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| A
study of MADAGASCAR GARNET |
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| Garnet
Crystal System: cubic. |
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| Red
- violetish: |
Hardness |
Density |
Ref.Index |
| Pyrope
Mg3Al2Si3O12 |
7,25 |
3,58
g / cm3 |
1,714 |
| Almandine
Fe3Al2Si3O12 |
7,50 |
4,32 |
1,830 |
| Rhodolite
Mg,Fe3Al2SiO12 |
7,25 |
3,78
- 3,90 |
1,74
-1,78 |
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| Orange
- yellow-brown : |
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| Spessartite
Mn3Al2Si3O12 |
7,25 |
4,20
- 4,25 |
1,78
- 1,81 |
| Malaya
(Mg,Fe,Mn,Ca)3Al2(SiO4)3 |
7,25 |
3,74
- 4,00 |
1,78 |
| Hessonite
Ca3Al2(SiO4)3 |
7,25 |
3,58
- 3,65 |
1,73
- 1,74 |
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| Green
:
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| Tsavolite
Ca3Al2(SiO4)3 |
7,25 |
3,60
- 3,68 |
1,73
- 1,74 |
| Uvarovite
Ca3Cr2Si3O12 |
7,50 |
3,85 |
1,87 |
| Dementoïde
Ca3F2SiO12 |
6,5
- 7 |
3,82
- 3,85 |
1,89 |
| In
a perfect crystal, when a face appears in the crystal in the process of
growth, all the faces appear with the same |
| development. |
| If
one of the symmetrical faces is less developed on a crystalline sample, or
exceptionally does not appear, that |
| comes
from the accidental actions of the external environment which opposed its
growth. |
| Temperature,
pressure, nature of the mineral solution, speed of the crystalline growth
and the direction of the |
| movement
of solution etc... represent the external influences on the crystalline
shapes. |
| The
frequency of the faces of the crystals is related to the reticular
density, the fast growth of some faces |
| influences
the crystalline shape definitively. |
| Garnet
thus crystallizes under the cubic system, which crystals are characterized
by the presence of three |
| quaternary
axes A4 joining the centers of the faces, four ternary axes A3 joining the
opposed tops, six binary |
| axes
A2 joining the mediums of the edges. |
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| ·
One
of the causes modifying the initial shape of crystals is truncation. |
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Truncation on corners. |
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| Cube |
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Dodecahedron |
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| Truncation cuts two different lengths on
adjacent corners. |
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| Cube |
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Tetrahexahedron |
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| Truncation cutting three equal lengths out of the three
adjacent corners. |
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| Cube |
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Octahedron |
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| Truncation cuts two equal lengths out of two
corners and a larger
length on the third. |
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| Trisoctahedron |
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Octahedron |
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| Truncation on the segment crosses, two equal lengths
out of two corners, a smaller length on the third. |
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| Cube |
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Trapesohedron |
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| Octahedron |
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Trapesohedron |
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| Dodecahedron |
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Trapesohedron |
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| Hexoctahedron |
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Dodecahedron |
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| Almandine
in matrix |
Pyrope-Almandine |
Almandine
in matrix |
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| Almandine
in matrix |
Almandine
in matrix |
Rhodolite
(Ambohitompoina) |
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| There is also a law
according to which certain crystals do not present modifications that on
half of corners, or of the |
| similar angles. |
| Here
is a truncation on a top cutting
three different lengths on corners, and which repeats only three times around |
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the ternary axis. |
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| Cube
and diplohedron |
Diplohedron |
Right
Gyrohedron |
Left
Gyrohedron |
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| The diplohedron is made of twenty-four irregular
quadrilaterals. The class plagiohedron which faces (HKL) are |
| arranged in
the spiral order. |
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| In
other cases, twelve irregular pentagons are formed by a truncation on one
sharp angle, on both adjacent angles, the |
| unequal
lengths, it is the pentagonal dodecahedron. |
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Positive |
Negative |
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| The regular tetrahedron
consisted four equilateral triangles forming between them an angle of 70°
31. |
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| Positiv
tetrahedron
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Négativ
tetrahedron
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Octahedron |
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| Positiv
tetrahedron
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Cube |
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| The
tetrahedron or triakistetrahedron consisted twelve faces which are
isosceles triangles, and the hexatetrahedron |
| with
its
twenty four triangular faces. |
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| Triakistetrahedron |
Hexakistetrahedron |
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| The trapezoidal dodecahedron consisted twelve
quadrilaterals deltoid and the tetrahedral pentagonal dodecahedron |
| are
formed by a truncation appearing on each top and cutting three different
lengths on angle. |
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right |
left |
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| Deltoid
dodecahedron |
Pentagonal
tetraedrical dodecahedron
. |
Almandine
in matrix
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| In Madagascar, one finds rhodolite
in a gneiss rich in
biotite, in which (almandite-pyrope) is presented in the form of |
| small
grains, or in the state of large porphyroblasts, generally deprived of
geometrical contours, plagioclase |
| (oligoclase
with andesine) is the feldspar dominating and sometimes exclusive.
