Madagascar Amethyst

Written by Alain Darbellay GGGems

( SiO2 ) 2004 All rights reserved. (Variety of Quartz) Crystallographic properties: Trigonal System (Subdivision ofrepparttar hexagonal system) Quartz forms hexagonal prisms atrepparttar 109295 blunted ends andrepparttar 109296 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 torepparttar 109297 presence of colour centers which come from substitution of ions of silicon by iron ions inrepparttar 109298 crystal lattice of quartz. The amethyst crystallizes at temperatures lower than smoky quartz for example. In Madagascar, we find it either in repparttar 109299 crypts of pegmatites, or inrepparttar 109300 quartzite veins in connection with those. The geodes ofrepparttar 109301 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 jointrepparttar 109302 tops ofrepparttar 109303 regular trihedrons, three normal binary axes A2 withrepparttar 109304 ternary axis joining repparttar 109305 meddle of opposite horizontal corners. Here elements of symmetry ofrepparttar 109306 Trigonal system with repparttar 109307 oblique shape,repparttar 109308 tetragonal scalenoedron. It is interesting to see what becomes this tetragonal scalenoedron in others classes whererepparttar 109309 disappearance of repparttar 109310 symmetry planes makes decrease of halfrepparttar 109311 number ofrepparttar 109312 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 Systems

Written by Alain Darbellay GGGems

2004 All rights reserved. 7 systems of crystallization: There are 32 possibilities of combinations ofrepparttar elements of symmetry (axis of rotation, symmetry plane, centers combined inversion and operations). Sincerepparttar 109294 discovery ofrepparttar 109295 crystals reticular structure, one could show by calculation that to these 32 classes which relate torepparttar 109296 shapes ofrepparttar 109297 crystals, correspond 230 types of network. One immediately realizesrepparttar 109298 complexity ofrepparttar 109299 reasoning which should be made to know allrepparttar 109300 possible combinations. The 32 classes are grouped in 7 systems. Here arerepparttar 109301 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 ofrepparttar 109302 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 : appearrepparttar 109303 following symmetrical operations: > > Rotation

Inversion by symmetry plane

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