 
Crystalline System: cubic. 





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 
1,741,78 




Orange
 yellowbrown : 



Spessartite
Mn3Al2Si3O12 
7,25 
4,20
 4,25 
1,78
 1,81 
Malaya
Mn3Al2(SiO4)3 
7,25 
3,74
 4,00 
1,78 
Hessonite
Ca3Al2(SiO4)3 
7,25 
3,58
 3,65 
1,73
 1,74 




Green
:




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
forms. 

The
frequency of the faces of the crystals is related to the reticular
density, the fast growth of some faces 

influences
the crystalline form definitively. 

Garnet
thus crystallizes under the cubic system, whose 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. 





·
One
of the causes modifying the initial form of crystals is truncation. 

Truncation on corners. 












Cube 


Dodecahedron 






Truncation cuts two different lengths on
adjacent corners. 





Cube 

Tetrahexahedron 




Truncation cutting three equal lengths out of the three
adjacent corners. 






Cube 


Octahedron 













Truncation cuts two equal lengths out of two
corners and a larger
length on the third. 







Trisoctahedron 

Octahedron 
























Truncation on the segment crosses, two equal lengths
out of two corners, a smaller length on the third. 







Cube 

Trapesohedron 


















Octahedron 

Trapesohedron 


















Dodecahedron 

Trapesohedron 


















Hexoctahedron 

Dodecahedron 











Almandine
in matrix 
PyropeAlmandine 
Almandine
in matrix 















Almandine
in matrix 
Almandine
in matrix 
Rhodolite
(Ambohitompoina) 








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 the ternary axis. 




Cube
and diplohedron 
Diplohedron 
Right
Gyrohedron 
Left
Gyrohedron 




The diplohedron is made of twentyfour irregular
quadrilaterals. The class plagiohedron whose faces (HKL) 
are
arranged in
the spiral order. 
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. 







Positiv 
Négativ 











Almandine
in matrix 
Tsavolite
(Madagascar) 
Spessartite
in pegmatite 


(Tsilaizina) 



The regular tetrahedron
consisted four equilateral triangles forming between them an angle of 70°
31. 



Positiv
tetrahedron

Négativ
tetrahedron

Octahedron 






Positiv
tetrahedron


Cube 












The
tetrahedron or triakistetrahedron consisted twelve faces which are
isosceles triangles, and the 
hexatetrahedron with
its
twenty four triangular faces. 


Triakistetrahedron 
Hexakistetrahedron 



The trapezoidal dodecahedron consisted twelve
quadrilaterals deltoïde and the tetrahedral pentagonal 
dodecahedron are
formed by a truncation appearing on each top and cutting three different
lengths on 
angle. 







right 
left 

Deltoïde
dodecahedron 
Pentagonal
tetraedrical dodecahedron
. 
Almandine
in matrix




















Spessartite
(Ambohimarangitra) 
Malaya
(Andoharano) 
Malaya
(Madagascar) 


















Rhodolite
(Ankilytokana) 
Hessonite
(Soakibany) 
Imperial
Malaya (Madagascar) 















In Madagascar, one finds rhodolite
in a gneiss rich in biotite, in which (almanditepyrope) is presented in 
the form of small
grains, or with 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
schistous 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 feldspar 
granoblastic, 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. 

Leptynites with amphibolopyroxenite intercalation rich in garnets of a
pale pink (almanditepyrope), with 
often rutile and graphite abound
in certain areas of Madagascar. The feldspar is orthoclase, associated 
with ogigoclasealbite feldspar and sometimes
with spindleshaped microperthite, there exists much of myrmekite. 
These rocks are with
fine grins, but they very often contain large regularly distributed crystals. 
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
opposed pole in which biotite prevails, with progressive disappearance of
feldspar. 










The Besafotra river
carry the 
spessartites onseveral
kilometers 
from their source, doubtless
a 
sodolitic pegmatite. A walk of
25 
kilometres among the mountains is 
necessary to reach this place. 













The tanety "grounds bordering
the river," are also 

the object of the orange
garnet's fever. 

















Sifting in river. 

Initially, the spessartite appeared in 

the Besafotra
river, searched out 

here near to its source. 








Ankilytokana, one of the fabulous rhodolite
deposit 

exploited
in a leptynite
vein on a sixteen 

meters depth. 



Leptynites
are primarily consisted in alkaline feldspars 

and
quartz. When these rocks are not ribboned, and 

that
is
frequent, it is often difficult to decide if a 

sample,
not
seen in place, belongs to a leptynite or an 

aplite, it
should
be noticed that in Madagascar, these 

last
contain
microcline and not of orthoclase. In this 

area,
one
observes graphite spangles in the leptynites.




















Malaya
garnet discovered into 

September
1998, in eluvium in 

a
broken up leptynite. 

The
modest depth of the deposit 

did
not
require
a significant work 

to
extract
it. 










This
stone shows an exceptional capacity to restore 

the light,
thanks in
particular to its high refractive


index, especially under
not very enlightened condition. 



All
of
the pictures on this site have been shot by Darbel du Bourg. 
©
1987  2009 Darbel du Bourg. All rights reserved. Alain
Darbellay
