Education and shape of crystals
Education means the set of all planes occurring on one crystal, and their habit means their development, which is revealed in size.
The crystals of some substances have many faces and come in various combinations. An example of a mineral, whose crystals are distinguished by the richness of the walls and the variety of combinations of forms is calcite. Its individual crystals may have over 100 walls. Depending on the conditions of formation, crystals of the same substances can be of different forms, e.g.. the mineral that forms the octahedrons can be in the form of cubes. The characteristic forms of South African diamonds are octahedrons, and Brazilian diamonds - diamond cubes and dodecahedrons. Sometimes it is possible to infer the conditions of their formation on the basis of the crystal forms, about admixtures of foreign bodies in them, etc..
Completely correct crystal forms, in which the analogous walls are of the same size, they arise only under special conditions. If the supply of the substance, from which the crystal is formed, it was uneven during its growth, some faces of the crystal may have become larger than others, analogous walls, which may make it difficult to recognize the true crystal form at first. However, according to the law of constancy of angles, the angles between the analogous crystal faces are constant, because the faces of the crystal grow parallel.
Different shapes of crystals: a - lamellar, b - plate, c - evenly educated, d - bar, e - needle.
The shape of the crystals depends on the ratio of their lengths, width and height. By observing, for example,. a crystal which is a combination of a tetragonal column and a base double-wall, one can sometimes find different growth in three main directions. With the uniform growth of the crystals in three directions, a form identical or close to a cube will be obtained. If, on the other hand, the crystal grows rapidly towards the quadruple axis, a bar or needle shape will be obtained, and in extreme cases - hair. If, on the other hand, in this direction, the crystal growth will be slower, it will be in the form of a plate or plate.
The shape of the crystals is an important distinguishing feature. When describing minerals, the characteristic shapes of their crystals are mentioned: plate, bar, needle roller, hairs or fibrous clusters.
Crystals of some substances sometimes form normal adhesions, which are called twin adhesions. The position of the two Crystals that make up the twin is symmetrical: one is rotated relative to the other by an angle of 90 ° or 180 °, or they are fused symmetrically with respect to the plane, called the twin plane. In addition to twins composed of two crystals, some substances form multiple twins, composed of a larger number of fused crystals. On twins there are sometimes concave angles, which single crystals never have. Sometimes twins are invisible to the unaided eye and can only be detected under a polarizing microscope.
Twins are found quite often in rubies and sapphires, especially from Siam. They are also formed by diamonds, spinels and other precious stones. When processing diamonds, the grinder must take this phenomenon into account, because the orientation of hardness and cleavage varies from one twin to another.
Twin crystals with concave angles: a - plaster, b — ortoklaz, c - quartz, d — rutyl, and - chryzoberyl, f — staurolit, g — fluoryt.