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Identity HelpLustrous black prisms with bastnäsite from Zagi Mountain

16th Jan 2020 21:32 UTCNick Gilly

01995550015792103512306.jpg
Well, I have finally bought a bastnäsite, after the dravite specimen last year that was mimicking one. I've literally only bought it tonight, so it'll probably be a couple of weeks before I have the specimen in my hand, but the matrix looks quite interesting. There are numerous black lustrous striated prisms in the matrix. I'm guessing these could be riebeckite looking a photos from Zagi Mountain on Mindat. Anyone else agree, or could they be something else? Just though I'd ask as they look quite distinctive. Pic included from the auction.

Thanks if anyone can help.

16th Jan 2020 21:35 UTCNick Gilly

02325790015792104973370.jpg
Another slightly better view of the black mineral.

16th Jan 2020 22:47 UTCAntonio Nazario

I would say either rutile or riebeckite. 

16th Jan 2020 22:14 UTCKevin Conroy Manager

Rutile?

16th Jan 2020 23:00 UTCJosé Zendrera Expert

Black crystals could be aegirine.
What about bluish clear crystals in second photo? Fluorite?

17th Jan 2020 01:51 UTCFrank K. Mazdab Manager

remember Nick, a small crystal discreetly chipped off the back of your rock, coarsely broken, and viewed up close with your hand lens will give you a wealth of cleavage/fracture information to easily differentiate the black minerals so far noted in this thread... :-)

17th Jan 2020 06:49 UTCNick Gilly

Thanks guys. It'll obviously be easier to give more information when the specimen arrives, but I was interested to see what others thought.

Re: second photo: I don't think those crystals are bluish, maybe a bit greyish. It's just the way the pic has come out. Pity the fine detail is masked by compression artefacts. As this is an Alpine-type deposit I presumed the other minerals were quartz and some sort of feldspar.

30th Jan 2020 18:10 UTCNick Gilly

Well, the specimen has arrived today, and on close inspection I suspect the thin black prismatic crystals are aegirine. There's no pics of aegirine from Zagi Mountain on Mindat but it looks very similar to other pics of this mineral.

The colourless to white crystals are presumably feldspar, maybe orthoclase, with a few quartz scattered about.

The bastnäsite crystals are nice, with lustrous prism faces and matte terminations for some reason. As has been mentioned before by others, there does seem to e a bit of a colour shift with these crystals too, as they appear a rich orangey-brown in incandescent light, and a more yellowish-brown under a daylight-type LED bulb.

Overall a pretty neat specimen!

30th Jan 2020 18:50 UTCUwe Kolitsch Manager

they appear a rich orangey-brown in incandescent light, and a more yellowish-brown under a daylight-type LED bulb.
 Such a colour-change behaviour is typical of REE minerals.

30th Jan 2020 20:27 UTCFrank K. Mazdab Manager

Aegirine is certainly a reasonable possibility, and it sounds like the overall [sodic pyroxene]±[sodic amphibole]+[alkali-feldspar]+[quartz] assemblage may be akin to typical peralkaline granite mineralogy... classic mineral collecting localities like Mt. Malosa in Malawi and the Mount Rosa granite in Colorado might be comparable examples if you want to keep other species in mind as you peruse your specimen with a hand lens (and on a petrologic note, if a peralkaline granite, then it offers some intriguing thoughts into what's perhaps gone on in the Zagi Mountain area in the geologic past... hmmm).

But in terms of your particular specimen, I still wouldn't discount the possibility that some of the black prismatic crystals could still be a sodic amphibole... as I suggested before, a tiny crystal discreetly chipped off the back, coarsely broken and then checked for cleavage with a hand lens might still be a worthwhile test (plus it's a good test to get practice with). Although the crystal terminations and cross-sections of aegirine and the sodic amphiboles are normally characteristically different, there may be some ambiguous examples so cleavage would be most definitive.

Regardless, pretty specimen in any case... the bastnäsite crystals make an otherwise nice rock into an even nicer mineral display piece! Nice find (on eBay... lol).

30th Jan 2020 21:34 UTCNick Gilly

Thanks Frank. I'm also guessing aegirine because there's a specimen in the archives on e-Rocks from Zagi Mountain with aegirine which looks very similar, from Mark Wrigley actually. He's termed the matrix Warsak granite with aegirine and albite.

Certainly another nice find at <$15 before shipping yes :-)
 
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