engleski

Symmetry in the world of minerals

Symmetry is very common in nature, even minerals are not an exception. Moreover, from the very definition of mineral, which states that a mineral is a naturally occurring homogeneous solid, formed by geological process, with a definite (but generally not fixed) chemical composition and a highly ordered atomic arrangement, it is clearly visible that symmetry is a very important characteristic of each mineral. The question is: Why does symmetry occur in minerals? The answer to that question is not a very simple one but it can be said that symmetry is internal, that it is a result of an ordered geometrical arrangement of atoms in the crystal structure. If a mineral grows freely it becomes a crystal whose faces are an external reflection of its internal symmetry. Internal symmetry is the result of the atoms' tendency to be in the state of the lowest possible energy. The simple symmetry operations in minerals are: rotation, reflection, and inversion, and each of these operations is performed by an element of symmetry: axis of rotation, mirror plane and center of symmetry, respectively. There are also a compound symmetry operations composed of two operations performed as a single event. Symmetry elements may be present on crystal polyhedral in limited number of different combinations. There are only 32 possible combinations of symmetry elements on crystals. These 32 combinations define the 32 crystal classes which can be grouped in six (seven) crystal systems. Due to the fact that additional symmetry elements (screw axes, glide planes) are present in crystal structures atoms in crystal structures can be arranged in conformity with 230 possible combinations of symmetry elements which are called space groups.

symmetry; minerals

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