Crytal Morphology Department of Geology晶体形态地质系.pptVIP

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 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 Vectorial Properties, cont. Vectorial properties of crystals are: Continous –varies continuously with vector direction Hardness Conductivity for heat and electricity Thermal expansion S