CORUNDUM


AL2O3	Sapphire found in a metamorphized limestone.
· Crystallographic properties: 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
Uniaxial and optically négatif.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: the 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. The 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 two different aspects:
1 stony and opaque crystals.
2 crystals of smaller size often transparent and usable as gem.
The stony corundum comes from mica schists metamorphized by the granite, as well as
granitic veinules endomorphized and more or less deprived of quartz which injects those. The silimanite
is a usual satellite of corundum in this type of deposit. One also finds some in eruptive rocks, syenites.
The gems are generally found in alluvium, but come either from basaltic slags, or from metamorphized
limestones or endomorphous feldspatic rocks.

1	2	3	4	5
One distinguishes two principal types of corundums:
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
of individuals.



6	7	8	9	10




11	12		13
Scalenoedron	Stacking with parallel axes,		Crystal supporting on one of its
	gutters at contours of the face.		basesa 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 the Malagasy corundum crystals very frequently shows scratches or triangular
figures in relief, limited by p.









Macle 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 alsoexist at the state of inclusions, associated with magnetite and pyrite.
Because of its hardness, its inalterability and its density, the corundum constitutes a standard mineral
of the alluvia.



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	The stone comes from	alluvium, the shape is rounded.
granit, its shape is intact.	a metamorphized	The origin of the stone is a
	limestone.	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 they depend on the network. This structure of the

crystalline material explains the law of the constancy of the dihedral angles.