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Crystal Morphology: External Form Klein, pages Geol 3055 Prof. Merle. - ppt download

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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 -8 cm)
Crystal Morphology: External Form Klein, pages Geol 3055 Prof. Merle
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..
Crystal form seems smooth to the eye because unit cell dimensions are in the Angstrom level (10 -8 cm).
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
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 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.
Continuous Vectorial Properties: Importance of crystal orientation Hardness: kyanite, diamond Thermal expansion: Ruby Electrical & heat conductivity: Ge & Si Speed of light: transparent crystals
–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 Rate of solution –Crystal faces are etch or pitted when attacked by a chemical solution.