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Needle Inclusions in tanzanite

Posted by Dawid Godziewski  
Dawid Godziewski May 08, 2012 08:20PM
I have tanzanite crystal (flat, size: 24 x 21 x 4mm, 3 grams weight) - of course from Tanzania. The crystal has a very thin needle - inclusions - size up to 16 mm. What could it be?

Rob Woodside May 08, 2012 09:24PM
Are these Jessica's molybdenite tubes??? Why do they appear more blue than the surrounding tanzanite?
Robert Simonoff May 09, 2012 03:36AM
I am sure Jessica and/or Mike would love to get that piece under a microscope to see if they are the same thing. Bernard Olivier in his "Geology and Petrology of the Merelani Tanzanite Deposit, NE Tanzania" mentions graphite needles. He mentions that they are in the surrounding gneiss as well as included in the tanzanite. Bernard's, paper then goes on to say that these needles were not tested, but assumed to be graphite needles upon visual inspection - a safe assumption since there is so much graphite there (it is everywhere!!!).

However, Jessica and Mike's project could not confirm graphite needles as none were found in that work. They did confirm molybdenite wires/needles.

Anonymous User May 09, 2012 12:22PM
I don't have an opinion graphite vs. molybdenite.
But what a nice crystal!
John A. Jaszczak May 09, 2012 06:45PM
Wonderful specimen! Raman spectroscopy can distinguish graphite from molybdenite- perhaps even inside the tanzanite if it doesn't fluoresce, non-destructively.
Ronald John Gyllenhammer May 09, 2012 07:10PM
Hi Dawid et Al,

I don't have access to this paper but was able to view the abstract.

Notes on fluid inclusions of vanadiferous zoisite (tanzanite) and green grossular in Merelani area, Northern Thnzania. E. Malisa, K. Kinnunen, and T. Koljonen, Bulletin of the Geological Society of Finland, Vol. 58, No. 2, 1986, pp. 53-58.

Here's an excerpt from the abstract, page 57, left side, halfway down the page.

"... Solid inclusions noted in tanzanite include calcite, gypsum, graphite, rutile, sphene, xenotime,
quartz, diopside, and tremolite-actinolite. Some of these minerals occur as acicular needles which
could impart a cat's-eye effect to some tanzanites."

Reiner Mielke May 09, 2012 08:32PM
I would say it is more likely tremolite-actinolite and looks blue because it is darker than the background.
Robert Simonoff May 09, 2012 09:03PM
Do you know where in Merelani this piece is from? Though the geology of the various areas seems to be similar, there have been different minerals reported from various blocks and pits.

Dawid Godziewski May 09, 2012 09:50PM
I do not know the exact location. I still viewed this specimen - needles are very thin and difficult to specify the color. They seem to be silver with a blue glow - as molybdenite. but this may be an illusion. I notice at the bottom of the specimen (where the needles grow), the surface is soft mineral, silver - just like molybdenite !
Alfredo Petrov May 09, 2012 10:14PM
It's very hard to discern the true colour of a mineral, especially a metallic one, when present as an inclusion inside quartz, tanzanite or any other transparent mineral, so other forms of ID are needed; discussion of apparent colours can be quite misleading. I've been frustrated by metallic inclusions in quartz for the same reason. Not sure about the physics involved in these apparent colour shifts at the boundary between inclusions and their host minerals, but I hope someone with better knowledge of optics can explain it to us.
Rob Woodside May 09, 2012 10:19PM
Thanks Dawid, Can you photo the base? As you say, Moly has a blue hue and graphite is black, so maybe a photo would be revealing?
Rob Woodside May 09, 2012 10:26PM
Alfredo, The blue of the Tanzanite certainly confuses things here and John's idea of a Raman is a good one. However I haven't noticed this problem with clear colourless, quartz, calcite, or fluorite. With photomicrographs of graphite whiskers and molybdenite tubes, a high power microscope might tell the tale. Other acicular possibilities should reveal themselves as well.
Robert Simonoff May 09, 2012 10:33PM
We certainly have studied many of the molybdenite wires under the optical microscope. How, this would be the first completely incuded in a tanzanite we have seen. This is also FAR more wires per square cm than we have ever seen. That said we have found these wires on a small number of pieces so who is to say. The fact that the base looks metallic (like moly) and graphite wires were assumed, but never proven, certainly make one consider molybdenite here. would certainly love to see the piece close up - you have something really nice there!

Dawid Godziewski May 10, 2012 01:31PM
The base photos. Small and green is probably - garnet - tsavorite.

Rob Woodside May 10, 2012 03:18PM
Thanks so much for those photos. That looks more like Moly than graphite, so that is an indication of what the needles are, but not yet a proof.
Ronald John Gyllenhammer May 10, 2012 04:06PM
Hi Dawid,

> "...I notice at the bottom of the specimen (where the needles grow), the surface is soft mineral, silver - just like molybdenite !"

Molybdenite and Graphite can appear to be very similiar and so can be a problem to distinguish between the two. If I may suggest, perhaps you can test this area at the bottom of the specimen you mention above as the "soft, silver mineral". Crystalline Molydenite and Graphite have different conductivity properties as I understand it, (when I have more time I'll check the literature on this). If you have a multimeter, perhaps you can check for both continuity and resistance with your leads in this soft silver area at the bottom of the specimen. Graphite should be quite conductive and be positive for a continuity check, with low resistance. Unless heated to high temperatures I don't think you will see these same properties with crystalline Molybdenite, of this I am unsure but again I will check the literature when I have more time.

