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	1,74-1,78
Orange - yellow-brown :
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	Pyrope-Almandine	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 twenty-four 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 (almandite-pyrope) 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 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.

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.

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