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Identity HelpMultiple minerals associated with chrome tourmaline from Tanzania
21st Sep 2017 13:33 UTCAymeric Longi
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.
21st Sep 2017 13:40 UTCAymeric Longi
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...
21st Sep 2017 13:46 UTCAymeric Longi
21st Sep 2017 19:34 UTCRob Woodside 🌟 Manager
21st Sep 2017 22:54 UTCPeter Chin Expert
21st Sep 2017 23:15 UTCIan Nicastro
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.
22nd Sep 2017 00:22 UTCAymeric Longi
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!
Cheers! :)
22nd Sep 2017 12:21 UTCAymeric Longi
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.
23rd Sep 2017 00:03 UTCIan Nicastro
23rd Sep 2017 02:47 UTCJohn A. Jaszczak Expert
and rotate it around. The dicrhoism may be striking.
27th Sep 2017 14:56 UTCAymeric Longi
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.
27th Sep 2017 15:12 UTCAymeric Longi
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
3rd Oct 2017 21:38 UTCPeter Slootweg 🌟
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. These specimens were seen in the stock of Joyce Dronkelaar of Kilimanjaro gemstones.
3rd Oct 2017 22:30 UTCIan Nicastro
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.
9th Oct 2017 15:09 UTCAymeric Longi
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.
9th Oct 2017 15:16 UTCAymeric Longi
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 ?
9th Oct 2017 15:17 UTCAymeric Longi
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.
9th Oct 2017 22:46 UTCIan Nicastro
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?
12th Oct 2017 16:46 UTCPeter Slootweg 🌟
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.
30th Dec 2017 16:49 UTCAymeric Longi
My, first of all, I'm very sorry to get back to the discussion so lately, the past 2 months have seen me having to deal with a nasty bout of accute depression and those thing have a tendency to greatly affect my communication skills and so my visit on forums...
It's a thing of the past now and I'm glad to read more of these very interesting informations you've shared here.
A zombie is half-alive so I wasn't THAT inactive whilst absent and beside adding a few more chrome dravite combinations to my collection (more on this later ;) ), I also add the opportunity to have a small chat with the person I got the specimen from.
It happens that he actually was on site when those were collected and most unfortunately he was unable to put on name on the locality. Only could he tell me that it wasn't Landanai, but a "small, baren hill 50-60km North-West of Arusha".
About the Usambara effect. Indeed, the small crystal I mentionned look like schorl at first, although both have a closed c-axis (colour-zoned crystals as well). But about that photo I posted above of a cross section, I'm not sure it actually shows the said effect, but rather that it's just the colour of the cristal .
For Usambara & Chelsea filter I use a MityLite torch with xenon bulb, works great!
Wow, thanks for the chlorophyl tip, I certainly wouldn't have thought about that, amazing!
About the last specimen photos and orange crystals (thanks for the photo comment Ian :) my old 5Mp still got it!) , indeed, I too think it is some iron oxide coating, as at the base of the tourmaline is a colorless growth of what seems to be the same mineral, about which I wonder wether it could be calcite or not as just underneath the group of crystals is a mass of milky calcite, which upper side surface shows same crystaline feature (in any case it is not mica). Gypsum sound like a better guess than my celestite, I hadn't thought about mica for the staircase, thanks a lot!
As mentioned before I've added a few more combinations to my collection, of which I'll post photos for the sake of sharing information and maybe eventually discovering what is this mysterious locality.
Among those is this one on photos, which beside the tourmaline features a somewhat botryoidal material, of such a green color that it made me grab my chelsea filter and red it turned indeed! Could this be Chrome Chalcedonia? I don't think it's been repertoriated in Tanzania so far, right?
Dimensions are 5,45cm x 4,45cm x 3,73cm
FoV - #1: 1,9cm - #2: 3,5cm
Does anyone know which type of mineral with cubic, translucent, coloress crystals could be found in the environment yielding the discussed specimen?
Thanks for the shared knowledge :)
Cheers!
24th Jul 2018 16:31 UTCAymeric Longi
My, I didn't thought this thread was that old, but since the discussion had strayed beyond the sole subject of chrome-bearing tourmaline it seems kinda more logical to update this one discussion here rather than starting a new one.
Because some Elbaite is popping up, I assume maybe there are lithium bearing pegmatites cutting across these metamorphic deposits.
Well, Ian, looks like you assumed quite well as Darrellhenryite has been found lately at Mwajanga and for the most part they look at first glance very similar to those blue-colorless & olive green terminated crystals. The person I got some from was on field trip these past weeks and his colleagues couldn't tell me much about these, but he's back now and I'll try to get more information in the coming days (at least to confirm it is indeed from Mwajanga...).