Further, I suspect that it may be graphite at the bottom of the specimen. Note the above referenced paper for this description "Crystals of tanzanite occur mainly in boudinaged pegmatitic veins and hydrothermal fracture fillings in a brecciated and hydrothermally altered graphite-bearing diopside gneiss. Associated minerals include quartz, diopside, graphite, calcite, hematite, and sphene, as well as green vanadian grossular garnet."

Now whether or not these acicular inclusions are indeed Graphite remains to be proven for this specimen but intuitively it would seem that there could be a correlation between the acicular needles and the "soft silver mineral at the base". All this speculation is just that, especially since this is just a silly "remote viewing" effort from only images. I have not personally seen Graphite as a acicular inclusion like this but then again, there's alot I haven't seen. Also, I am always hesistant to disagree with Rob since he is very often correct. :-) Someone like John Jaszczak who has studied Graphite in depth may have a better perspective than me on the possibility of Graphite as an acicular inclusion here. BTW Dawid, It's a truly beautiful and wondrous specimen. Good luck with it and all the best.

Rob Woodside May 10, 2012 04:51PM
Thanks Ron, like you I'm not convinced of anything about these needles. Moly tubes are almost too good to be true and on that basis alone should be doubted. I don't know about the relative conductivities, but since moly is used in Li ion batteries, it probably has a decent conductivity. However, if the needles break the surface one can use your idea to see if there is an electrical connection with the base. I doubt that rutile, diopside, or tremolite would show any conductivity. Although the base looks to me at a distance to be more like Moly than graphite, I would not put money on it. Ron you are a wise man and your voice of caution is much appreciated.:-)
Robert Simonoff May 10, 2012 05:23PM
Molybdenite wires HAVE been positively identified from this location, graphite wires have not. So, I would assume molybdenite for the wires until proven otherwise.

The silver, on the bottom, could be graphite, just not associated with the acicular mineral, but instead coincident. I agree with the testing, especially, since this does look like mauled graphite that is so common on the matrix of these pieces.

I see 2 other minerals at the bottom of this piece, one is tan and could be titanite or zoisite - as per the Mineral News paper. The greenish mineral could be garnet or diopside - again the Mineral News paper found both to be in similar green colors in association here. The colors appeared similar especially when the crystals were small, such as that one is.


Edited 2 time(s). Last edit at 05/10/2012 05:30PM by Jessica and Robert Simonoff.
Ronald John Gyllenhammer May 10, 2012 07:10PM
Hi Bob,

I was unaware of this paper on which Jessica is a listed author, Simonoff J., Wise M (2012) A closer Look at Merelani Minerals., Mineral News Vol 28 No 2. I do not have this issue but will make an effort to get it and look forward to reading it. Concerning the Molybdenite wire, was this found as an inclusion or otherwise? All the best.

Robert Simonoff May 11, 2012 02:32AM
The molybdenite was found in several different. Including shallow inclusions - just below the surface of the tanzanite.

Rob Woodside May 12, 2012 05:51PM
I agree with Bob that at this point the specimen could be fairly labeled "Moybdenite? in Tanzanite", but it is certainly possible to nail the identity of these needles.

Does anyone know the cause of the blue colour in Tanzanite?

Could it be trace Molybdenum? Does the heating change the oxidation state of Mo to give the blue colour? Molybdenite and Ilsemannites are blue and the blue appears more intense around the needles in this tanzanite. This is rank speculation; but if true, then Mo should show up in an ICP analysis.

Edited 1 time(s). Last edit at 05/12/2012 05:54PM by Rob Woodside.
Robert Simonoff May 13, 2012 02:29AM
But the silver mineral at the bottom is still unidentified, but I would bet on graphite.

Rob, the color of tanzanite, according to Bernard Olivier's thesis on Merelani, is largely Vanadium (V3+ substituting for Al3+) within the crystal structure (blue violet coloring). the range seems to be between 972 and 6196 ppm in his study. He used spectroscopic and EPR analysis, which also shoed that the Ti4+/Ti3+ ratio within the structure was also important. Heating tanzanite causes an exchange:

Ti3+ V4+ -> Ti4+ V3+ (which increases the amount of blue/violet coloring)

Green zoisite seems to have more Ti than the other colors of zoitite
Blue ziosite seems to have the highest concentration of V
Colorless has the lowest concentration of V
Orange zoisites seems to have Sr substituting for Ca (not entirely tested in the paper) and Nd3+ substituting for Al3+

Note: I do find that the same work did identify tremolite-actinolite needles included in some tanzanite. I am not sure, however if this was tested or assumed (as the graphite needles were assumed).

If anyone wants to read this tome, it can be found here:

Ralph Bottrill May 13, 2012 05:18AM
Very unusual to find moly and graphite together. When I was taught mineragraphiy I was told they are very hard to tell apart in polished section but luckily occur in quite different parageneses. So much for that theory!

Rob Woodside May 13, 2012 02:57PM
Thanks Bob. The black streak of graphite should be quite distinct from the bluish grey streak of molybdenite and it wouldn't damage the base of the piece. So that should settle the identity of the metallic mineral at the base.
John A. Jaszczak May 15, 2012 02:29AM
Considering how much graphite there is in the world, molybdenite associated with graphite is probably rare; nevertheless,
I have molybdenite on graphite from Lime Crest quarry, NJ and graphite crystals on a molybdenite crystal from Moshi, Tanzania.
The latter is not far from Merelani. (Both are TINY)

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