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Identity HelpSturmanite on Ettringite N'Chwaning II

30th Dec 2011 17:21 UTCMaggie Wilson Expert

Calling all South African specialists!

We have a thumbnail labelled "sturmanite coating ettringite" from N'Chwaning II Mine in South Africa. I couldn't find anything quite like it in the database, so before I upload the photos, I wouldn't mind you opinions. Is it as labelled? Is it a pseudomorph? Or is it "door number 3"? Something else entirely?

Thanks in advance

Maggie

30th Dec 2011 19:28 UTCRolf Brandt

Hi Maggie, as far as I know is it only possible by XRD to determine whether it is sturmanite or ettringite. Some people say, of course they can see the difference, but I rather believe the mineralogists who worked on it. Charlesite (colourless) in many cases forms the core of the sturmanite/ettringite crystals. Regards Rolf

31st Dec 2011 04:54 UTCDebbie Woolf Manager

I agree with Rolf & it's exactly what is stated in the book Minerals of South Africa. Hope it's stored air-tight as they do disintegrate. Label it as Sturmanite.

31st Dec 2011 05:31 UTCBruce Cairncross Expert

Hi Maggie,

Your problem is as Rolf states but can be even more problematic.

Sometimes individual crystals are zoned and contain may contain both etrringite and sturmanite in a single crystal. Sturmanite and ettringite form a complete solid solution series. Work that was carried out by von Bezing and others and published in the Min Record in 1991 shows that most specimens labelled as sturmanite are in fact ferroan ettringite. In addition, charlesite and jouravskite are close chemical relatives and also complicate identification. Having said all that, your particular specimen is very similar in size, habit and colour to ones I have in my collection that came out during the late 1980s, so it looks like a fairly old specimen, and it shows the colour change that ocurs in many "sturmanite" specimens. As there's 25 H2O molecules present, its widely believed that they dehydrate with time.

Regards,

Bruce

31st Dec 2011 09:31 UTCAlfredo Petrov Manager

In addition to dehydration, it can also absorb carbon dioxide and partially alter to carbonate, so don't breathe on it. So, as Debbie mentioned, it should be stored in an air-tight container.

31st Dec 2011 12:56 UTCMaggie Wilson Expert

THANK-you all for your replies! I will update my catalog. Thank goodness for cut and paste!

I've also loaded the photos to the Mindat database quoting excerpts from this thread. http://www.mindat.org/photo-434398.html

Happy New Year!

Maggie

31st Dec 2011 13:12 UTCReiner Mielke Expert

Thank You Bruce,

You are right this specimen is from the 80's and was acquired from Frank and Wendy Melanson. I would like to point out that this specimen has a pale yellow core with a brown crust as can be seen in the small broken crystal on the side. I have not noticed any deterioration but then I coated it with mineral oil to prevent that.

(arrow points to it).

23rd Jun 2014 01:19 UTCVandall Thomas King Manager

Rolph, the unit cell dimensions are probably not different enough to distinguish different members of the ettringite group by x-ray diffraction. Charlesite, as it is turning out, does not have a formal identification published for South Africa. It seems more like a "guess" until data are actually provided.

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