Transcript Crystal Morphology

Crystal Morphology: External
Form
Klein, pages 189-192
Geol 3055
Prof. Merle
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.
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)
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)
Lattice points
Plane Lattice
• Crystal faces have a
direct relationship
between them.
• Steno’s 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.
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 crystal’s physical properties
Na
Cl-
Vectorial Properties, cont.
• Vectorial properties of crystals are:
– Continous –varies continuously with vector direction
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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
Continuous Vectorial Properties:
Importance of crystal orientation
• Hardness: kyanite, diamond
• Electrical & heat conductivity: Ge & Si
• Speed of light: transparent crystals
Thermal expansion: Ruby
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