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 Post subject: Vesuvianite to Chromiferous vesuvianite Kenya near Tanzania
PostPosted: Sun Feb 28, 2010 12:07 pm 
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I recently did a study on yellow-green to deep green vesuvianite from Kenya (near the town of Namanga at the Tanzanian border).
Let me give you some data here.
Hope you'll like this work.

Quote:
E1:Image E2:Image E3:Image E4:Image E5:Image

Name: Vesuvianite (Idocrase)
Color: Green-yellow to Green to Deep green
Locality: Kenya, "Mile-tisa" (near Namanga, Tanzania)

Polariscope: Anisotropic with iridescent «syrup like" texture.

Pleochroism:
Net to weak. The darker the green is, the weaker the pleochroism is (cf photo here under)

E1:Image E3:Image E5:Image



Luminescence:
UV Long wave: Inert
UV Short wave: Inert

Refractive index:
Ne=1.712 to 1.722
No=1.713 to 1.723
DN=0.001 to 0.003
Uniaxial +/-
Note: No correlation between refractive index and the intensity of the green color.


Specific gravity:
Hydrostatic weighing:
3.33 to 3.45
Note: No correlation between specific gravity and the intensity of the green color

Hand spectroscope (prism and diffraction grating):

E1:Image | Image


E3:Image | Image


E5:Image | Image

The line at 461 nm is best seen in green-yellow stone. The line at 529 nm seems to follow the same behavior.
Sharp and weak lines in the red have only been seen in the most strongly green colored sample (E5 Deep green).

Spectrometer:
Image

The green-yellow color (E1) is gave by iron III (Fe3+) in octahedral coordination (with eventual influence of the intervalence charge transfer Fe2+ -> Fe3+;Fritsch et Rossman, 2001).

The green color of this material is more intense (deep green) as the visibility of forbidden transition bands, that are associated to chromium (689 et 697 nm), is evident as well as the 610 nm band comes stronger (E4 et E5).
Absorption bands near 450 nm, 610 nm and peaks of forbidden transitions at 689 nm and 697 nm are given by chromium.

There is, on this sampling, a correlation between the color intensity and the intensity of chromium signals.

Intense green chromiferous vesuvianite and more classical green-yellow stones colored by iron seem to have cohabited in the same deposit.

Inclusions:
Well formed crystals (euhedral) apparently colorless with acute angles.
Image
Euhedral crystals with “rounded” shape.
Image
According to the bibliography (Gubelin et Koivula, 1992) these inclusion could be diopside and/or apatites. Moreover the presence in this deposit (marble type) of yellow diopside, suggest that this inclusions are most probably diopsides.

Bibliography:
Fritsch E., Rossman R., 2001, Revue de Gemmologie A.F.G. n°143: L'Origine de la couleur dans les gemmes (4eme partie), pp 26-37

Gübelin E.J., Koivula J.I., 1992, Photoatlas of inclusions in gemstones, Vol.1


Best thanks to W. Radl , Mawingu Gems, Germany


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Jean-Marie Arlabosse
GemInterest.com


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 Post subject:
PostPosted: Sun Feb 28, 2010 1:15 pm 
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Thank you for sharing your very informative data on the Vesuvianite material from Kenya.
It made my day!


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 Post subject:
PostPosted: Sun Feb 28, 2010 3:52 pm 
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Thanks for sharing Jean-Marie!

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 Post subject:
PostPosted: Sun Feb 28, 2010 5:08 pm 
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Fascinating!
Do you know when this deposit was discovered? Did you pick up the samples in Tucson?

I have read reports on vesuvianite found in Kenya in 2002, south of Nairobi, but it was NOT chromium bearing.

Around the same time (2002) some vesuvianite was discovered in Madagascar which was found to contain Cr+3 (presumably in an octahedral site) based on a series of weak and poorly defined bands at 688 and 704 (using polarized UV-Vis_NIR absorption spectroscopy). The inclusions in this material were also unique. Magnetite and calcite/aragonite were both identified which had never been seen before in vesuvianite

The only other location for chrome bearing vesuvianite (to my knowledge) is Quebec. These xls were discovered in 2008 Jeffrey Mine, Asbestos, Quebec: (notice color is not unlike vesuvianite colored solely by iron)
Image
Courtesy of Northwest Minerals


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 Post subject:
PostPosted: Mon Mar 01, 2010 2:32 am 
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Yes I have had the sample in Tucson.
It seems this is an old discovery. May be the 2002 discovery you talk about?
As the chromium bearing samples seem to be very few and small, may be author 2002 did not talk about the most green colored?

I didn't know about Cr-vesuvianite from Madagascar. I am interested to have bibliography reference (if it exist).

Yes jeffrey mine had furnished strong green crystals of Cr-vesuvianite!
(I used this kind of sample as reference in this study ;-) )
Image

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Jean-Marie Arlabosse
GemInterest.com


Last edited by Gemça on Tue Mar 02, 2010 9:03 am, edited 1 time in total.

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 Post subject: Madagascar Vesuvianite
PostPosted: Mon Mar 01, 2010 11:09 am 
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Morning!
re:I didn't know about Cr-vesuvianite from Madagascar. I am interested to have bibliography reference (if it exist).

My reference is G&G, Spring 2002, page 101-102.


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 Post subject:
PostPosted: Thu Mar 25, 2010 3:05 am 
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Hi Jean-Marie,

Just thought you might like to see a photo of some of the material in calcite host from one of our mines in Kenya. Note accompanying yellow diopside.

Paul

Image

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 Post subject:
PostPosted: Fri Mar 26, 2010 4:14 am 
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Excelent!
Thank you Paul

Could this mine be the origanal one? I think so.
The host (calcite), de yellow diopside..from Kenya.. all this facts match with what it has been reported for my samples.

Thank you very much.
Do you happen to have the Geo Localization (GPS coordinates) of this mine (that you could give to me by mp)?
Juts to do a virtual trip with Google earth :)

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Jean-Marie Arlabosse
GemInterest.com


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 Post subject:
PostPosted: Tue Mar 30, 2010 5:52 am 
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Outstanding report,
My father was one of the fathers of CSI in the USA for Law Enforcement
Your attention to detail is brillant. You help to identify a gem, show its habit, and point out spectra for both prism and defraction grating, but the "coup de ristance" is the GPS coordinates as well as a shot of the mineral in situ at the mine site.
This kind of research result is very important to appraisers, because finally we will begin to place gems at their proper locales rather than where a salesman who may use puffery claims origin?
It is important to miners to discover shrinkage of their own inventories, or if their stone rough is being touted somewhere else to defeat rarity and dilute price.
Are there any geologic maps or tectonic charts of the area? any other references? Govermental minerological records, industrial references or commercial records. This will help greatly to see the extent of the find as well as show that at this mine the crystaline samples are true to known mineralogic formations of similar stratas.
Of times, gemstones or the crystaline habit of gem rough is but a minor part of a larger commercial project for the extraction of ore from another commercial operation. Sometimes, gemologists get so detailed in their world, that they miss the forrest when examining a small cluster of trees. Knowing what is being mined for commercial ventures in the area, will also help identify original mines and expose miracle sites where similar commercial ventures have never existed. The lack there of, may be a case of puffery or like finding diamonds in Canada, a missed opportunity that commercial developers missed? Anyway, the report is superb, thanks for sharing....w


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