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 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.
  
                     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    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.

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   Remarquable macle of
    blue corundum.      corundum from Maromby.   red corundum.
               
Gem ruby from Ambatovaravarana.
 
                              The alluvial ruby deposit of Ambatovaravarana in the high plateaus of 
                               Madagascar, is in the left center on the picture, under the hill.
Macle of blue sapphire.      Macle of ruby 60 grms.
All of the pictures on this site have been shot by Darbel du Bourg.
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