Madagascar AmethystWritten by Alain Darbellay GGGems
( SiO2 ) © 2004 gggems.com All rights reserved. (Variety of Quartz) · Crystallographic properties: Trigonal System (Subdivision of hexagonal system) Quartz forms hexagonal prisms at blunted ends and head finishing in hexagonal pyramids. · Physical properties Hardness 7 Density 2.65  2.66 Refractive Index: 1.54 1.55 + 0.009 positive uniaxial Glare: vitreous · Chemical Composition: Faceted Amethyst Silicium Dioxide ( SiO2 ) The colouring of amethysts is due to presence of colour centers which come from substitution of ions of silicon by iron ions in crystal lattice of quartz. The amethyst crystallizes at temperatures lower than smoky quartz for example. In Madagascar, we find it either in crypts of pegmatites, or in quartzite veins in connection with those. The geodes of siliceous nodules of basalts contain some too. The first have an hexagonal network, although their pattern of crystallization is only of ternary order. The seconds have a ternary network. The elementary mesh is a rhomboedron, i.e. a parallelepiped consisted six equal rhombuses. A ternary axis A3 joint tops of regular trihedrons, three normal binary axes A2 with ternary axis joining meddle of opposite horizontal corners. Here elements of symmetry of Trigonal system with oblique shape, tetragonal scalenoedron. It is interesting to see what becomes this tetragonal scalenoedron in others classes where disappearance of symmetry planes makes decrease of half number of faces. We obtain a trapezohedron (One notices that these two trapezohedrons are not superposable. It is said that they are two enantiomorphism shapes. They are symmetrical compared to a symmetry plane.) One speaks about right trapezohedron and left trapezohedron (just as we have a right hand and a left hand, nonsuperposable thus enantiomorphism).
  Crystalline SystemsWritten by Alain Darbellay GGGems
© 2004 gggems.com All rights reserved. 7 systems of crystallization: There are 32 possibilities of combinations of elements of symmetry (axis of rotation, symmetry plane, centers combined inversion and operations). Since discovery of crystals reticular structure, one could show by calculation that to these 32 classes which relate to shapes of crystals, correspond 230 types of network. One immediately realizes complexity of reasoning which should be made to know all possible combinations. The 32 classes are grouped in 7 systems. Here are figures. CUBIC SYSTEM QUADRATIC SYSTEM HEXAGONAL SYSTEM Go to Garnet Go to Zircon Go to Apatite Diamond, garnet, spinell. Zircon Beryl, Apatite  TRIGONAL SYSTEM ORTHOROMBIC SYSTEM MONOCLINIC SYSTEM (Subdivision of hexagonal system) Go to Topaz Go to Orthoclase Go to Corundum Go to Tourmaline Calcite, corundum, tourmaline, quartz. Chrysoberyl, topaz Orthoclase  TRICLINIC SYSTEM The symmetry of crystals : appear following symmetrical operations: > > · Rotation · Inversion by symmetry plane
