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Identity HelpTsumeb piece.

26th Mar 2020 00:51 GMTGabriel Plattes

02138210015851837184963.jpg
I wonder if anyone can tell me what the terracotta-orange coloured crystal at the apex is? It is a Tsumeb piece, the white is mimetite, the green conichalcite, and the matrix appears to be galena. The shaft of the crystal is six-sided. The specimen measures 7.5 cm.

26th Mar 2020 02:27 GMTKevin Conroy Manager

Quartz and mimetite are both possibilities for the crystal in question.   A close-up may REALLY help.

The matrix is possibly tennantite.

26th Mar 2020 02:57 GMTGabriel Plattes

09773560015851913274015.jpg
Tennantite, nicely spotted, ta indeed. It compares to others here, not coated in druse (or rather, in far less).
This image is the best I can do for the time being, until I manage to figure out how to work these cameras better. I hope it is of some help.

26th Mar 2020 03:07 GMTGabriel Plattes

For qtz, the Min Rec issue of Tsumeb records: 'Colorless to milky and gray crystals reaching nearly a cm have been found...' (p. 30). A pity, none of this colour, if it is one, was mentioned.

26th Mar 2020 03:12 GMTGabriel Plattes

Pfw. Taking a look at the mimetite entry: '... wide variety of colors and habits. The crystals range from minute needles... large, prismatic crystals with pinacoid and/or pyramid terminations. The color ranges from nearly colorless to pale yellow, pale brown,... orange, red, green...'

26th Mar 2020 03:15 GMTKevin Conroy Manager

There are plenty of folks who have more experience that I with Tsumeb minerals, so let's see what they think.   I've seen quartz with other colors than what the Min Rec states (although some were colored by other minerals).

26th Mar 2020 03:08 GMTKevin Conroy Manager

My gut is telling me it's quartz.   It seems to be more opaque than the mimetites that I've seen from Tsumeb.   Also, the angle of the termination faces seem a bit too steep for mimetite.

26th Mar 2020 03:20 GMTGabriel Plattes

Very good indeed, thank you sir.

26th Mar 2020 09:01 GMTPaul De Bondt Manager

Could be quartz or mimetite but with already much mimetite present, I would tend to that.
But the matrix is tennantite and not galena.
Look at the triangular form of the crystals.

26th Mar 2020 09:38 GMTGabriel Plattes

In the drawer I had it sitting between galena and tennantite specimens, should have spotted what you blokes picked up on. One learns, no?
After Kevin's reply, I looked at the white coloured mimetites through my loop, and in a few places, they carry a little red-brown colour. 
In the dark cavity on the lower right of the image, sits another crystal as that at the apex, but perhaps 2/3ds its size. 

26th Mar 2020 09:47 GMTPaul De Bondt Manager

From the specimens I've observed, tennantite is sometimes associated with quartz but then the crystals are very slender and mostly water clear. In this association, no secondary mineral is present. Just the quartz. Mimetite should not fit in that tennantite/quartz association.
But with Tsumeb, you never know.
Always something new to discover, even if the mine is closed since almost 30 years.
That's why it's the greatest mineral deposit on earth !

26th Mar 2020 09:55 GMTGabriel Plattes

I think I have some like that here, as you describe, with qtz. The tennantite is barely coated with those. Just small clusters of qtz, no? The tennantite mostly bare. 

26th Mar 2020 10:19 GMTGabriel Plattes

First I loved the fey Whim Creek, then Tsumeb, now leaning Cornwall... Then, when the Keweenaw expert piped in, I began reading material on that... I was a bibliophile, irritable at folks who pushed books back on a shelf, to place minerals in front of them (still hold to this, books should sit as forward as possible on a shelf; also, loathe cross-shelving); but now, a love of minerals has firmly joined that. A geologist associate of mine, predicted this would happen. A fascinating world you all move through. 
Oh, Paul (& Dale), I must send through an image of a moss-copper from Cornwall, dating late c18, early c19. Delicate, and plum gorgeous. I imagine the mine cannot be picked, but lovely to think it could have been Dolcoath. 

