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Multiple minerals associated with chrome tourmaline from Tanzania
Posted by Aymeric Longi
Aymeric Longi September 21, 2017 01:33PMGreetings everyone,
This past summer have seen me becoming the happy owner of a few chrome tourmaline on matrix from Tanzania and beside the superb color I've been, and still am, quite delighted at the mucho large variety of minerals which do come along the lovely greenery. There's just one thing though, beside the calcite and pyrite, I just can't figure out what are those minerals as my knowledge in Tanzanian mineralogy is still much limited. I've done sone search here, and beside a post of mine about inclusions in other chrome tourmalines I haven't found anything at all.
One first thing, the seller told me those specimen where from "near Loliondo" and when I suggested nani Hill he answered positively. I kinda have some doubts about the locality and, after doing some internet search, I'm actually tempted to think they could from Landanai.
Here is one first photo showing the "mineralogical set up" of most specimen.
The base is made of what looks like greenish quartzite, then comes a layer of orange-yellow color, with pyrite and some micaceous material. Note that when scratched with a metalic point the material releases a strong scent of sulfur (this is what makes me think about Landanai, since the locality is know for this feature, well, Merelani too actually...).
Above the orange layer comes a crust of calcite where are found most of the tourmaline crystals (most of the time partial crystals, broken before being embedded in the calcite layer).
Some specimen such as this one shows a fourth layer, which is made of a tiny bits of 4 to 6 different minerals tightly agglomerated together.
I have 3 specimen, each showing different minerals in association with the chrome tourmalines.
Edited 1 time(s). Last edit at 09/21/2017 06:35PM by Aymeric Longi.
Aymeric Longi September 21, 2017 01:40PMThis first speciimen is 1,22cm x 1,24cm x 1,24cm
It is partially covered with a colorless to white to yellow-ish crust, which shows botroydal features here and there. It shows no reaction to the acid test, so calcite it is not.
I've thought about fluorite, I just don't know...
Aymeric Longi September 21, 2017 01:46PMOn the same specimen, there's also a small group of at least 4 different minerals, one being some acidular crystals, another also metalic but just small grains/crystals. 2 other type of crystals are also presents (short prismatic, tabular/blade-like). All are completely embedded in the crust mentionned above, but the two non-metalic ones might be featured on another specimen which will be discussed after this one.
Ian Nicastro September 21, 2017 11:15PMI have not seen this yellowish crust before on tourmaline from Tanzania, have you tried scratching it to determine the hardness? I am not familiar with the geology around Landanai, but I believe you are correct that the original location given was not correct. Come to think of it.... Landanai seems to not even be listed on mindat. It seems like the only place I can find that location listed for tourmaline is the Marin Minerals website. The stuff I have seen attributed to Landanai looks a lot like the stuff from the Merelani Hills. As Rob suggested yellow Prehnite comes from the Merelani Hills. The crust could also be opal-AN, or Calcite, or some sort of Zeolite, etc...
Previously, I had talked a bit with Jochen Hintze, a german dealer who has visited Tanzania, as well as Cal Graeber and Peter Slootweg, who are all very knowledgeable about Tanzania. However, the focus of those conversations was on the new tourmaline coming out of the hills around Mwajanga, near the village of Komolo in the Simanjiro District of Manyara Region. I've gotten to handle a fair bit of matrix pieces from the Commander Mine, as well as matrix pieces from the new blue capped white tourmaline with dark bases from that area, and the matrix on all of these is very interesting... a lot of albite feldspar and calcite, with minimal quartz. Here is what Jochen had to say about Mwajanga: "It is not really a village but a mining area about 15 km far from the masai village of Komolo. There are several hills full of so called foxhole mining. All pockets are bound to a metamorphic dolomite, but very important: at the beginning of mining tourmalines were found in and with natrolite crystals (!!), now the tourmalines are found in calcite."
