| Popis: |
Minerals very often appear in the form of crystals, i.e. homogeneous regular geometric bodies with a certain degree of symmetry, which is a reflection of the internal structure of minerals. However, the morphology of minerals depends also on the crystallization conditions and crystal growth. Therefore distorted crystals which seem to have lost their symmetry are common. This apparent asymmetry is not real because the angles between equivalent faces of crystals of the same substance are constant. Although at first sight the crystal is not symmetrical, precise measurement of angles between faces and their strereographic or gnomonic projection will show all elements of symmetry that crystal has as a result of mineral structure. In reality minerals are not ideal. A perfect crystal, with every atom of the same type in the correct position, does not exist. All crystals have some defects. Imperfections or defects in their internal structure are common. Such imperfections affect basic properties of crystalline materials. Imperfections in crystal structures are generally classified according to their geometry as point defects, line defects, or plane defects. Point defects are places where an atom is missing or irregularly placed in the crystal structure. Point defects include vacancies, interstitial atoms, substitution impurity atoms, and interstitial impurity atoms. Line defects are commonly called dislocations, and represent groups of atoms in irregular positions concentrated along linear features in a crystal structure. Plane defects are interfaces between homogeneous regions of the material. Plane defects include grain boundaries, stacking faults and external surfaces. |
