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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 conchoidal.
Hardness 9. Density 3,95 to 4,1 Refractive index: 1.76 - 1.77
Uniaxial and optically negative.ng 1,7675; Np 1,7593
The faces a1 frequently show the 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 the ruby, due to chromium oxide, on the 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 the
irregularity of the 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 the face. a small rhombohedron p in parallel position.
Type 2: Characterized by the association of the prism d 1 (1120) at a broad base, with which can associate the
isosceles ones, among which e 3 is most frequent, as well as the 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
Ruby in amphibolite.
The crystals engaged in rocks, rich in mica have rough faces, encrusted of biotite or muscovite, minerals which
also exist at the state of inclusions, associated with magnetite and pyrite.
Because of its hardness, its inalterability and its density, corundum constitutes a standard mineral of alluvia.
Ruby Giant Sapphire in a metamorphic dolomitic limestone.
Polychrome Sapphire Blue Sapphire ( origin: Blue Sapphire ( origin: Ilakaka 1999 )
( origin: Iankaroka 1990 ) Andranondambo 1992 ) Found in the sandstones of Isalo in
This sapphire comes from a granit, The stone comes from alluvium, the shape is rounded.
its shape is intact. a metamorphized limestone. The origin of the stone is a limestone.
· The structure of crystals.
It is R.J.Haüy who put forth the first assumption, confirmed 200 years later, on the structure of the crystallized
material. He lets drop accidentally a calcite crystal which broke into small rhomboedrons. He supposed whereas
the crystals were consisted of stacking small rhomboedrons elementary (that he called the "integral molecule"),
he could reconstitute all known shapes of calcite. In a crystal lattice, three atoms or groups of atoms (nodes),
which is not on the same line, determine a plan called reticular plan. But there is a great number of these
plans: it is enough to choose nodes not strictly nearby. As the network is infinite, there is even an infinity of
possible plans. The dimension of the faces does not play of role. In the course of growth, a face can develop
more quickly than another. The angle that the faces make between them remains constant, since it
depend on the network. This structure of the crystalline material explains the law of the constancy of the
dihedral angles.
Triangular figure on a Scratches on a rare Stacking in macle Triangles on a translucent
colorless corundum. colorless corundum. of a ruby. corundum.
Triangular figure on a Triangles in step on a blue Remarkable macle of
blue corundum. corundum from Maromby. red corundum.
Sapphire & Ruby Safari Gem ruby from Ambatovaravarana.
The alluvial ruby deposit of Ambatovaravarana in the high plateaus, is in the
left center on the picture, under the hill.
Macle of blue sapphire. Macle of ruby 60 grms.
Information ¦ Cutting Styles ¦ Characteristics ¦ Crystalline Systems ¦ Madagascar Sapphire
Andranondambo Sapphires ¦ Blue Sapphire Crystal ¦ Ruby Crystal ¦ Sapphire Slice
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