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Identity HelpJeanbandyite, Natanite.

21st Oct 2007 15:29 UTCBill Gordon

Hi All,

When I collected this from Hingston Downs Quarry I was told that I had Natanite and that it was an alteration of Jeanbandyite. On their respective pages in Mindat I don't see a specimen of the same appearance as this. Could someone put me right please.

22nd Oct 2007 17:24 UTCPeter Haas

What does the matrix consist of ? Tennantite/Bornite ?

22nd Oct 2007 17:58 UTCBill Gordon

Hi Peter,

predominately Arsenopyrite but the vein that these specimens came from had an admix of various sulphides.

I was wondering, with the info given to me, if it was one species altering to the other.

22nd Oct 2007 19:48 UTCSteve Rust Manager

Bill I belive that both phases at Hingston can be in the same crystal. Some form of zoneing, not sure if any of the crsystals are pure end members. Simular to crystals from Penberthy Croft.

22nd Oct 2007 20:48 UTCKnut Eldjarn 🌟 Manager

I have alleged Jeanbandyite in dark yellow crystals on arsenopyrite crystals from the locality. Jeanbandyite and natanite ar both iron-tin-hydroxides the former being tetragonal in the stottite-group and the latter cubic in the schoenfliesite group. Thus these two phases would not fit in the same structure with traditional zoneing as could be the case with intermittent layers of 2 end-members in the same series in one crystal. But these 2 minerals with a different structure could of course be intergrown in some other way. I have been especially interested in minerals of the schoenfliesite and stottite-groups from different localities but have never seen or read that minerals in one group could alter to the corresponding mineral in the other group. On my specimen the crystals are distinctly "octaedral" or pseudooctaedral (as tetragonal crystals of the stottite-group may be). There are no signs of a macroscopic 2-phase intergrowth or alteration.

22nd Oct 2007 23:08 UTCLefteris Rantos Expert

Hello,

Have a look at this delightful little specimen:

http://www.mindat.org/photo-79258.html

Tetrawickmannite and Wickmannite have the same relationship with Jeanbandyite and Natanite, respectively. A similar epitaxial intergrowth could also occur between the Fe-Sn-memebers of the same groups, and it could be the case in Bill Gordon's specimen (although this remains to be proven in a more certain way).

Also the latter species in such an epitaxial intergrowth could fully envelop the earlier crystal, forming a kind of a "zoned" crystal (although not technically such, since the two species have different structures). In this context, see also Jason Smith's comment about "obvious" Wickmannites occurring at the Foote mine, in the caption of the above photo. Jason's oppinion could be useful in this thread, in case he has studied similar intergrowths of tetrawickmannite/Wickmannite at the Foote.

Btw, thanks for the great (and educational!) photo, Jason!

Lefteris.

22nd Oct 2007 23:25 UTCLefteris Rantos Expert

"I have been especially interested in minerals of the schoenfliesite and stottite-groups from different localities but have never seen or read that minerals in one group could alter to the corresponding mineral in the other group."

Knut,

Wickmannite and Tetrawickmannite, which are dimorphs belonging to the Schoenfliesite and Stottite groups respectively, also occur together at the Langban mines (apart from the Foote mine, as stated above).

The great book on the locality - "Langban, the Mines, their Minerals, Geology and Explorers" - notes that the two species occur together on a handful of known specimens (one is pictured, even, with xls of both dimorphs in direct association), but there is no reference in any kind of intergrowth or paragenesis/alteration between them.

23rd Oct 2007 08:22 UTCKnut Eldjarn 🌟 Manager

Lefteris,

The question is not if specimens in the Shoenfliessite group and stottite group may occur together, but if this can be as a result of alteration or in zoned crystals as for end-members in a series.

I have specimens where wickmannite/tetrawickmannite occur together from Långban and also (probably) from a locality in Norway and have posted a picture in mindat of the latter. My points related to the question of alteration raised by Bill was this:

1. Epitaxial overgrowth of one mineral on another is not usualy a process of alteration of the former mineral but an ordered growth of the new mineral based on the structure of the underlying mineral. There must be some structural relationship but not necessarily a chemical relationship between the two minerals ( i.e. rutile on hematite).

2. Zoneing in crystals of different end memebers in a series ( ref. the comment from Steve) requires that the minerals have the same crystal structure but differ in chemical composition.

3. I have never seen specimens or publications refering to specimens of minerals in the shoenfliesite and stottite-groups altering to the mineral in the other group as would i.e. be the case with pseudomorphs.

23rd Oct 2007 15:01 UTCRichard De Nul Expert

The Mineralogical Magazine, Vol. 62 (5), 707 - 712 gives a very good description about the differences between these two minerals. Nantanite tends to be greenish while jeanbandyite is more of an opaque yellow-brown colour. Steve is correct when he says that an intergrown is possible at Hingston Down.

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