26th Mar 2020 10:15 GMTFrank K. Mazdab Manager

Are all the abundant scattered little crystals that are of about the same color as the big one also little pyramid-tipped hexagonal prisms? (I can't quite tell because the image is too blurry). Then maybe the big one and the little ones are the same mineral? If you can convince yourself of that, discreetly chip off one of the small crystals from the non-display side of the specimen and try some tests on it (luster on a broken surface, tenacity/hardness [if possible to measure on such a small grain], fusibility on charcoal [+ garlic odor from As minerals], solubility in HNO3, density [if you have a pycnometer bottle and a sensitive balance]).

26th Mar 2020 10:24 GMTGabriel Plattes

Nice Frank. No, the crystal shape is different. I'll chip naught off it sir! ;) The clear-white ones are mimetite, its label records this. But some of the clear-white ones have some of that red-brown colour, partially coloured in that fashion. Still mimetite, but two periods of growth? First the red-browns (the two pyramid-tipped ones), then the clears? Some of the clears have grown on the apex pyramid-tipped one.

26th Mar 2020 10:37 GMTFrank K. Mazdab Manager

Another likely more costly alternative is finding a university or museum with an environmental SEM (which doesn't require a sample to be carbon-coated) that accepts external users, and scheduling a session. Many modern instruments have chambers large enough for your sample.

The sample goes in as-is (ideally with a flat surface of the crystal of interest as close to perpendicular to the electron beam path as possible), the chamber evacuated until the sample just starts to charge, and then an EDS analysis is done... even a rather poor, totally non-quantitative result should easily differentiate quartz from mimetite.

26th Mar 2020 11:11 GMTGabriel Plattes

Indeed, shall see what I can arrange. For now, I'll record mimetite on its label in square brackets (denoting educated guess) for the red-browns. 

26th Mar 2020 12:59 GMTGabriel Plattes

Frank, [I must insert this, should we take bets ;)], they were not elongated (the clear-whites), but they could have been stubby... From my naive recollection, they were dissimilar (I'd put no trust in this). 

26th Mar 2020 12:18 GMTJolyon Ralph Founder

I'm 90% sure it's quartz based on the photo. More photos would help

26th Mar 2020 12:45 GMTGabriel Plattes

Holy smokes, the founder! Queequeg, grab your harpoon! 
I've an associate with a Raman, I'll contact him, and we'll find out (assuming a Raman can do this, Frank?). I'm backin' in Paul, are we taking bets? ;)
In the meanwhile, I'll arrange for my colleague to take a detailed image through the microscope. 

26th Mar 2020 13:37 GMTFrank K. Mazdab Manager

Raman will work... the only issue may be in having a set-up available that will accommodate a specimen of that size.

30th Mar 2020 05:23 BSTGabriel Plattes

02081640015855421885144.jpg

30th Mar 2020 05:24 BSTGabriel Plattes

02639460015855422438615.jpg

close-up

30th Mar 2020 05:25 BSTGabriel Plattes

06320030015855422987372.jpg
From the first image, it would appear that the colouring is a coating. Qtz wins?

26th Mar 2020 12:47 GMTRolf Luetcke Expert

Gabriel,
Read the responses above and my first thought was to do a tiny scratch test on a small spot near the bottom of the crystal in a place where a scratch won't be seen.  A tiny needle will suffice but it will tell you if it is a quartz.   I know breaking off something I would not do either but a tiny scratch at the base would not hurt things much and it would tell.

26th Mar 2020 13:01 GMTGabriel Plattes

What would I need to look out for either way, if I did that Rolf? What sort of needle, just a regular one?

26th Mar 2020 15:10 GMTAlex Homenuke Expert

What about smithsonite?

27th Mar 2020 03:08 GMTGabriel Plattes

01616040015852783158009.jpg
Alex, hrmm, I could not comment on this.
I'm now leaning towards Jolyon and Kevin (but will stick with Paul for form), after having taken a gander at this Tsumeb piece here (one of those that Paul mentioned). Do please pardon the image, I shall work on my skills during next week. 6.7 cm.

27th Mar 2020 06:35 GMTKeith Compton Manager

Well that one is tennantite and quartz

30th Mar 2020 06:19 BSTKevin Conroy Manager

Thank you for posting these close ups.  I think that quartz is right.

30th Mar 2020 07:31 BSTGabriel Plattes

No, thank you mate.
 
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