In association can be found albite, a white and very soft mica (polylithionite?) and quartz, small and clear crystals of it, seems to be rather common.
Trying to show the different shades of pink here...
This crystal is actually mostly facet grade, deep rubellite color.
Some of the blue caps are themselves topped with a "toupé" of acidular crystals, are those tourmaline too?
24th Jul 2018 17:01 UTCIan Nicastro
2nd Aug 2018 13:49 UTCAymeric Longi
Interesting, I have never heard of Darrellhenryite before now.
Same for me, until a month ago I had never heard about Darrellhenryite and had I not been intrigued by the name I still wouldn't... I have seen earlier this year some material of pink colour that I found in a small lot of rough of "yellow dravite". Vivid pink colour, but with open c axis of intense yellow-orange colour. I'm really surprised that there are so few Tanzanian tourmaline to be found around (beside the chrome-dravite of course), there's a such a variety!
The 2nd photo you posted, I posted photos from that same find before
Is it the same find? According to seller the Darrellhenryite were found in March 2017 and he thinks/hope he got the whole pocket?
the crystals usually have blue caps at one end, white-colorless middles, and black to lime green caps at the other end...
The bi-terminated pieces look very similar indeed, although there is some peachy hue to the greenish terminations.
Seller told me it's a mix of Darrellhenryite and Elbaite. The person ifn charge of the lab for analysis is currently on vacation, so I guess I can get more detailed information after his return later this month. I'll sure post about it!
I'm glad I didn't toothbrushed this one... neat little quartz crystals on what must be achroite tourmaline. FoV a little more than 1cm.
2nd Aug 2018 20:22 UTCPeter Slootweg 🌟
Interesting post. Darrelhenryite is new to me too. I had to look it up and was surprised how little the difference with elbaite is. I guess it wil take some sophisticated equipment to tell the difference. I tend to be skeptical about the Darrelhenryite designation. Especially from Mwajanga as the geology there is so different than the type locality Nova Ves, Bohemia, Czech Republic.
I too noticed the bit if pinkish/peach color in the dark portions of the Mwajanga blue cap crystals when backlit. I thought it to the confirmation that the black cores were dravite instead of common schorl. Ian, New Era Gems and others all confirmed the colorless, green and blue sections to be elbaite. I’m not sure however how easy it is to tell the difference between elbaite and Darrelhenryite. Maybe it is Darrelhenryite, but I feel it is unlikely.
It sounds a bit like a marketing ploy to generate more interest in the material. Let’s hope I’m wrong.
4th Aug 2018 14:43 UTCAymeric Longi
It sounds a bit like a marketing ploy to generate more interest in the material. Let’s hope I’m wrong.
Well, the seller I got these from trades in quite large volumes and the small lot of Darrellhenryite represents litterally a small drop of the same volume. I can't see what benefit he would get doing so. As for the person who bought the lot in question (me), he's an absolute maniac when it comes to species, localities, descriptions and what-not. Beside the fact that it would be extremely unhonest (and so a potential business-killer), that would mean inflicting to myself some real mental torture at trying to explain or convince and what-not.
Especially from Mwajanga as the geology there is so different than the type locality Nova Ves, Bohemia, Czech Republic.
As Ian mentionned it at the beginning of the discussion, aren't elbaite a sign that there might be some pegmatite over there?
I too noticed the bit if pinkish/peach color in the dark portions of the Mwajanga blue cap crystals when backlit.
No, no, when backlit, the colour is of bright peachy to orange-red with a pink hue added. Those crystals with the black part showing a pink hue are doing so under directed light, not transmited.
I thought it to the confirmation that the black cores were dravite instead of common schorl.
Looks like there are different configurations to be found actually. Some crystals (opaque or not) show a thin (a tad more than 1mm) pink rim envelopping a thick core which seems to be of the very same colour than the regular Mwajanga Dravite (Root-beer ones with chatoyant termination). At least the specks of colour that are visible at the base of crystals in questions are absolutely similar to what can be seen on, or rather under, those dravite. See first photo of this post.
For some crystals, the core seems to be glassier, there's a very faint pinkish hue to the black and instead of the rather numerous orange-red specks that can be observed with dravite (or what i think to be so) there are just a few couple specks of a vivid blue colour.
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.
Same here, only associates are albite, quartz (lots of tiny, clear crystals) and white mica, very, very soft one.
Green terminations with a pink tip
Crystal mostly gemmy and of deep pink colour only, no colour-zoning of any kind, this seems to be the rarest variation.
Crystal "section" showing faint pink rim around red-orange-ish-specked core
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