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Polytype pseudomorphs

Posted by Reiner Mielke  
Reiner Mielke March 01, 2012 02:14PM
In looking into the magnetic properties of pyrrhotite I checked my pyrrhotite crystals to see if they were attracted to a magnet and they were all strongly attracted. According to what I have read, the hexagonal polytypes are supposed to be antiferromagnetic and the monoclinic polytypes ferrimagnetic. Does this mean that my crystals are actually monoclinic pseudomorphs of hexagonal pyrrhotite? Are there any euhedral monoclinic pyrrhotite crystals?
Alfredo Petrov March 01, 2012 02:33PM
As far as I recall, Reiner, "troilite" is the only pyrrhotite polytype that is not ferromagnetic, all the rest being attracted to a hand magnet to greater or lesser degree.
Rob Woodside March 01, 2012 05:29PM
In an antiferromagnet the magnetic dipoles aline in opposite pairs so there is no net magnetic dipole moment. So if you bring a piece of non magnetic soft iron close by there will be no attraction. However if you bring a magnet close by it will disrupt the alinement and induce a net dipole moment which can repel or attract. In a ferrimagnetic material again two sets of dipoles are alined in opposite pairs, but now one set is stronger than the other leading to a net magnetic dipole moment that will attract.

I suspect that most hexagonal looking xls of pyrrhotite are twinned monoclinic xls (as in mica). To know what polytype you have, you really have to x-ray the sample. It would be cute if you could spot a hexagonal polytype with its zero net magnetic moment. The pyrrhotites from Henderson # 2 are perhaps the most x-rayed pyrrhotites. They are long slender monoclinic prisms that look nothing like pyrrhotite except for the colour. There are presumably other localities.
Reiner Mielke March 01, 2012 05:54PM
Hello Alfredo,

Troilite is not considered a polytype of pyrrhotite, it is a separate species. According to Dekkers, M.J. 1988, Magnetic Properties of Natural pyrrhotite, Physics of the Earth and Palnetary Interiors, 52,376-393, Elsevier Science Publishers. Three hexagonal polymorphs 5C,11C, and 6C are antiferromagnetic ( not attracted to a magnet) and monoclinic 4C is ferrimagnetic ( attracted to a magnet).

Hello Rob,

Thanks for the info on the Henderson xls, sure would like to get one of those monoclinic crystals. I guess any hexagonal magnetic Pyrrhotite would have to be a twin then. Do you know of any non-magnetic hexagonal crystals? There appears to be a discrepency between the list of polytypes in Mindat and the published data. I questioned this on a different thread a while ago and got no response,7,251419,251899#msg-251899 Do you know anything about this?

Edited 1 time(s). Last edit at 03/01/2012 05:56PM by Reiner Mielke.
Alfredo Petrov March 01, 2012 06:26PM
Reiner, I suppose attraction (or not) to a magnet is another one of those cases like hard vs soft, light vs dark, and soluble vs insoluble: no sharp boundary, so actual measurements would give us a scale with an infinite number of possible degrees of attraction, rather than a yes or no answer. Perhaps the greater the iron deficiency (which varies by polytype - pyrrhotite being one of few species with different chemical formulae for each polytype) the greater the magnetic attraction?
As for the status of troilite, "considered" may be the important word here. It's really pyrrhotite-2H, and doesn't deserve species status, but it gets "considered" independently, much like several other "species" (eg; augite, actinolite...) for historical convenience more than any particular mineralogical reason. Eventually the historical inertia gets overcome and these things tend to get properly reclassified (like the intermediate plagioclase feldspar varieties finally were, much to the horror of petrographers), but it can take a while - I prefer to jump the gun, so I don't have to change all my labels later :-)
Ronald John Gyllenhammer March 01, 2012 09:45PM
Hi Reiner,

> "According to what I have read, the hexagonal polytypes are supposed to be antiferromagnetic and the monoclinic polytypes ferrimagnetic."

It's safer to ask this question in the context of "phases" of these polytypes or combinations of them rather than imagining homogenous macro crystals of Pyrrhotite that are one polytype or another. I think you are mostly correct to say that hexagonal polytype (Fe-rich) phases are generally antiferromagnetic and that the monoclinic polytype, (Fe deficient) is ferrimagnetic. Likewise I think you could usually say that most monoclinic Pyrrhotite is higher in sulfur than hexagonal Pyrrhotite.

> "Does this mean that my crystals are actually monoclinic pseudomorphs of hexagonal pyrrhotite?"

The truth is, I think most naturally occuring Pyrrhotite specimens or samples can have multiple phases, that is both; monoclinic and hexagonal superstructures and possibly even a rare orthorhombic phase at the same time as well. This is probably the best way of looking at variable degrees of Pyrrohotite magnetism. So as Alfredo suggests, generally it's probably true that the greater the presence of Fe deficient phase occurence in the sample, the greater the degree of magnetism and of course conversely; the greater the presence of Fe rich phase occurence in the sample, the lesser the degree of magnetism in the sample.

> "Are there any euhedral monoclinic pyrrhotite crystals?"

Most Pyrrohotite crystals are characterized as "monoclinic". I think of it this way, I see Troilite as "stochiometric" Pyrrohotite (FeS) and hexagonal right? Perfectly occupied Fe layer, S layer, Fe layer, etc, etc. Ok, so if these Fe layers are (Fe cation) deficient, the circumstance changes a couple of things. Not only could it maybe make the phase "more magnetic" but it also distorts the overall structure, effectively changing it to a more monoclinic structure and although some layers may not be deficient the overall structure of the crystal will then tend to almost always be monoclinic. Now this is a simplistic view but at least reasonable I think. Pick a location anywhere in the world, pick a Pyrrhotite sample from there, analyze it and you may likely have a signature combination of these phases that potentially distinguishes it from samples elsewhere. This because some samples will be single phase and some multiple phases many with varing proportions of the two or possibly even three phases. So it goes, you could have many combinations; a single phase that's magnetic, a single phase that's non magnetic, multiple phases where one is magnetic the other non magnetic and even multiple phases where there are two or more polytypes that are non magnetic, etc.

As for some asides, I love Rob's discussion on dipole moment and Alfredo makes a good analogy to the similar complex nature of feldspar phases, etc. Pyrrhotite is actually pretty involved and there are also some items on that need to be reviewed or updated for accuracy. For example: The Pyrrhotite page shows the polytype; pyrrhotite-5H, Fe9S10 as hexagonal. If you scroll down the page and click on the link to the Pyrrhotite-5H page , it shows it as orthorhombic. All the best.

Reiner Mielke March 01, 2012 09:49PM
Thanks Ron!
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