Crystal Morphology - Department of Geology晶体形态-地质系

• Hardness: kyanite, diamond
Thermal expansion: Ruby
• Electrical & heat conductivity: Ge & Si • Speed of light: transparent crystals
Discontinuous Vectorial Properties: Importance of crystal orientation
– Low density nodes grow so fast that the faces developed there disappear
– ቤተ መጻሕፍቲ ባይዱig. 5.24
• Rate of solution
– Crystal faces are etch or pitted when attacked by a chemical solution
• 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
About crystal morphology…
• Crystal morphology – form of a crystal
• Aspects related to crystal morphology are: angular relationships, size, and shape of faces.
– Discontinous- only to certain definite planes or directions
• Growth rate • Solution rate • X-ray diffraction
Continuous Vectorial Properties: Importance of crystal orientation
• 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.
Na
Cl-
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
• 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.
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.
• 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
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 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)
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