GENETIC SIGNIFICANCE OF VARIATIONS IN THE FACES OF THE SIMPLE FORMS OF TOPAZ CRYSTAL FROM CHAMBER PEGMATITES OF THE KOROSTEN PLUTON (UKRAINIAN SHIELD)

Abstract

Topaz crystal morphology and habit distortion has been studied in various mineral-structural zones of chamber pegmatites of the Korosten pluton, which is located in north-western part of the Ukrainian Shield. It was assumed that the symmetry of the crystals obey the Curie principle. This means that only the symmetry elements common to the crystal and the medium in which it is formed will remain on real polyhedrons. The types of symmetry that contain the axes of infinite order are reduced to the following groups: 1) ∞L∞∞PC is a ball; 2) ∞L∞ is a ball filled with an optically active liquid; 3) L∞∞L2∞PПC is a cylinder; 4) L∞ПС is a rotating cylinder; 5) L∞∞P is a cone; 6) L∞∞L2 is a twisted cylinder; 7) L∞ is a rotating cone. Symmetry of the real fluid-dynamic situation of the mineral-forming medium of topaz-bearing parageneses often evolves in the following way: ∞L∞∞PC → L∞∞P → P. In this case, the flow of the mineral-forming fluid has the symmetry P. The resulting topaz crystals can have P symmetry if their symmetry plane coincides with the flow symmetry plane, otherwise they have no symmetry elements at all. In particular, it is shown for the first crystals that the upper faces grew faster, and their size is smaller than that of the lower ones. Growth was limited by the supply of the necessary fluid to the growing crystal faces. Hence, it follows that the fluid flow was in the direction from top to bottom. If the planes of symmetry of the fluid flow and of the polyhedron do not coincide, then visually triclinic crystals of the second type are formed. They are much more abundant than the ones of the first type. In addition to these two types, polyhedra with external symmetry L2 are found. It is difficult to imagine an environment with such symmetry because; it is unlikely that an attached crystal would grow between two fluid streams moving in opposite directions. Nevertheless, polyhedra flattened along the faces M {110} and less often along l {120} are frequent. That is, they grew in the environment in which the fluid flow moved in a direction parallel to the {110} faces (and less often {120}), in the direction from the smaller faces of a simple forms to the larger ones. The direction of fluid flow is more difficult to establish, with more or less the same development of the faces of the simple form of the topaz crystal.