Needle Inclusions in tanzanite

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?

Are these Jessica's molybdenite tubes??? Why do they appear more blue than the surrounding tanzanite?

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.

Bob

I don't have an opinion graphite vs. molybdenite.
But what a nice crystal!

Wonderful specimen! Raman spectroscopy can distinguish graphite from molybdenite- perhaps even inside the tanzanite if it doesn't fluoresce, non-destructively.

Hi Dawid et Al,

I don't have access to this paper but was able to view the abstract. [gia.metapress.com]

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."

Ron

I would say it is more likely tremolite-actinolite and looks blue because it is darker than the background.

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.

Thanks
Bob

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 !

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.

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?

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.

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!

Bob

The base photos. Small and green is probably - garnet - tsavorite.

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.

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.

Ron

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.

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.

Bob



Edited 2 time(s). Last edit at 05/10/2012 05:30PM by Jessica and Robert Simonoff.

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.

Ron

The molybdenite was found in several different. Including shallow inclusions - just below the surface of the tanzanite.

Bob