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.
Madagascar gem safariWritten by Alain Darbellay GGGems
© 2004 gggems.com All rights reserved. MADAGASCAR GEM SAFARI 2 Sapphire from Maromby in a metamorphic limestone. ( Andranondambo area.) Looking for sapphire on ground in 1992. Sapphires from Andranondambo, discovered in 1991. ( South-East of Madagascar ) In 1658, Flacourt, governor of Fort-Dauphin mentions sapphires in south of Madagascar. In 1808, Barthélemy Hugon writes that in valley of Ambolo there is a so beautiful stone and so rare that lapidaries have not been able to recognize it and it was above of all that is not diamond. ... In 1991, at time of great dryness in south, sapphire samples are found, several centuries after their first discovery. The son of Mrs. Dauphine, innkeeper in Tranomaro, main village of valley, was among prospectors. Actually, one of most beautiful quality in world is just leaving ground. The area of Tranomaro is characterized by an extreme calcic and magnesian paragneiss abundance: gneiss with diopside, plagioclasites, pyroxenites, wernerites, wolastonite limestones, with varied leptynites, generally with garnet, kinzigites, quarzites. One finds there rather frequently syenitic gneisses or lamboanites. Gneisses of injection are very frequent. The typomorphic minerals are: basic diopside, scapolite, calcite, plagioclases, wollastonite, garnet, cordierite and corundum. East of Tranomaro, arteritic and eyesys injections assign gneisses at leptynites. Cordierite gneiss bands are deeply injected bed by bed, injection has introduced magnetite in abundance. Wollastonites form ptymatic lenses with Marohotro, 18 km. south from Andanondambo. folds very tortured at middle of limestone. Note: The sapphire on left comes from Ambandanira, and aluminous anorthites or sakenites. that on right-hand side from Maromby (area of Andranondambo) Sapphire from Behataza plateau (Andranondambo area) Sapphires from Ilakaka. One of various holes in sandstone forming ( South-West of Madagascar ) deposit of Ilakaka discovered in to 1998. Blue sapphires , pink and violet, Ilakaka area. SANDSTONES OF ISALO. The white sandstones of Isalo present imposing stratifications intersected with gravel beds and small conglomerates. These soft sandstones resisted to erosion and taken a ruiniform relief because presence of braces siliceous veins. The mineral-bearing deposits include : a deposit of generally classified gravels, richest part and larger elements being at base of bed - rock. MORE ABOUT ILAKAKA