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Crystal Morphology: External Form

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.

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Crystal Morphology: External Form

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  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. • 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.

  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 crystal’s 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

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