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| A
study of MADAGASCAR CORUNDUM |
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| AL2O3 |
Sapphire found in a
metamorphic marble.
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| · Crystallographic
properties of corundum: |
| Trigonal.c = 1,3630 pp 0 86°4' ; pa1
= 122°26' |
| Macle
according
to p (1011), often polysynthetic,
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| accompanied
by plans of separation, similar plans are also |
| observed
according to a1 (0001) unequal break to conchoïdal. |
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| 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.
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| · 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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In Madagascar,
corundum shows two different aspects: |
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1
stony
and opaque crystals. |
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2
crystals
of smaller size often transparent and usable as gem. |
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In
Madagascar, stony corundum comes from mica schists metamorphosed by granite, as well as |
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endomorphosed
granitic
veinules and more or less deprived of quartz which injects
those. Silimanite is |
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an usual
satellite
of corundum in this type of deposit. One also finds some one in eruptive
rocks, |
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syenites. |
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Gems are generally
found in alluvium, but
occur either from basaltic slags, or from metamorphic |
| 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 |
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by facets p
(1011).
Represented by figures 1 to 11, but which often become complicated in
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consequence of the
irregularity
of the development of some of their faces and by stacking with |
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parallel
axes of a great number of
individuals. |
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 6 |
 7 |
 8 |
 9 |
 10 |
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 11 |
 12 |
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 13
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Scalenoedron |
Stacking
with parallel axes, |
Crystal
supporting on one of its bases |
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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. |
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The
base of Malagasy corundum crystals very frequently shows scratches or
triangular figures in |
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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.
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| Because
of its hardness, its inalterability and its density, corundum
constitutes a standard mineral of
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| 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
metamorphic marble. |
The
origin of the stone is a marble. |
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| ·
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 |
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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 any role. During its |
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growth, a face can develop more
quickly than another. The angle that the faces make between them |
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remains
constant, since it depends
on the network. This structure of the crystalline material explains the |
| law
of the constancy of the dihedral
angles. |
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| 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 highlands, |
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is located in the center on the
picture, under the hill.
©
Image J. Darbellay |
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Your
guide to GGGems |
 |
©
All of the pictures on this
website have been shot by
gggems.com
|
Alain
Darbellay |
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Text written by Alain Darbellay.
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