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© 2003-Present, John C. Bean |
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But crystals are known as "crystals" because they exhibit symmetrical facets. This model has such facets. It is a "cubic symmetry" crystal. Cubic crystals have three perpendicular axes. Facets are realigned by rotations of 90, 180, or 270 degrees around these axes.
Here the facets are labeled by "Miller Indices." For cubic crystals, these indices are the x, y, and z components of a short vector perpendicular to the facet. A vector with components (1, 1, 0) is perpendicular to the front facet of this crystal, making it a [110] facet. For the inclined facet above it, the perpendicular vector is (1, 1, 1), making it a [111] facet. Other facets are similarly labeled.
But for crystals with cubic symmetry, the atomic structures of certain facets are identical. They are thus called members of a " family." For instance, here the (100), (010), (001), (-100), (0-10), (00-1) facets make up the {100} family.
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