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Nsutite question

Posted by Rolf Luetcke  
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Rolf Luetcke September 13, 2018 08:09PM


I purchased this specimen from the micro mounters room at the Tucson mineral show and the label said nsutite and the specimen had these long, black crystals that were silvery metallic from the side. Sure doesn't look like any of the nsutite photos and the Tres de Mayo Mine in Arizona doesn't have the mineral listed. Since it is a tiny specimen, an analysis is not happening. Hoping someone has a bit of an idea what these crystals may be.
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W. Richard Gunter September 13, 2018 10:09PM
They could be another Mn-oxide such as ramsdellite. The Mn-Fe oxides minerals within the Chihuahua, Mexico quartz geodes have been investigated and the results and photographs published in the Mineralogical Record. The elongate ramsdellite from the geodes looks similar to the black prisms in your photos.
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Keith Compton September 13, 2018 10:35PM
Rolf

I can't really comment on either the brown or the black material but it is unclear to me which you think may be nsutite. If you are asking if the black needles are nsutite then from the literature I would presume no.

The brown material may be nsutite:

From the following reference: Zwicker, W.K., Groeneveld Meijer, W.O.J., Jaffe, H.W. (1962) Nsutite - a widespread manganese oxide mineral. American Mineralogist: 47: 246-266.

It is commented that: "It is doubtful whether nsutite can be identified from a megascopic examination alone. Shrinkage cracks, however, which are somewhat diagnostic for nsutite in polished sections, give massive specimens, such as that from Tartana, a strongly developed columnar jointing. The Morro da Mina [Lefaiete District, Minas Gerais] material is a soft, brown' earthy mass, whereas numerous samples from the Nsuta mine are fairly hard, metallic gray, porous aggregates".

"Characteristic of many occurrences of nsutite is its proximity to manganese caronbate. In some occurrences manganoan nsutite forms replacement veinlets in manganese carbonate rock. It then appears to be among the first oxides forming from the oxidation of manganese
carbonate along with birnessite".

Although the brown on your specimen looks more like a coating on something.
Then again the black blebs in the lower left of your second photo?
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Alfredo Petrov September 14, 2018 09:03AM
25 years before being described as Nsutaite, the same species had been described from Japan as Yokosukaite. There it formed black or blackish brown compact masses with light brown to yellowish brown rhodochrosite in the exposed oxide zone of a metasedimentary rhodonite-spessartine deposit, which concords with what Keith quoted above about the Mn carbonate association.

The brownish crust on Rolf's photo is certainly not nsutite, and it might be rhodochrosite. The black needles might be a different Mn oxide, perhaps a member of the cryptomelane group? "Psilomelane" seems to be abundant there, according to the mines' description on Mindat, and "psilomelanes" are most commonly cryptomelane group members.
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Rolf Luetcke September 14, 2018 12:17PM
This is the problem with purchasing specimens with a labeled mineral. I had to do a bit of guesswork with this piece. The purchase price was all of $3 so that is why I had commented about not getting it tested. The brown looked to me like an iron oxide and I could only assume that the needles is what was meant. As I had also said, the identification was suspect from the start since the same person had identified brackebuschite from this location, which turned out to be calderonite. Richard, I know what you mean about the needles in geodes, I have some also with the similar needles.
I realize this piece is one that should be among my ? specimens since there is little I can do but thought the long needles were quite interesting. I wanted to see what others think and I sure appreciate the input.
Alfredo, I had thought of psilomelane also but that is normally much thinner and I had never seen psilomelane in such nice and stiff crystals. An interesting mystery piece.
Thanks all.
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