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Identity HelpType locality for pyrrhotite-7H

14th Jan 2016 21:01 UTCReiner Mielke Expert

What is the source of the sample that was used to define the polytype? Or easier yet at what locality can I find this polytype? Someone must know as it appears as a valid polytype on Mindat. http://www.mindat.org/min-3328.html

15th Jan 2016 00:58 UTCRob Woodside 🌟 Manager

Sorry I don't know the source. However polytypes are accidental growth features and so a pyrrhotite from a polytype source may not be that polytype. The only honest polytype specimen is the x-ray mount that gives the polytype diffraction pattern. :-(

Thanks for working on this and I hope you can track it down.:-)

15th Jan 2016 01:16 UTCReiner Mielke Expert

"polytypes are accidental growth features" Bad choice of words. They are slightly different atomic arrangements that are not accidental. Most of the pyrrhotite from Voisey's Bay is non-magnetic, not your usual -4m polytype, and not accidental. I am not sure exactly which polytype it is so I am trying to get a sample suitable for structural analysis. However it is true that a meteorite impact could change the polytype and in that case it would be accidental.

15th Jan 2016 01:26 UTCRob Woodside 🌟 Manager

There will be localities where there is a single polytype, but I wouldn't count on it. How many specimens must be analysed and the same polytype found before one can make that claim?

Why do you consider a meteor impact to be more accidental than the slight impurities or the chemistry of the growth medium that ensure this polytype has the slightly lower Gibbs free energy?

15th Jan 2016 01:39 UTCReiner Mielke Expert

Hello Rob

Yes there are localities with a single polytype and I do count on it. These are not just random structures that vary randomly. They are unique to a specific set of pressures and temperatures and not the result of impurities. Synthesis experiments have shown that each polytype has a specific stability field that has nothing to go with impurities.

15th Jan 2016 01:47 UTCReiner Mielke Expert

"How many specimens must be analysed and the same polytype found before one can make that claim?" You do realize that Pyrrhotite-4M is a polytype. It is the only magnetic polytype( and by far the most common) and it very simple to determine if it is the only type at a locality with a magnet or are you suggesting that there is no such thing as pure 4M?

15th Jan 2016 01:50 UTCRob Woodside 🌟 Manager

How do you account for a single Sphalerite xl containing lenses of Wurtzite? I don't believe a polytype is a thermodynamic phase, as you are suggesting, but I'm willing to learn.

15th Jan 2016 02:52 UTCReiner Mielke Expert

Hello Rob,

Here you are http://www.geologinenseura.fi/bulletin/Volume58/sgs_bt_058_1_pages_293_305.pdf The reference to hexagonal pyrrhotite is a bit outdated. At the time it was thought that all non-magnetic pyrrhotite was hexagonal.

15th Jan 2016 02:57 UTCReiner Mielke Expert

"How do you account for a single Sphalerite xl containing lenses of Wurtzite? " How do you account for a single sphalerite crystal containing no wurtzite lenses?

15th Jan 2016 03:11 UTCRob Woodside 🌟 Manager

Slow down Reiner. The first of your last three posts was posted while I was writing my last post. I started replying to that and your second to last post came and I started replying to that. Then your last post arrived. My next post will try to deal with your last three. Please don't post any more until you see my next post.

15th Jan 2016 04:15 UTCRob Woodside 🌟 Manager

1) Yes I "do realize that Pyrrhotite-4M is a polytype". The -4M is the give away. Yes there probably is pure 4M, but it occurs in mixtures with varying degrees of magnetism.

2) From the abstract of your Killerud ref.:

"Hexagonal pyrrhotite takes a small amount of oxygen in solid solution. This oxygen may be responsible for the formation of hexagonal superstructures and may be the cause of the metastable behavior of supersaturated hexagonal pyrrhotite."

So the suggestion is that the polytypes (superstructures) are accidentally created by oxygen impurities.

However, do you have a copy of Wang H, Salveson I (2005) A review on the mineral chemistry of the non-stoichiometric iron sulphide, Fe1-xS (0≤x≤0.125): polymorphs, phase relations and transitions, electronic and magnetic structures, Phase Transitions, 78, 547-567 ?