These gneisses contain sometimes |
| pegmatic
beds very rich in crystals. |
| One very finds
also garnetiferous gneisses containing little biotite, hardly directed. |
| Kinzigites. The gneisses which have been just enumerated have a very clear
schisteous structure, which had with the |
| biotite abundance. A rather
frequent type is approximately blocks and presents a compact aspect,
thanks to the |
| prevalence of large garnets without geometrical form,
associated quartz and granoblastic feldspar, biotite is not |
| very abundant.
The structure points out that of corneal micaceous of contact of the
granite. This gneiss can be |
| compared with the kinzigite
of the Black Forest. |
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| Leptynites with amphibolo-pyroxenite intercalation rich in garnets of a
pale pink (almandite-pyrope), with often |
| rutile and graphite abound
in certain areas of Madagascar. The feldspar is orthoclase, associated
with |
| ogigoclase-albite feldspar and sometimes
with spindle-shaped microperthite, there exists much of myrmekite. |
| These rocks are with
fine grins, but they very often contain large regularly distributed crystals. |
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Usually garnet does not have a geometrical form, but it takes clear faces
in more quartzose zones. |
| Leptynites derive from the granites by
disappearance of the mica; the garnetiferous mica schists constitute the |
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opposed pole in which biotite prevails, with progressive disappearance of
feldspar. |
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The Besafotra river
carry out spessartites |
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on several
kilometers from their source, |
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doubtless
a sodolitic pegmatite. |
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A walk of
25 kilometers among the mountains |
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is necessary to reach this place. |
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| SPESSARTITE
GARNET |
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| The tanety "grounds bordering
the river," are also |
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| the object of the orange
garnet's fever. |
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Sifting in river. |
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Initially, the spessartite appeared in the |
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Besafotra
river, searched out here near to |
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its source. |
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Ankilytokana, one of the fabulous rhodolite
occurrences |
|
exploited
in a leptynite
vein on a sixteen meters
depth. |
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RHODOLITE GARNET
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Leptynites
are primarily consisted in alkaline feldspars |
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and
quartz. When these rocks are not ribboned, and |
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that
is
frequent, it is often difficult to decide if a sample, |
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not
seen in place, belongs to a leptynite or an aplite, it |
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should
be noticed that in Madagascar, these last |
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contain
microcline and not orthoclase. In this area, |
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one
observes graphite spangles in the leptynites.
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| Malaya
garnet discovered into |
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| September
1998, in eluvium in |
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| a
broken up leptynite. |
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| The
modest depth of the deposit did |
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| not
require
any significant work to |
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| extract
it. |
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This
stone shows an exceptional capacity to restore |
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the light,
thanks in
particular to its high refractive index,
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especially under
not very enlightened condition.
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| Malaya
Garnet Discovery |
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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.
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