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| AL2O3 |
Sapphire found in a
metamorphized marble.
|
| · 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 conchoïdal. |
|
| Hardness
9. Density 3,95 to 4,1 Refractive index: 1.76 - 1.77 |
| Uniaxial
and optically négative.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 story |
| 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. |
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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 |
|
endomorphized
granitic
veinules and more or less deprived of quartz which injects
those. Silimanite is |
|
an usual
satellite
of corundum in this type of deposit. One also finds some one in eruptive
rocks, |
|
syenites. |
|
Gems are generally
found in alluvium, but come either from basaltic slags, or from metamorphized |
| marble or
endomorpheous
feldspatic
rocks. |
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|
 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. |
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|
 6 |
 7 |
 8 |
 9 |
 10 |
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| |
 11 |
 12 |
 |
 13
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Scalenoedron |
Stacking
with parallel axes, |
Crystal
supporting on one of its bases |
|
gutters
at contours of
the face. |
a
small rhombohedron p in parallel |
|
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|
position. |
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| 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. |
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| Macle
of blue
corundum. |
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| Translucent violet - pink sapphire |
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Ruby in amphibolite. |
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| 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. |
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|
 |
 |
 |
| 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 marble. |
The
origin of the stone is a marble. |
|
|
| ·
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 depends
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. |
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|
 |
|
|
The alluvial ruby
deposit of Ambatovaravarana in the high plateaus, |
|
is located in the center on the
picture, under the hill. |
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|
 |
|
Your
guide to GGGems |
 |
All of the pictures on this
website have been shot by
gggems.com |
Alain
Darbellay |
|
Text written by Alain Darbellay. |
|
|
