Ilakaka Madagascar ( Sapphire rush )Written by Alain Darbellay GGGems
© 2004 gggems.com All rights reserved. ILAKAKA · The plateau of Ilakaka in 1992. Ilakaka is located on road to Tulear, south-west of Madagascar. The discovery of sapphire in current of 1998 transformed area. · Ilakaka in 1999. It is in accesses of downwards brook named Ilakaka that first sapphires were found in 1998. The existence of gemmiferous minerals, including corundum was known for a long time in sandstones of Isalo. French geologists having undertaken research in 1946 in particular. If their discovery did not cause particular interest at time, several reasons can be advanced : political instability which reigned after second world war in colony, and fact that majority of corundums coming from area of Ilakaka must be heated to be marketed, indeed, a blue stone, but cloudy, could not inevitably let suppose to its discoverers that it would become pure and transparent after a suitable heat treatment. Moreover, knowledge in this field remained summary at that time. In Ilakaka, arrival of merchants coming from Thailand and from Sri-Lanka changed things, because this group of purchasers became experts in "the " Tsy misy mandronono, tsy misy prix" enhancement of corundum." Thus, cloudy stones of This sentence is none by everyone in better quality will be often sold as (untreated Sri-Lankan Ilakaka. sapphires) on international market, this because of It means that : " if stone is not milky, difficulties in detecting heating treatment in a stone its owner will not get price for it." from Ilakaka not containing other inclusion only one haze of fine titanium drops, reabsorbed after heating. ( Note: that true untreated Sri-Lankan sapphires are rare.) The excavations follow gravel deposits which Impressing holes bore sandstones of Madagascans call lalam' bato, (the way of stones.) Ilakaka testifying to fever of sapphire. Ankaboka, one of many gemmiferous places of area. · Maromiandry is located The sivany (sifting of gravels) in Maromiandry. 40 kilometers north of Seldom practiced dry as in this place distant trunk road between from a river. Ilakaka and Sakaraha. The sifting of gemmiferous gravels in river close to Maromiandry.
| | Madagascar GarnetsWritten by Alain Darbellay GGGems
© 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 crystal in process of growth, all faces appear with same development. If one of symmetrical faces is less developed on a crystalline sample, or exceptionally does not appear, that comes from accidental actions of external environment which opposed its growth. Temperature, pressure, nature of mineral solution, speed of crystalline growth and direction of movement of solution etc... represent external influences on crystalline forms. The frequency of faces of crystals is related to reticular density, fast growth of some faces influences crystalline form definitively. Garnet thus crystallizes under cubic system, whose crystals are characterized by presence of three quaternary axes A4 joining centers of faces, four ternary axes A3 joining opposed tops, six binary axes A2 joining mediums of edges. · One of causes modifying 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 three adjacent corners. Cube Octahedron Truncation cuts two equal lengths out of two corners and a larger length on third. Trisoctahedron Octahedron Truncation on segment crosses, two equal lengths out of two corners, a smaller length on 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 similar angles. Here is a truncation on a top cutting three different lengths on corners, and which repeats only three times around 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 spiral order. In other cases, twelve irregular pentagons are formed by a truncation on one sharp angle, on both adjacent angles, unequal lengths, it is 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 hexatetrahedron with its twenty four triangular faces. Triakistetrahedron Hexakistetrahedron The trapezoidal dodecahedron consisted twelve quadrilaterals deltoid and tetrahedral pentagonal dodecahedron are formed by a truncation appearing on each top and cutting three different lengths on angle.
|
