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GeneralTarnowitzite vs plumboan aragonite

7th May 2019 09:04 UTCStefan Koch Expert

Hi there,

I would like to know if there is a difference between Tarnowitzite and plumboan aragonite. I always thought one is a synonym for the other!?

But why both are listed for Tsumeb Mine?

And on the other hand why is only plumboan aragonite listed for Touissit?

Best regards,

Stefan

7th May 2019 11:06 UTCPaul De Bondt Manager

Hi Stefan,

There are some more inconsistencies for Tsumeb like the Zincian olivenite and zincolivenite and

gallium-rich plumbogummite and galloplumbogummite.

Cheers.

Paul.

7th May 2019 11:28 UTCJolyon Ralph Founder

The distinction is described on the plumboan aragonite page:

"Much 'tarnowizite' has been identified as a mixture of aragonite and cerussite, but plumboan aragonite with up to 3% PbCO3 may be homogeneous. "

Now, the question is how much of the 'plumboan aragonite' listed on that page is actually the mixture of aragonite and cerussite? Can we tell? Does it even matter?

Perhaps they should be merged together with a note that much is a mixture but some is homogeneous up to 3% PbCO3 in aragonite.

7th May 2019 21:03 UTCRichard Gunter Expert

Hi Jolyon:

I wonder if "mixture" is the proper term for two crystallographically related phases such as aragonite and cerussite? A mixture implies a random contribution from both phases and an intergrowth of crystallographically related phases tends not to be random. There are several investigations into the intergrowths in the tetrahedrite-tennantite series that illustrates the non-random nature of the intergrowths. I have not seen any microprobe data on an oriented section of tarnowitzite but it may look similar.

7th May 2019 21:40 UTCJolyon Ralph Founder

Sounds reasonable!

7th May 2019 21:40 UTCAlfredo Petrov Manager

Could it be that the tarnowitzite is a single phase at high temperature but exsolves into aragonite + cerussite on cooling?

7th May 2019 22:37 UTCRalph S Bottrill 🌟 Manager

It sounds very much like it Alfredo.

8th May 2019 00:13 UTCRichard Gunter Expert

Hi Alfredo:

You may be correct but the temperature of the single phase cannot be very high as tarnowitzite is almost always the latest phase in a suite of secondary minerals. At Tsumeb it occurs after smithsonite and some of the hydrous copper arsenates. There is some data that the aragonite phase in the Mendip Pb carbonate suite is plumbian, and thus tarnowitzite.

8th May 2019 01:24 UTCFrank K. Mazdab 🌟 Manager

Not only what Richard noted, but more critically that aragonite rapidly converts to calcite at higher temperature, so "tarnowitzite" couldn't occur as a high temperature mineral. Evaluating how much Pb can go in CaCO₃ at high temperature would therefore end up being a Pb in calcite study. A low-T high-P experiment might be more successful to see what the aragonite-cerussite solid solution looks like, but it doesn't seem there would be many natural geologic environments (at least on Earth) that would be well-modeled by such an experiment.

Since the 6-coordinated calcium site in the calcite structure would be much less accommodating for a big ion like Pb as might be the 9-coordinated calcium site in aragonite, there's definitely a miscibility gap between calcite and cerussite. In my own work, I've so far found up to about 3.3 wt% Pb in calcite, in a sample from Jakobsberg, Sweden.

Going back to the original post, it seems "tarnowitzite" and plumboan aragonite should not be both used in the same mineral list... we should commit to one or the other, and my suggestion would be to minimize propagating varietal names and stick with actual minerals... so ideally, aragonite var. Pb-enriched.

8th May 2019 15:33 UTCRichard Gunter Expert

Hi Frank:

I agree with you on the aragonite var. Pb-enriched. Tarnowitzite was a useful name for what is not "normal" aragonite. There are times when specific varietal names of common minerals can convey more information than the mineral alone, even though their use should be minimized. It is interesting that both calcite (Pb and Zn-bearing) and aragonite (Pb and non-Pb-bearing) should exist at Tsumeb and other Pb-bearing suites such as the Mendips. If we had more information on locations within the Tsumeb Mine might the "tarnowitzite" be a thermometer?

8th May 2019 22:54 UTCŁukasz Kruszewski Expert

No difference. They are equal.

13th May 2019 23:25 UTCStefan Koch Expert

-- moved topic --

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