Madagascar corundums ( Sapphire & Ruby discovery )Written by Alain Darbellay GGGems
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AL2O3 Sapphire found in a metamorphized limestone. · Crystallographic properties of corundum: Trigonal.c = 1,3630 pp 0 86°4' ; pa1 = 122°26' Macle according to p (1011), often polysynthetic, accompanied by plans of separation, similar plans are also observed according to a1 (0001) unequal break to conchoïdal. Hardness 9. Density 3,95 to 4,1 Refractive index: 1.76 - 1.77 Uniaxial and optically négative.ng = 1,7675; Np = 1,7593 The faces a1 frequently show phenomenon of asterism, generally due to reflexions within separation p. Strong polychromatism , with following maximum ng. · Chemical properties: corundum is composed of pure alumina; its colouring is due to metallic oxide traces or inclusions. The color of sapphire, due to iron and titanium oxide, gives its best effect under daylight. Electric light makes it often dark. The color of ruby, due to chromium oxide, on contrary shows its most luminous red under electric lighting. The Corundum shows in Madagascar two different aspects: 1 stony and opaque crystals. 2 crystals of smaller size often transparent and usable as gem. In Madagascar, stony corundum comes from mica schists metamorphized by granite, as well as granitic veinules endomorphized and more or less deprived of quartz which injects those. Silimanite is a usual satellite of corundum in this type of deposit. One also finds some in eruptive rocks, syenites. Gems are generally found in alluvium, but come either from basaltic slags, or from metamorphized limestones or endomorpheous feldspatic rocks. 1 2 3 4 5 One distinguishes two principal types of corundum in Madagascar: Type I : isoceloedric, more or less acute accompanied or not by a small face a1 (0001) and more rarely by facets p (1011). Represented by figures 1 to 11, but which often become complicated in consequence of irregularity of development of some of their faces and by stacking with parallel axes of a great number of individuals. 6 7 8 9 10 11 12 13 Scalenoedron Stacking with parallel axes, Crystal supporting on one of its bases gutters at contours of face. a small rhombohedron p in parallel position. Type 2: Characterized by association of prism d 1 (1120) at a broad base, with which can associate isosceles ones, among which e 3 is most frequent, as well as rhombohedron p. The base of Malagasy corundum crystals very frequently shows scratches or triangular figures in relief, limited by p. Macle of blue corundum. Translucent violet - pink sapphire
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).