From that paper:

Formation of superstructures does not appear to affect the short-range atom configurations, although detailed structural refinements of many superstructures are yet to be undertaken. Many of the hexagonal superstructures have very close stoichiometries but they appear to behave as separate phases (or polytypes) and were normally treated as such when delineating phase diagrams. Nakazawa and Morimoto [68, 69] sorted the numerous superstructures into five categories according to their multiplicities of the NiAs sublattice. The five categories of the superstructures are: (1) 2C for troilite, (2) 4C for monoclinic pyrrhotite, (3) NC (a ¼ 2A; c ¼ NC, N varies continuously between 5.0 and 11.0 [68–71], (4) MC (a ¼ 2A; c ¼ MC, M varies between 3.0 and 4.0), (5) NA (a ¼ NA; c ¼ 3C, N varies continuously between 40 and 90). Note A and C are respectively the a and c parameters of NiAs subcell. The most well-studied superstructures in nature, 5C (Fe9S10), 6C (Fe11S12), and 11C (Fe10S11), belong to the NC category [70, 71]. Although 5C, 6C, and 11C are crystallographically distinguishable, they are treated as one phase, NC, in the studies of phase relations. This is because they act as a single phase during phase transitions, and that their compositions fall into a very narrow range (47.37–47.83 at.% of Fe) [68–71]. These superstructures are best described in terms of stacking of fully occupied and ordered defective iron layers normal to the c-axis. Each structure is characterized by a regular succession of such layers, corresponding to an integral supercell multiplicity N, where c ¼ NC. The multiplicity N is related to the general chemical formula Fem–1Sm (m > 8) by N ¼ 0.5m (when m has an even value), or N ¼ m (when m has an odd value) [71]. This formulism only serves for the convenience of describing the superstructures in pyrrhotite. In fact, pyrrhotites with non-integral multiplicity superstructure are more common, as the m (as in Fem–1Sm) is not necessarily an integer and changes continuously with composition and temperature [71]. Stacking disorder between filled and vacancy bearing layers gives rise to non-integral N values, and thus to an apparently incommensurate c-axis repeat [72–74]. These pyrrhotites are often referred to just as hexagonal pyrrhotite, and have compositions in the region between troilite and monoclinic pyrrhotite. These superstructures can either be expressed as Fe9S10, Fe10S11, Fe11S12 or as a mixture of the stoichiometric phases with troilite or with monoclinic pyrrhotite [36, 68, 75–77].

The paper is a nice review.

From the first red bit. I take it that there are stoichiometric layered structures (polytypes) that behave as separate phases I think "separate phases (or polytypes)" is a "Bad choice of words". I would have thought that a stoichiometric themodynamic phase should be a mineral species if found in nature.

In the second red bit occurring after we hear of continuously changing compositions we read that, " 5C, 6C, and 11C " ..."act as a single phase during phase transitions, and that their compositions fall into a very narrow range (47.37–47.83 at.% of Fe)" which is what I was vaguely remembering.

In the third red bit we have "These superstructures can either be expressed as Fe9S10, Fe10S11, Fe11S12 or as a mixture of the stoichiometric phases with troilite or with monoclinic pyrrhotite". This is Killerud's "Hexagonal" solid solution island.

OK a stoichiometric layered structure, a polytype, can be a thermodynamic phase as you suggest. So maybe this is why Smythite survives as a species when it is just one of pyrrhotite's polytypoids. I still think it is an accident of impurity or environment that produces polytypes and that would be a reason to deny them species status. Maybe a pro might comment.

3) "How do you account for a single sphalerite crystal containing no wurtzite lenses?" The accident creating the wurtzite polytype never happened. It is a rare accident.

15th Jan 2016 13:35 UTCReiner Mielke Expert

Hello Rob,

What you are highlighting are the shortcomings of synthesis. When they quench the mixtures to determine the phases present they retain their metastable configuration and thus do not properly represent natural phases ( although this has been somewhat mitigated by high temperature XRD methods). The whole issue is still up in the air with respect to how to properly interpret the results of such experiments that is what 2) refers to. To be more specific this metastability could lead one to conclude that non-magnetic pyrhotite is hexagonal which has been one of the underlying reasons that this non-magnetic-hexagonal myth came about.

In such experiments the various non-magnetic poltypes are treated as one phase because it is too difficult and not worth the effort to distinguish them. Most of this research is motivated by the metallurgy of nickel which is concerned with the magnetic separation of pyrrhotite to improve grade and understanding the mechanisms of oxidation which affects recoveries in flotation.