So basically we have dolomitic marble in this area around Mwajanga that has produced yellow, yellowish-brown, brown (some of which had steel blue fibrous caps), olive green pencils (associated with natrolite), and deep green tourmaline (Commander Mine) which have all been characterized as Dravite from the dealers who had for sale. The new blue capped white on dark tourmaline tested as Elbaite, and previously there was a very small find of thin blue capped pale pink and yellow tourmaline from the area that also appeared to be Elbaite. Because some Elbaite is popping up, I assume maybe there are lithium bearing pegmatites cutting across these metamorphic deposits. Although this might not be where your specimens are from, I imagine the geology of Mwajanga could be similar to where your stones are from.
I've been able to track down research articles on the geology of Merelani previously while looking up info on the Tsavorite deposits around Merelani that may be of use to you:
*The geology and petrology of the Merelani tanzanite deposit, NE Tanzania. Available from: https://www.researchgate.net/publication/44138046_The_geology_and_petrology_of_the_Merelani_tanzanite_deposit_NE_Tanzania
*Evidence of evaporites in the genesis of the vanadian grossular 'tsavorite' deposit in Namalulu, Tanzania (PDF Download Available). Available from: https://www.researchgate.net/publication/231589989_Evidence_of_evaporites_in_the_genesis_of_the_vanadian_grossular_'tsavorite'_deposit_in_Namalulu_Tanzania:
"The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li2O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite."
*New aspects and perspectives on tsavorite deposits (PDF Download Available). Available from: https://www.researchgate.net/publication/236592700_New_aspects_and_perspectives_on_tsavorite_deposits.
Edited 1 time(s). Last edit at 09/21/2017 11:17PM by Ian Nicastro.
Aymeric Longi September 22, 2017 12:22AMGreetings everyone,
Thank you Rob & Peter for your suggestions, I hadn't thought about prehnite indeed!
Wow, Ian, thanks a lot for all this information, that is great!
Indeed, Landanai seems to be Marin's specialty, although I remember to have seen the locality mentioned in some mineral show report, in a section about some chrome tourmaline new find in Landanai, I'll try to find the link and post it.
I know about Mwajanga, lots of lovely things coming from there indeed, I got a few of those amber-ish red/orange/yellow ones with chatoyant terminations (attached photo), so beautiful !
About hardness test I've got a "side piece" which can take some scratching without risking some damage, will try and see tomorrow then report here. I also will post pics of the second specimen so as to further this interesting discussion :) Thanks again for all the information!
Aymeric Longi September 22, 2017 12:21PMFound the link, it was in St-Marie Sainte-Marie Mineral Show Report 2012
Ian, have you heard anything about color-zoned chrome tourmalines, such as this one pictured here ? The very few I've seen on sale were also labelled as from Landanai.
Ian Nicastro September 23, 2017 12:03AMI've not see one like that from Landanai before with yellow-brown at the base. Thanks for the link... and come to think of it, I recall seeing Landanai labeled material with Cal Graeber as well, so that is 3 different dealers that have had material labeled from there. A lot of the Commander Mine stuff had yellow-brown bases, so it seems quite likely that it's a deposit with similar geology. I'm pretty fascinated by all of the weird Tourmaline that Tanzania produces. I'm keeping my fingers crossed for someone to find blue Dravite there someday. Check your Tanzanian tourmaline with UV (both SW and LW), some of them are fluorescent.
Aymeric Longi September 27, 2017 02:56PMGreetings everyone,
Ian, indeed, there are quite a few dealers offering material from Landanai. I've got more pictures to look at and "explore". While selecting these I actually found more unidentified crystals (real tiny-tiny red ones) :)
Blue Dravite ? That sure sounds lovely :p Chromian or not, those Tanzanian tourmalines are quite fascinating indeed and in my opinion they are not getting the place they deserve. I used to think of dravite as those classic nepalese brown and often dull crystals, but Mwajanga has re-written it all I think and set new standards as well.
John, this crystal pictured above shows no notable dichroism actually. I have some which show two different emerald green tones but the color-zoned has a closed c-axis.
About this color-zoned one it appears that the yellowish part shows very very small reaction, if any, under chelsea filter, but the bright green termination does turn a nice red color.
I have another color-zoned crystal, but inverted with a yellow-ish termination, and here two, it's the greener part that reacts under chelsea filter.
