Madagascar,Garnets,2004,gggems education Madagascar Garnets

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© 2004 gggems.com All rights reserved. Crystalline System: cubic. Red - violetish: Hardness Density Ref.Index Pyrope Mg3Al2Si3O12 7,25 3,58 g / cm3 1,714 Almandine Fe3Al2Si3O12 7,50 4,32 1,830 Rhodolite Mg,Fe3Al2SiO12 7,25 3,78 - 3,90 1,74-1,78 Orange - yellow-brown : Spessartite Mn3Al2Si3O12 7,25 4,20 - 4,25 1,78 - 1,81 Malaya Mn3Al2(SiO4)3 7,25 3,74 - 4,00 1,78 Hessonite Ca3Al2(SiO4)3 7,25 3,58 - 3,65 1,73 - 1,74 Green : Tsavolite Ca3Al2(SiO4)3 7,25 3,60 - 3,68 1,73 - 1,74 Uvarovite Ca3Cr2Si3O12 7,50 3,85 1,87 Dementoïde Ca3F2SiO12 6,5 - 7 3,82 - 3,85 1,89 In a perfect crystal, when a face appears in the crystal in the process of growth, all the faces appear with the same development. If one of the symmetrical faces is less developed on a crystalline sample, or exceptionally does not appear, that comes from the accidental actions of the external environment which opposed its growth. Temperature, pressure, nature of the mineral solution, speed of the crystalline growth and the direction of the movement of solution etc... represent the external influences on the crystalline forms. The frequency of the faces of the crystals is related to the reticular density, the fast growth of some faces influences the crystalline form definitively. Garnet thus crystallizes under the cubic system, whose crystals are characterized by the presence of three quaternary axes A4 joining the centers of the faces, four ternary axes A3 joining the opposed tops, six binary axes A2 joining the mediums of the edges. · One of the causes modifying the initial form of crystals is truncation. Truncation on corners. Cube Dodecahedron Truncation cuts two different lengths on adjacent corners. Cube Tetrahexahedron Truncation cutting three equal lengths out of the three adjacent corners. Cube Octahedron Truncation cuts two equal lengths out of two corners and a larger length on the third. Trisoctahedron Octahedron Truncation on the segment crosses, two equal lengths out of two corners, a smaller length on the third. Cube Trapesohedron Octahedron Trapesohedron Dodecahedron Trapesohedron Hexoctahedron Dodecahedron Almandine in matrix Pyrope-Almandine Almandine in matrix Almandine in matrix Almandine in matrix Rhodolite (Ambohitompoina) There is also a law according to which certain crystals do not present modifications that on half of corners, or of the similar angles. Here is a truncation on a top cutting three different lengths on corners, and which repeats only three times around the ternary axis. Cube and diplohedron Diplohedron Right Gyrohedron Left Gyrohedron The diplohedron is made of twenty-four irregular quadrilaterals. The class plagiohedron whose faces (HKL) are arranged in the spiral order. In other cases, twelve irregular pentagons are formed by a truncation on one sharp angle, on both adjacent angles, the unequal lengths, it is the pentagonal dodecahedron. Positive Negative Almandine in matrix Tsavolite (Madagascar) Spessartite in pegmatite (Tsilaizina) The regular tetrahedron consisted four equilateral triangles forming between them an angle of 70° 31. Positiv tetrahedron Négativ tetrahedron Octahedron Positiv tetrahedron Cube The tetrahedron or triakistetrahedron consisted twelve faces which are isosceles triangles, and the hexatetrahedron with its twenty four triangular faces. Triakistetrahedron Hexakistetrahedron The trapezoidal dodecahedron consisted twelve quadrilaterals deltoid and the tetrahedral pentagonal dodecahedron are formed by a truncation appearing on each top and cutting three different lengths on angle. right left Deltoid dodecahedron Pentagonal tetraedrical dodecahedron . Almandine in matrix Spessartite (Ambohimarangitra) Malaya (Andoharano) Malaya (Madagascar) Rhodolite (Ankilytokana) Hessonite (Soakibany) Imperial Malaya (Madagascar) In Madagascar, one finds rhodolite in a gneiss rich in biotite, in which (almandite-pyrope) is presented in the form of small grains, or with the state of large porphyroblasts, generally deprived of geometrical contours, plagioclase (oligoclase with andesine) is the feldspar dominating and sometimes exclusive. These gneisses contain sometimes pegmatic beds very rich in crystals. One very finds also garnetiferous gneisses containing little biotite, hardly directed. Kinzigites. The gneisses which have been just enumerated have a very clear schisteous structure, which had with the biotite abundance. A rather frequent type is approximately blocks and presents a compact aspect, thanks to the prevalence of large garnets without geometrical form, associated quartz and feldspar granoblastic, biotite is not very abundant. The structure points out that of corneal micaceous of contact of the granite. This gneiss can be compared with the kinzigite of the Black Forest. Leptynites with amphibolo-pyroxenite intercalation rich in garnets of a pale pink (almandite-pyrope), with often rutile and graphite abound in certain areas of Madagascar. The feldspar is orthoclase, associated with ogigoclase-albite feldspar and sometimes with spindle-shaped microperthite, there exists much of myrmekite. These rocks are with fine grins, but they very often contain large regularly distributed crystals. Usually garnet does not have a geometrical form, but it takes clear faces in more quartzose zones. Leptynites derive from the granites by disappearance of the mica; the garnetiferous mica schists constitute the opposed pole in which biotite prevails, with progressive disappearance of feldspar. The Besafotra river carry out the spessartites on several kilometers from their source, doubtless a sodolitic pegmatite. A walk of 25 kilometers among the mountains is necessary to reach this place. SPESSARTITE GARNET The tanety "grounds bordering the river," are also the object of the orange garnet's fever. Sifting in river. Initially, the spessartite appeared in the Besafotra river, searched out here near to its source. Ankilytokana, one of the fabulous rhodolite occurrences exploited in a leptynite vein on a sixteen meters depth. RHODOLITE GARNET Leptynites are primarily consisted in alkaline feldspars and quartz. When these rocks are not ribboned, and that is frequent, it is often difficult to decide if a sample, not seen in place, belongs to a leptynite or an aplite, it should be noticed that in Madagascar, these last contain microcline and not of orthoclase. In this area, one observes graphite spangles in the leptynites. Malaya garnet discovered into September 1998, in eluvium in a broken up leptynite. The modest depth of the deposit did not require a significant work to extract it. This stone shows an exceptional capacity to restore the light, thanks in particular to its high refractive index, especially under not very enlightened condition. Malaya Garnet Discovery Cutting Styles¦Characteristics¦ Crystalline Systems ¦ Madagascar Sapphire ¦ Corundum data ¦ Malaya Garnet ¦ Rhodolite Garnet ¦ Spessartite Garnet ¦ Hessonite Garnet ¦ Your guide to GGGems All the pictures on this site have been shot by gggems.com Alain Darbellay © 1987 - 2004 gggems.com All rights reserved.