FMF - Friends of Minerals Forum, discussion and message board

[John S. White]

For me one of the more interesting things to appear on the mineral market in recent years is the golden kyanite from Sangulungulu, Loliondo, Tanzania. The color is extraordinary, the pieces, although usually small, can be gem qualtiy and some have been faceted. Those that appeal to me most are the "V" twins as seen in this photograph. This piece is 4 cm across. Few are ever terminated, but that is typical for kyanite. This is also the locality for the spessartines of the same color which occur in fine large euhedral crystals showing the dominant trapezohedron, at times slightly modified by tiny octahedron faces.

[robynahawk]

I have to agree John - there was a vendor selling it as Orange Kyanite at the Riverpark - it is definitely one of my favorites from Tucson!

Robyn

[Pete Richards]

Very pretty specimen, John. But here's the comment that you might have anticipated from me: I don't see that this is obviously a twin. Perhaps it's a question of the photo. But how are the two parts of the twin oriented in the photo?

Has anyone determined the twinning relationship? Is it consistent from specimen to specimen? Dana's System vol. 8, with all its problems, does not list a twining relationship for kyanite that would produce v-shaped twins, it seems. Can you elaborate?

[John S. White]

Well Pete you may have caught me making an unsupportable assertion. I have assumed twinning because this relationship is so common with this new kyanite. I have two examples myself that appear to have the identical angular relationship. The Handbook of Mineralogy describes lamellar twinning on {100}, twin axis normal to {100} or parallel to [010] or [001], common, but I doubt that this would produce the butterfly relationship we see here. The fact that these "butterfly" twins appear quite common makes me want to believe that there is yet another twin law for kyanite heretofore unrecognized. Why not?

[Pete Richards]

It's entirely possible that these kyanite crystals display a previously unrecognized twin law. The fact that there are many examples with the same apparent angle helps make the case for twinning. I have not seen any of these personally, so I can't say that I'm convinced they're NOT twins, but as John points out, the twinning described in the texts would not seem to lead to such a morphology. I'll hope to see some examples at the Rochester Mineralogical Symposium.

[Luiz Menezes]

Hi John and Pete:

I think these "V"-shaped clusters of golden kyanite crystals from Tanzania are twins; I bought several of these crystals at the Riverpark from Werner Radl (Mawingu Gems, probably the same dealer that Robyn has mentioned), and the angles look consistent. My specimens are in my office and I will only be able to photograph them on Monady, I will try to put 2 of them side by side to try to demonstrate that they show the same angle.

Confirming John's information that they came from the same locality for the nice orange spessartine crystals that have been recently offered to the market, I am annexing below a photo of a nice specimen that I bought at the 2009 Mineralientage München from Werner Radl.

Regards

Luiz

[John S. White]

A really wonderful specimen Luiz. It certainly answers the question about whether or not the two minerals occur at exactly the same place.

[Jim Prentiss]

Hello Gentlemen,

Just today, at a semi-local gem and mineral show, I became acquainted with the orange Kyanite. It is not sepecially pretty but it is translucent with a nice amber-gold color in transmitted light. That said, where might I find more information on the occurrence in Tanzania.

Thank you,

Jim Prentiss

[John S. White]

The following was found on mindat.org:

A new locality for fine spessartine garnets. Located about 35 km NE of Loliondo proper "in the middle of nowhere." A village called Lemisikio is nearby.

"This deposit was visited and studied by Mr. Jochen O. Hintze, German geologist. He described it as an approximate 100 meter wide quartz mass or vein with weathered detritus. The quartz is criss-crossed with veinlets of hematite. The spessartine garnets are bound to the hematite in the veinlets. Most work to date has been digging into the loose weathered material to retrieve the garnet crystals. Only a little work has been done on the intact quartz mass. (priv. comm. J. Hintze to C. Lemanski, Tucson, 2008)."

The place name is new and was assigned by the local Massai tribesmen. The spelling was graciously provided by Mr. Hintze from his field notebook. The locality (new village) is growing rapidly with the arrival of new residents responding to the "Garnet Rush."

References:
Lapis (2008): 33(7-8).