As for the striking dichroism, I have a cristal section showing a yellow-ish green colour sideway but when looking through c-axis, the colour turns darker, at tad amber-ish, but with striking red flashes. I don't think this is some usambara effect though.
Aymeric Longi September 27, 2017 03:12PMTo continue with the mysterious locality's mineralogy :)
I've got another specimen that is packed with non-tourmaline minerals (4 of metallic appearance, 4 non-metallic), unfortunately my old 5Mp meets its limits with some of them as some are quite tiny.
The photos joined here focus on the metallic ones.
- one is of a flaky appearance, covering in layers some parts of the specimen. Streak test gives an orange-ish brown color, quite ocre-like.
- Another presents itself as a roughly botryoidal mass, with surface covered with tiny spherules (at the base of this mass I found a small group of reddish crystals, too small to get a clear picture alas). Streak test produced very same color than flaky material.
- Another is a crystal with bi-pyramidal geometry (unless it's actually two pyramidal crystals joined together...). Nearby is a light color area made of transparent crystals of a cubic shape (not really colorless, more of a cream-ish pink-ish hue). Crystal is about 0,8mm long
- and last one is more of a flatened wire shape. longest is about 2mm.
FoW photo 1 is about 1cm
Peter Slootweg October 03, 2017 09:38PMHi Aymeric, Tanzanian tourmalines are very interesting indeed! As Ian pointed out, there are several area’s in Northern Tanzania producing this magic stuff.The mineralogy for these is quite simple comparing to the Merelani mines although they are all part of the same geology. Merelani however is a special case that has yet to produce these green tourmalines as far as I know. There are several tourmalines attributed to Merelani here on mindat but I suspect they come from the greater area south of the Tanzanite locality.
These dravites are all form marbles. Calcite/dolomite is the most common association together with pyrite, graphite, rutile, mica, quartz, feldspar and all the iron oxides formed by decomposing pyrite. Sulfur is found but rarely as crystals but mostly present as H2S in the matrix as you noticed by the bad smell when broken.
The coating on the tourmaline fragments is likely to be a low temperature silicate. Chalcedony or opal is a good guess. The other minerals on the third specimen could well be tourmaline fragments of a light color, there appears to be a small colorless tourmaline crystal in the mix as well, all covered by the mystery coating. The long black needle is rutile. I have seen similar needles in the commander mine material with were undoubtly rutile due to the twining shown.
Dravites with strong color zoning from yellow to dark green to colorless are believed to be from Landanai. It is not uncommon (https://www.mindat.org/photo-572047.html). The specimen with the striking dichroism to red shows the real Usambara effect, no doubt. Most of the chrome tourmalines do. If they turn red under the Chelsea color filter they have this property. You will only need a very big crystal to show it in normal lighting conditions. Not many are so chromium/vanadium enriched to show it in small specimens like the ones from the Umba valley north of the Usambara mountains.
I use a homemade Usambara filter to check these stones. A flat piece of very dark material with a thickness that is just under the tipping point for color change. If you put a very light green chrome dravite over the transmitted light it will turn the stone pink to red.
I think your last specimen show a host of iron oxides in different phases coating or replacing pyrite, mica and feldspar.
Thank you for posting this interesting material
Ian, Its a pity the new blue caps turned out to be “just” elbaite. No blue dravite yet. I have a blue dravite , Raman confirmed, but its from a bit south. Northern Mozambique to be precise. Ill see if I can make a picture for you. The blue caps from Mwajanga can be quite big as shown by the lot is saw lately. Surprising to find these are on a typical pegmatite matrix as opposed to the marble occurrences. the only associates were albite, quartz and a white mica.
Here is also an unusually big and rich specimen from the Commander find. It’s about 20 cm wide. Dravite with calcite, green and white mica, rutile, minor quartz on a graphite schist.
Ian Nicastro October 03, 2017 10:30PMNice to hear from you again Peter, great information on the accessory minerals. I agree with the Usambara effect being witnessed in the above specimen of Aymeric's. I had no idea that green Dravite attributed to Merelani was incorrect... but I guess that makes sense as I have never seen any specimens of Tanzanite, Cr/V Diopside, Tsavorite or Prehnite combined with Cr/V Dravite. Whereas all the previously mentioned minerals can be found together.
