MADAGASCAR SPHENE

 

Titanite crystallographic properties: Monoclnic.
   Ca Ti (O ISiO4)
  Hardness 5 - 5.5 
  Density 3.52 to 3.54
  Refractive index: 1.885 - 2.050
  Bi-refraction: 0.105 to 0.135
  Dispersion: 0.051
Colors : Yellow, greenish-yellow, brown-orange, green (chrome)
Chemical Properties: Calcium Titanium Silicate
   Often includes (Nb, Cr, F, Na, Fe, Mn, Sn, V or Yt).       
 Occurrences: In the pegmatites (Madagascar). Intrusive metamorphic rocks.

 

 

<<< Chrome Sphene 15,30 carats. with red sparkles 
The birefringence of sphene is twice that of the zircon, which
facilitates its identification. This stone shows a exceptionally
high fire. Unlike the same zircon color, it is distinctly dichroic.
Its refractive index is beyond the scale of the refractometer.
In this calcium and titanium silicate, rare earth replace calcium up to 14%, aluminum,
iron and chromium may also be present.

 

In Madagascar, we find gem
quality sphene in the North East,
and Center Northwest.
Brown and opaque minerals
are associated with scapolite in
Southeast.
Sphenes Specimen associated
to scapolites. (South East.) >>
Group of Daraina. Regional geology and metamorphism.
Since the first geological reconnaissance conducted in Madagascar, one reported in the extreme north of the island a
group of porphyry facies or schisteous characterized by the abundance of epidote and amphibole. A. Lacroix (1922),
which samples were provided, observed sometimes porphyritic with automorphic plagioclase or their relics,
granolépidoblastics common schistose structures. These remarks of structural order and some chemical analysis, led
him to attribute these facies to the metamorphism of old basic igneous rocks, the transformation would have been
the cause of some chemical changes. The hypothesis of a" ancient volcanic series" was then formulated with its major
arguments. Some consecutive reports within the fifties brought a good description of the main petrographic facies. We
then distinguished three distinct geological units in the middle of the crystalline basement of the North of the Island:
- Series of  Mangily (East) R. Dormois 1949
- Series of  Daraina (North) R. sleeping 1949

- Complex of Milanoa (South) P. Brenon 1950

 

<<< Typical color of a sphene from northern Madagascar. 26.64 cts.
Lithology and summary petrography :
The series of Daraina includes acid rock and intermediate or basic rocks, usually shale. The most common 
acid facies is a feldspar sericite schist often pyriteous. The presence of small feldspathycs tonsils
oriented in a frame packed finely schisteous where dominate sericite evokes both at the naked eye than at 
the microscope the structure of some mylonites. There are also benches of microgranites with big
euhedral orthoclase which are probably relics of the original formations, volcanic rocks
or eruptive acids.
Basic or intermediate facies, of varied texture and composition, all contain a high proportion
of amphybole and epidote, with the exception of some chloritic varieties. All intermediates are represented
between porphyritic textures, oriented or not, at large euhedral plagioclase and granoblastic textures 
or homogeneous lépidoblastiques.

 

Typical crystals of sphene with a top
beveled. >>>
The term Sphên "splitting wedge" is
referring to the shape of crystals, then
that the word "titanite" refers to
its chemical composition.
Crystals, generally well shaped, have
different facies, sometimes flattened in house
double roof (typical case of intrusive and
metamorphic rocks), sometimes elongated and
twinned by penetration with an aspect of
channe.
(typical case of sphenes from alpine slots).
We can found large crystals in
pegmatites.
The monoclinic system in which the sphene develops, has three crystallographic axes of unequal lengths,
two intersecting at an oblique angle, the third being perpendicular to the plane of the other two.
The monoclinic crystals are biaxial.
Metamorphism of basic igneous rocks.
The Group of Daraina is an outstanding example of a precambrian eruptive system with varied terms,
metamorphosed under medium metamorphic intensity conditions.
The group of Daraina also stands with a significant copper mineralization which has been the subject of research
in 1954. It turned out that we can not separate metallogenic activities from méthamorphic processes. In
copper belt itself, in the valley of Antsahandrevo, the most important mineralization  ( massive chalcosine lens of
Androtsara-Matsaborivaky) is within a few meters depth of magnifier doleritic loupe,
flushing at the side of a hill between amphibole gneiss and epidote.

 

 
Exceptional formation of chrome 
sphene crystals in their gangue.
(This piece of collection weights 1290 grams.)
©  Image A. Darbellay

 

Metamorphism of basic igneous rocks. (Continued)
The blue-green color under the microscope, which characterizes most of the amphiboles of this complex
is not accompanied by substantially different optical properties from those of common hornblende. Note that 
the refraction and birefringence of the blue-green amphibole are particularly low. These characters closer
to the sodium hornblendes of the group of cataphorites, French geologists are used to a chemical analyze
in order to indicate the composition of that amphibole. According to the ionic composition reported in 96 (4x24)
oxygen, amphibole can be regarded as a mixture of the following three varieties defined by Winchell:
Ferriferous tschermakite, Edenite, and a chemical member of the Edenites' family. Possible impurities are epidote
and sphene that could have remained in inclusion within amphibole grains. Note that the presence of sphene
is likely to significantly alter the figures for titanium. The blue-green amphibole in thin lamella is due to the high
proportion of ferric iron and not the soda content denied by the analysis.
Sphene occurrence in northern Madagascar.  ©  Image J. Darbellay
It is at a modest depth that the stones are usually extracted.

 

Chrome Sphene 9.92 cts.

 

Sphene is very popular in the collectors' field for its remarkable appearance.

( Stronger dispersion than that of diamond, strong pleochroism, high birefringence ).

8.71 cts 6.72 cts. 4,16 cts 18.74 cts, Andoariana 10,14 cts
Orangy-braun Sphene Yellowish Sphene. Typical color of a (Center North-West.) Chrome Sphene  
sphene from the North Rare

 

Giant Sphene weighting 51.57 carats.

 

Sphenes / Visual Catalog / Safari
Geological references: A. Lacroix, P. Brenon, R.Dormois, H.Bessairies,
                                  H. de la Roche, A Poldervaart. Your guide to GGGems
Text and images :
A. Darbellay
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