
  to lattice planes that have a high density of
  lattice points (nodes).
• Frequency of a given face is roughly proportional
  to the number of nodes it intersects the lattice
  more nodes, more common is the face (Law of
  Bravais)

5
Lattice points
Plane Lattice
6

• Crystal faces have a direct relationship between
  them.
• Stenos law of the constancy of interfacial
  angles
• The angles between equivalent faces of crystals
  of the same substance, measured at the same
  temperature, are constant this is why the
  morphology of a crystal is a valuable tool to
  identify a mineral.

7
Vectorial Properties

• Different atomic arrangements along planes and
  directions within a structure gives differences
  in atomic environments
• This can change the magnitude of some of the
  crystals physical properties

Na
Cl-
8
Vectorial Properties, cont.

• Vectorial properties of crystals are
• Continous varies continuously with vector
  direction
• Hardness
• Conductivity for heat and electricity
• Thermal expansion
• Speed of light
• Discontinous- only to certain definite planes or
  directions
• Growth rate
• Solution rate
• X-ray diffraction

9
Continuous Vectorial Properties Importance of
crystal orientation

• Hardness kyanite, diamond Thermal
  expansion Ruby
• Electrical heat conductivity Ge Si
• Speed of light transparent crystals

10
Discontinuous Vectorial Properties Importance
of crystal orientation

• Rate of growth-
• depends on density of plane lattices low
  density, fewer nodes for growth, rapid growth.
• Energy in high density planes is less than lower
  density planes, more stable planes
• Low density nodes grow so fast that the faces
  developed there disappear
• Fig. 5.24
• Rate of solution
• Crystal faces are etch or pitted when attacked by
  a chemical solution