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| A
study of MADAGASCAR TOURMALINE |
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| (Na,
Li,Ca) (Fe2,
Mg, Mn, Al)3
Al6 I(OH)4 I
(BO3)3ISi6O18I |
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| ·
Tourmaline
crystallographic
properties: Trigonal. |
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| Hardness
7 - 7.5 |
| Density 3.02 to 3.26 |
| Refractive index: 1.62 - 1.64 |
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| ·
Chemical
Properties: Alumina borosilicate with fluorine. |
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Occurences:
In the acid magmatic rocks and associated |
| pegmatites,
limestones and schists, placers. |
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| Coloured
tourmalines are related to sodolithic
pegmatites. |
| One
distinguishes two principal types of tourmalines: the hexagonal type and
the triangular one. |
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The
hexagonal type is especially characterized by the presence, in
the prismatic zone, of |
| the
well developed S1 faces, and those of L always narrow. When
the crystals Bi-terminated, |
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| one
finds with the one of the ends of the ternary axis the rhomboedron R,
and with the other |
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| same
the rhomboedron combined with that (0111) = P. |
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The
triangular type, here is never finished at the two ends, it is
characterized by the |
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| prevalence
of faces I in the prismatic zone, while those S are very
narrow just as those S2 |
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| which
accompany them. The crystals on the finished end carry the base C,
combined with |
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| the
rhomboedron R; in the same way sometimes with the rhomboedron P
and the base C |
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 1 |
 2 |
 3 |
 4 |
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Fig. 1& 2 : Triangular
type |
Fig. 3 & 4 :Hexagonal
type |
Very
rare type of |
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Rubellite from Antsongombato |
tourmaline:
white head |
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and
red body. |
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5 |
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 6 |
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| Crystal
of the hexagonal type finished |
Polychrome
tourmaline
being in Tsilaizina, Madagascar. |
| at
a peak frequently found in Tsilaisina. |
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 |
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 |
 |
| Liddicoatite Tourmaline |
Deep-PinkTourmaline |
Blue-greenTourmaline. |
Indigolite |
| from
Anjanabonoina. |
Antsongombato type. |
(
Kunar valley, Afghanistan.) |
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Colors
of tourmalines: Blue is due to bivalent iron, (magnesium gives a clearer
blue) the manganese produce tone |
| from
pink to red, chromium and vanadium is in green tourmalines, the yellow and
the brown one is produced by |
| trivalent
iron. |
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| Diagrammatic
provision of
a complete crystal cut parallel
to the ternary axis. |
Growth zoning and
geometrical drawings in a
tourmaline
sliced in section from Vohitrakanga. |
SLICED
TOURMALINE |
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Rubellite |
Chrome Tourmalines |
Indigolite from
Anjamiary. |
Macle of
green
tourmaline |
| Antsongombato type. |
from
Tsaniria. |
( Madagascar ) |
Estaknala ( Pakistan ) |
| ( Madagascar ) |
( Madagascar ) |
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| Rubellite
from Antsongombato: |
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The
crystals of this remarkable mineral are very
variable in size, small one are
homogeneous only and of a beautiful pigeon blood red color lighter or
darker. Those larger almost always have a complex structure and are formed
by concentric zones. At the time of the deposit discovery, very
large crystals were quite exceptional and were still inhomogeneous.
In some rubellites, alternating zones are all
colored in red, more or less vivid, the center is such an intense purplish
red, middle and peripheral zones are red or pink in various shades and
even colorless. In other stones the center is composed of yellow or brown,
and the periphery composed by rubellite. The reverse also occurs on large
crystals that appear black while the central parts show of a beautiful
red. When
color zoning are numerous they are very thin. The
optical properties of tourmaline from Antsomgombato examined on blood-red
pieces, lighter red and zoned crystals show that they are all slightly
biaxial.
In Tsilaisina, zoned crystals are also very
common. Some crystals show some zones of alternating color and which are
not concentric, but arranged in parallel layers over the height of the
prism. Here pink tourmaline is distinguished from others by its low
content of manganese and a high proportion of lime.
The main fact that emerges from the observation of
tourmalines from the
above occurrences, is their undeniable relationship. All
tourmalines contain manganese in varying proportions, and sometimes
substantial, all are of a remarkable poverty in magnesia and iron oxide
with the exception of black tourmaline. The silica and alumina will
undergo changes that reach only 2%, and the total amount of alkali is
between 1.04 and 3.48%. Fluoride, always constant, oscillating between 1.4
and 0.73% boric acid and between 9.46 and 11.03%. The most alcaliferous
tourmaline is the red of Antsongombato, 3.48%. The alkalies do not seem to
have a direct influence on the coloration. Here, manganese is probably the
main chromogenous factor, but the color also depends on the elements that
accompany it. The combined presence of lime appears to be the cause of the
pink or red. |
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| Sketch
of the Antsongombato mine ( Antsirabe area) |
|
| at
the beginning of the 20th century. The attack |
Mine
of Alakamisy Itenina ( Fianarantsoa
area, |
| measured
about fifteen meters, the coal face was to |
Madagascar). |
| hillside
10 meters above the brook, it showed the |
This
mine is worked in alluvium, the first stones were |
| presence
of a large sill of hard pegmatite, laid out in |
discovered
in 1989 in a rice plantation. It produced |
| benches
lities by a secondary cleavage. To the wall |
rubellites
and remarkable tourmalines
with geometrical |
| one
sees a schistous limestone,
greenish and broken up, |
drawings.
©
Image J. Darbellay |
| to
the roof, one distinguishes only clay. This |
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| occurrence
was famous for its red-ruby
tourmalines. |
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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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