[Gerhard Niklasch]

I got this stone on the cheap at a recent auction---- oops, wrong thread! ;)

Ok, kidding aside: Just after this present thread took off, I serendipitously discovered that such an orange presumed-twin Kyanite was being offered on auction by a very respectable vendor (Marcus Oglieri). My curiosity was piqued, and I won it at a very reasonable price. It's now in my collection as 10TZK-001.

Over the course of the last couple of weeks, I've attempted to figure out the hypothetical twinning law, and I'm going to report on my findings in this post and the next.

But first another reference:
Karan M Chadwick and George R Rossman, Orange kyanite from Tanzania, Gems & Gemology (2009) 45#2, 146-147. The abstract is available online at
http:(slash)(slash)minerals(dot)caltech(dot)edu(slash)Manuscripts(slash)2009(slash)Orange_Kyanite(slash)Index(dot)html
The color is attributed to Mn^3+.

The little two-winged beauty is just 20mm tall and about 14mm wide, so all of the following photos were taken through the stereo microscope. All the stereo pairs are crossed-eyes. (If you prefer parallel viewing, you can use your browser's 'Save image as...' function to download and save the image, and use your favorite image manipulation software to cut it in halves and stick them back together the other way round.)

[Gerhard Niklasch]

I thought it would be a very simple exercise the establish the opening angle between the two wings, but my investigations were severely hampered by the uneven surfaces. The best I can say with confidence is that the angle is between 117° and 123°!

I tried a variety of methods:
* Simply looking down along the hinge line onto a sheet of paper and drawing the projection,
* taking photographs viewing along the hinge line and measuring the projected angle in the images,
* jerry-rigging a horizontal single-circle goniometer out of an old theodolite, with a white LED a few meters away as the signal, and with the crystal placed on the alidade with the hinge line upright,
* and finally, using several of the prism angles of two solid apatite crystals for comparison, holding the kyanite wings against the prism faces and wiggling them for best contact whilst viewing along the edge with one eye.

The results in each case were reasonably repeatable and reasonably in agreement with each other, but there are simply too many shallow hillocks and smaller-scale roughness on the inner faces of the wings to pin down the angle better than to +-3°.

But fortunately, the opening angle isn't the only parameter we can read off!

[Gerhard Niklasch]

Whoops. For all prior proofreading, now I see that I've misspelt Marcus Origlieri two posts up. My apologies!
Pleading DST lag...

Cheers, Gerhard

[John S. White]

Excellent work Gerhard. Perhaps now we can convince Peter Richards that these pairings are indeed twins and stimulate him to work on them himself. As mentioned, I have two examples that I will let him examine and Carl Francis reports that Harvard University has a "twin" in blue kyanite that shows the same relationship, and he will also lend this to Pete.

[Pete Richards]

This is a very nice analysis! I will look forward to doing some measurements on my optical goniometer to confirm that the twin law is {12-1}. Working with just the previously posted photos and trial and error using SHAPE, I came up with {121} as a tentative twin law. This gives angles close to those produced by {12-1}, but the difference should be easily detected when some specimens are in hand!

[Elise]

I was very happy to see a continuation of this discussion in greater detail in the latest Rock's & Minerals magazine "A Re-discovered Twin Law in Kyanite from Africa"
by R. Peter Richards, John S. White and Peter B. Leavens Vol. 87, No. 2 March/April 2012
http:(slash slash)www.rocksandminerals(dot)org/Back%20Issues(slash)2012/March-April%202012(slash)twin-law-abstract(dot)html

It inspired me to dig out my treasures from the last Tucson trip (still haven't unpacked). On the last day I ran across this specimen marked down from $40 to $5 and was happy to add it to my study collection. It is not quite as transparent as the one in Gerhard's photos above, but it has wonderful inclusions of several types. The red crystals are the most interesting -- they seem to be isotropic and many are angular and are reminiscent of a euhedral garnet - these are perhaps spessartine given the locality, but it is very hard to tell (rutile has also been reported). It was difficult to manipulate the whole specimen under crossed polars and observe the inclusions' reaction at the same time, that is why I am unsure of their nature (slightest trembling of fingers made the inclusion jump in and out of the field of view). It would be easier to just snap that thin section of the right blade off and play with it under the microscope - I'm just kidding, it is a great little specimen paired with this thread and the R&M article! I'll just have to stop drinking coffee and steady my hands to figure those red crystals out!

This is what makes FMF fun!

Best wishes,
Elise