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GeneralTwinned wulfenite

21st Aug 2009 08:44 UTCChristian Auer 🌟 Expert

Comming back from Mezica-Slovenia where I visited the great mining museum (+mineral exhibition) once again, I saw many of those fascinating Mezica wulfenites. Many different habits and twinnings - quite common are sandwich type xtls for example.

http://www.mindat.org/photo-183722.html shows such combinations.

My questions would be:

1) Can someone explain such a growing? Why is there no orientation visible?

2) Are there other localities where such growings occur? One is http://www.mindat.org/photo-152792.html (waaay smaller than Mezica)

Thanks for any info!

21st Aug 2009 10:44 UTCUwe Kolitsch Manager

Wulfenite is hemimorphic, thus it's possible that only one side of a tabular crystal shows pyramidal crystals of a second generation.

The recent, large article on Mezica (Rečnik, A. et al. (2009): Mežica in Slowenien. Mineralien-Welt 20 (4), 40-83.) gives a good explanation.

21st Aug 2009 14:16 UTCChristian Auer 🌟 Expert

Thank you Uwe,

I know that wulfenite is hemimorph. Still I wonder about the lack of orientation. I had assumed the xtls grow all to the left for example due to whtaever influences. But they don`t. Guess I`ll read that articel.

21st Aug 2009 15:52 UTCDonald Peck

According to both Mindat and WebMineral, the symmetry of wulfenite is 4/m and the space group is I 4(1)/a, which I believe translates to 4/m. Doesn't that make wulfenite Tetragonal Dipyramidal, not hemimorphic? I seem to remember (and I am not good at that anymore!) that it used to be considered hemimorphic, but the classification had been changed due to better data.

21st Aug 2009 17:16 UTCUwe Kolitsch Manager

Don: good question.

Our Mindat page has a comment that says that the space group could also be I-4.

A more detailed discussion can probably be found in the two recent papers with structure determinations (from 2000 and 2008). Maybe modern neutron or synchrotron studies are necessary?

21st Aug 2009 17:17 UTCJohan Kjellman Expert

I guess these articles can shed some light:

Hibbs, D.E., C.M. Jury, P. Leverett, I.R. Plimer, and P.A. Williams (2000) An explanation for the origin of hemihedrism in wulfenite: the single-crystal structures of I41/a and I4 wulfenites. Mineralogical Magazine, 64, 1057–1062.

Secco, L., Nestola, F. and Dal Negro, A. (2008): The wulfenite–stolzite series: centric or acentric structures? Mineralogical Magazine, 72, 987-990.

cheers

21st Aug 2009 18:20 UTCJohan Kjellman Expert

see also "chillagite", evidently a high tungstenian wulfenite that crystallize in I4.

the structural differences did not warrant the division in two minerals:

"Further advice from Dr Joel Grice, the Chairman of the CNMMN, has clarified the appropriate nomenclature to be used for these

species. Examples crystallizing in space groups I41/a and I4¯ are to be called wulfenite-I41/a and wulfenite-I4¯ , respectively." (Hibbs et al. p. 1058)

cheers

21st Aug 2009 18:24 UTCJohan Kjellman Expert

from Secco et al. 2008:

ABSTRACT

Three natural crystals of the wulfenite (PbMoO4)-stolzite (PbWO4) series were investigated by singlecrystal

X-ray diffraction. The results indicate that the symmetry is I41/a from nearly pure wulfenite to

intermediate compositions, in contrast to previous work which claimed a symmetry change to I4¯

symmetry (acentric) for intermediate compositions compared with I41/a (centric space group) for the

end-members. The results reported here show that the reflections violating I41/a symmetry observed in

this work and in a previous study are related primarily to l/2 effects, even if Renninger effects are not

excluded. Consequently, we find that the I41/a symmetry is retained throughout the wulfenite-stolzite

series.

21st Aug 2009 18:45 UTCRob Woodside 🌟 Manager

Thanks Johan. Is there any explanation in these papers for the obviously hemihedric specimens when the underlying symmetry is centric?

21st Aug 2009 19:59 UTCJohan Kjellman Expert

Anyone interested can PM me with their e-mail and I'll forward the two papers.

cheers

1st Nov 2014 12:35 UTCMark Holtkamp

Hi all,

Sorry to bring this old topic up again, but this wulfenite problem has been bugging me for some time and I found this topic only today.

The wulfenite in Christians picture are pyramidal (not bipyramidal as in common 4/m wulfenites).

I have a wulfenite from Morocco that has a mix of very distinct pyramidal and bipyramidal crystals, like on these photographs:

wulfenite
wulfenite
wulfenite

And like on these drawings (pyramidal and bipyramidal):

The spacegroup I-4 that is proposed to explain the hemihedrism of wulfenite translates to point group -4 (the tetragonal-disphenoidal class). And there are no pyramids in point group -4 (only in point groups 4 and 4mm). Wulfenite crystals in pointgroup -4 that would show their point group symmetry would have tetragonal sphenoids, not pyramids.

Any thoughts?

Mark.

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