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Flat faces are the characteristic feature of ionic crystals; nevertheless, among them there are curvefaced ones, up to crystals with cylindrical shapes. They occur more often among the minerals with a layered structure or minerals with a higher tendency to structural defectiveness (cylindrite, franckeite, asbestos, jamesonite, boulangerite, and etc.). Molecular crystals, especially carbon and hydrocarbon crystals, as a rule, are curvefaced, up to cylindrical and spherical. The examples of curvefaced crystals are exotic cylindrical ones of molybdenite from the pegmatite veins of the Slyudyanogorsky deposit of muscovite in the Middle Urals. The crystals are cylindrically shaped, sometimes with sharpened or flattened wedge-shaped edges. Their length is 2-10 mm, diameter - 0.8 mm. At a cross section, the crystals have concentrically-laminar composition with a tabulated axled canal 120 mkm in diameter. Thin layers form packages with the thickness of about 15 mkm. In the section, the crystals are fixed up in six such packages that can be observed by eye or by X-ray phase-dispersed introscopy.
According to the data of monocrystal and powder X-ray, molybdenite represents a trigonal polytype 3R-R3m with an insignificant quantity of a hexagonal polytype 2H-P63/mmc. Twisting of leaf-like crystalline germs is one of the reasons for cylindrical crystal formation due to the syngenetic shistosity of containing rocks with crystallization.
Curvefaced crystal character can be growth (interlayer gethermometry, dislocational structure, growth deformations, etc.) or deformational (plastic deformations). Curvefaced character of molecular crystals is connected with the peculiarities of molecular packages (spheroidization, fibralation, and helicoidization) and the action of surface forces.
In connection with curvefaced crystal study, the problem of their systematization becomes a very vital one. It's evident that like for flatfaced crystals, this classification should be based on the symmetry system and should be unique for both flatfaced and curvefaced forms.
The derivation of curvefaced forms on the basis of the original sphere deformation in taxon directions of the flatfaced crystal symmetry system is suggested.
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