It's interesting you noted that the accessory minerals you saw with the new blue cap find looked like typical pegmatite associated minerals. Cal Graeber had a bunch of large matrix pieces from this find, and he pointed out that only a couple of matrix pieces even had quartz crystals associated with them (I will attach a photo of one of the samples that had quartz points). Most of his matrix pieces had albite matrix, some with what appeared to be calcite present, but I can't confirm that it was in fact calcite.
As for the tourmaline identity, Cal had some of his tourmaline tested by a lab in Europe and his results showed that the dark blackish sections of the crystal were Dravite, while the whitish sections and blue caps tested as Elbaite. I believe that when Attard tested Steve at New Era's samples he said the blackish sections were mostly Schorl, with some Dravite component. When I had a single sample tested by mass spec (and I should note the sample had a gemmy clear section compared to most other milky white samples) the blue and clear section were Elbaite and the black part tested as Schorl.
Edited 1 time(s). Last edit at 10/03/2017 10:32PM by Ian Nicastro.
Aymeric Longi October 09, 2017 03:09PMGreetings everyone,
Thanks for jumping in and sharing your knowledge Peter, this is very interesting information :)
About the coating (all 6 pictures show one & single specimen actually), after a bit of thinking I realised that prehnite was quite unlikely considering that the material in questions shows different colors (white, colorless, green, orange/gold) which in my guess would be improbable with prehnite. I managed to scratch it very lightly with a quartz tip. One of the specimen I have harbours a small vug (1cm wide at most) which is partially filled with some botryoidal material, although thicker than just a coating, colorless and totally water-clear. In some areas, the said material (along with some colorless crystals as yet to le identified) contains small inclusions, which are also botryoidal but with a metalic sheen and strong iridescence. I haven't managed to get some clear photo so far, it is quite small and my old 5Mp is reaching its limits here.
About Usambara effect, I certainly didn't know that all, or most at least, chrome-bearing dravite can display it. Thanks for the home-made filter tip, would some radiography film do the trick?
From what I can see from my small collection, it seems that the size of crystal has nothing to do with the display of discussed effect. I would rather implicate first & foremost the colour saturation. I have two crystals (one is 0,75cm at its widest point, the other 1,30cm) with a level of color saturation that is such as they look like schorl at first glance, but if I get them close to my xenon torch, whole sections turn red and for the larger of the two the whole crystal look as if seen through chelsea filter. They too react under direct sunlight.
About the color-zoned crystals, the Usambara effect seems to be mostly located around the dark stripe separating the yellow-ish & emerald-ish greens. Isn't that odd? How come??
Is the particularly high chromium/vanadium content a specificity that distinguishes the dravite coming from Umba/Usambara from those of other localities?
About specificities, the first crystals I've added to my collection happen to bear different types of inclusions, mostly pyrite/Pyrite-to-Limonite along with tiny white crystals. It seems rather uncommon as I haven't seen much of these, be it here on mindant or on the internet retailers' pages. The seller I bought them from told me it was from Usamabara Mnts, without other detail. Do you know about any locality producing crystals with inclusions ? attached photos show one of these.
Aymeric Longi October 09, 2017 03:16PMOtherwise, to continue with associations, the following photos show details of a specimen which harbours I think at least 3 different minerals beside the tourmaline. A set of colorless ones, some of which kindof reminding me of some celestite crystals I have. Beside those and growing on the crystals along c-axis is a small "staircase" of a pale golden green-ish color, and last but not least, at the base of the tourmaline is a rather large group of mostly transparent crystals of a golden orange hue. I'm wondering if the greensih and orange ones are not the same actually.
The salmon-pink coloured mass at the base of crystals is probably same as the orange crystals, for the whole surface is well crystalized, with same features as orange crystals.
Amazing specimen by the way, 20cm large, that is sure some humongous specimen for chrome dravite!