Why is it difficult to distinguish these polytypes you ask? If you just run a standard XRD on this material it produces a pattern that looks hexagonal, hence the early thinking that non-magnetic pyrrhotite was hexagonal. However if you run high resolution XRD and with proper analysis you find they are not hexagonal. A perfect example of how difficult this analysis is can be found in this paper: Lilies, D. C., Villiers, J. P. R. D. (2012): Redetermination of the structure of 5C pyrrhotite at low temperature and at room temperature. American Mineralogist 97, 257-261 In it they highlight the method they used to determine the monoclinic structure of Pyrrhotite-5C which only two years early they had determined was orthorhombic, and which earlier researchers had thought was hexagonal.

"I still think it is an accident of impurity or environment that produces polytypes and that would be a reason to deny them species status." No one is suggesting giving polytypes species status are they? and you have not proven that these polytypes are due to impurities have you? As for accident, unless you believe in a God that controls everything then everything is an accident so as such I will have to agree with you this is all an accident. LOL.

15th Jan 2016 17:10 UTCUwe Kolitsch Manager

"... each polytype has a specific stability field that has nothing to go with impurities."

Wrong. Any (non-neglectable) impurity will influence the thermodynamic stability of any phase.

15th Jan 2016 20:35 UTCReiner Mielke Expert

Hello Uwe,

Rob was implying that it is the impurities that cause the different polytypes. I was addressing that, not the broader issue of multi-element systems. I was attempting to point out that it is not essential to have "impurities" to have different polytypes. One can have different polytypes with no impurities whose stability fields exist without impurities.

15th Jan 2016 20:50 UTCUwe Kolitsch Manager

Ok, then I misread that statement.

15th Jan 2016 21:21 UTCRob Woodside 🌟 Manager

" One can have different polytypes with no impurities whose stability fields exist without impurities."

So we have geologically occurring thermodynamic phases with a well defined chemistry and structure. Isn't that a mineral species?

I was saying that polytypes are accidental structures. By accidental I mean they are small departures from the ideal mineral species defined by the IMA. They are growth features that accidentally depart from ideal symmetry.

When I was a kid they were called stacking faults and were supposed to result from a defect in nucleation that showed up in the growth step and propagated through the xl by a spiral dislocation. Certainly some polytypes form by thermal relaxation rather than xl growth. Now a polytype is considered a layered structure with no talk of spiral dislocations. They are now described with groupoids which are so general that any structure can be so described.

15th Jan 2016 22:39 UTCReiner Mielke Expert

"So we have geologically occurring thermodynamic phases with a well defined chemistry and structure. Isn't that a mineral species?" Yes it is. I do not know why they are not considered species but I am glad they are not, there is already enough subdividing as far as I am concerned.

15th Jan 2016 22:50 UTCRob Woodside 🌟 Manager

That's been my position, but you have me wondering.:-S

23rd Jan 2016 07:53 UTCRalph S Bottrill 🌟 Manager

As I understand it polytypes can vary from completely ordered to completely disordered. With some sphalerites and wurtzites, foe example, careful XRD will enable you to determine the relative proportions of the polytypes. I guess that why these days they tend to be lumped together as one species despite obvious crystallographic differences. Similar to the microcline- orthoclase-sanidine series, could be considered one species with varying order/disorder.

23rd Jan 2016 12:59 UTCReiner Mielke Expert

Hello Ralph,

The pyrrhotite polytypes at low temperature are ordered they are not mixtures of polytypes although some pyrrhotite can contain more than one polytype as intergrowths which occur as distinct domains, for example much like pentlandite and pyrrhotite. Much of the literature on pyrrhotite polytypes is mainly concerned with magnetic vs. non-magnetic pyrrhotite and as such, in the past, there has been little effort put into distinguishing the non-magnetic polytypes ( of which there are three) .This has led to the thinking that these non-magnetic polytypes are mixtures of ill defined structure which they are not.

I don't know how it is with other minerals as I have not spent much time on them but this is the situation with pyrrhotite.

23rd Jan 2016 21:57 UTCRalph S Bottrill 🌟 Manager

Probably so, but pyrrhotite typically forms at high temperature, not sure how rapidly it inverts. I have certainly seen inter growths of different pyrrhotites in polished section, as shown by flame structures.

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