Ian, your elbaite on dravite is gorgeous, what a blue ! Is the large green crystal on matrix a chrome-bearing one ?
Aymeric Longi October 09, 2017 03:17PMLots of photos for this one, shot under different light conditions and at different angles. As for the scale, the tourmaline crystal is 0,35cm at tits widest.
I'll try to get better pics of the small vug I talked about and in any case another specimen will follow, with a different association of course, small crystals which look a lot like topazes actually.
Ian Nicastro October 09, 2017 10:46PMThat stout shape and very very deep saturation suggests it's from either the finds near Landani or the older finds from the Umba Valley, based on what I have seen online.
I would assume that the recent find of the blue capped elbaite-dravite/schorl don't have a high chromium content, because they aren't very green. The dark part of the crystal is mostly blackish. The pinacoid (flat) terminations are the ones with the blue cap and white layer under, while if you come across a DT crystal, the pyramidal termination ends up having a thin lime green layer/rind.
The orange crystals are weird. I'm having trouble IDing it, although these are good photos. Do we think the flat colorless crystals are Natrolite or Calcite?
Peter Slootweg October 12, 2017 04:46PMRegarding the usambara effect. The specimens who show the effect most clearly are the dark ones hat look like schorl. The saturation of the chromophores must be quite intense to show the effect in small specimens. The effect is most notable along the c-axis (usually the darkest color in any tourmaline) and depends much on the light used. The more red in the light the easier it is shown. If you take one of these white LED’s that is almost blue light, it might not show at all. The effect depends on the ratio of thickness versus saturation. I have specimens that show the effect with only 5 mm thickness. Others need up to 23 mm to show the effect (personal observation). Even dark ones may be color zoned with the top showing less usambara effect than the bottom parts. About the filter, I not familiar with radiography film, so I do not know if it will help. What do helps is a simple plant leaf. The darker the better. Chlorophyle, the coloring agent in plants shows this effect as well because they have a similar light absorption pattern. If you put al leaf between the stone and the light source you will notice the effect gets stronger. See also: http://www.nordskip.com/usambara.html
The high chrome/vanadium content for showing the effect is not restricted to the Umba/Usambara area. I have seen specimens form all over north east Tanzania with the effect. They have also been reported from Kenya and as far south as north Mozambique. It has also been known for al long time but was never described properly. I was surprised to find a parcel of dark green dravite in the stock of an old gem cutter who bought the parcel in the seventies, reportedly from Kenya, with a clear usambara effect. The stones are usually to dark to facet so the never did get any attention.
Many dealers will tell their stones are from Usambara mountains. The mountains themselves never produced any Usambara material as far as I know. The actual mine/location is a place called Nchongo in the Grevi hills situated north of the Usambara mountains in the Umba valley. This locality became famous for the fine crystals found there. Considering the inclusions, Calcite, feldspar, mica, pyrite and rutile are common. I would guess it would be one of the first three. Hard tot tell from a photo.
The combination on the last specimen is interesting. I’m not sure the brown color is actually the minerals color or iron oxides from the decomposing pyrite. In my opinion the brown minerals would be iron stained mica (staircase crystals) and calcite with colorless (pointy) gypsum crystals. That may sound strange, but gypsum would be expected as a late stage mineral in a sulfur/calcium rich environment. I have been personally offered gypsum specimens with graphite at the Merelani mines were it is a common constituent of the rocks and indicator mineral for the miners (I never bought one for they were unrealistically expensive and heavily damaged). I noted that in many cases the mica in direct contact with the chromium dravite is also green while other mica on the specimen remain colorless or are brown from iron oxides.
Check for cleavages and hardness on the brown minerals to be sure. A tiny drop of HCl may help as well to figure out if there are any carbonates among them.
Ian, your right the green side on the blue caps is not colored by chromium, at least it does not respond to the Chelsea or my “Usambara” filter. Since they are elbaite, that may prevent the chromium to enter the structure or something. It is interesting to note that all chrome tourmalines appear to be dravite/uvite. I have yet to see a report on a chromium elbaite. If anyone is aware of a chromium elbaite I’d like to know. The material has been reported from Mogok but never been validated.
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