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Improving Mindat.orgPyrrhotite polytype errors?

3rd Feb 2012 14:53 UTCReiner Mielke Expert

According to Mindat http://www.mindat.org/min-3328.html; 6C is Monoclinic and 11C is Hexagonal but if you look at the IMA list here: http://rruff.info/uploads/AM62_411.pdf ; 6C is Hexagonal "p y r r h o t i t e - H 2 a 2 a 6 c ( 6 C t y p e p y r r h o t i t e ) and 11C is orthorhombic "p y r r h o t i t e - O R 2 a 2 b l l c ( l l C t y p e p y r r h o t i t e )" ?? I have seen another publication that refers to 6C as hexagonal but have never seen 11C as being called orthorhombic. Who is right?

10th Feb 2012 18:39 UTCReiner Mielke Expert

This paper refers to 6C as being hexagonal http://igitur-archive.library.uu.nl/geo/2007-1126-201212/dekkers_88_magnetic.pdf

6th Jan 2015 01:09 UTCReiner Mielke Expert

According to this : http://rruff.info/uploads/AM62_411.pdf shouldn't this http://www.mindat.org/min-46090.html be Pyrrhotite-6M? However why is this http://www.mindat.org/min-40440.html there, it has the same formula and structure? Isn't it a duplication? Also shouldn't this http://www.mindat.org/min-39654.html be Pyrrhotite-6O or -6OR? Sorry I don't have access to the most recent publications: de Villiers J P R, Liles D C (2010) The crystal-structure and vacancy distribution in 6C pyrrhotite, American Mineralogist, 95, 148-152 and 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 so things may have changed? Is there a new convention I am not aware of?

6th Jan 2015 01:16 UTCJolyon Ralph Founder

I tried a while back to get references for all different polytypes of pyrrhotite. I found, as you did, the data were inconsistent and the change I made at the time were based on the best references I could find (see references listed on each page.

6th Jan 2015 01:23 UTCReiner Mielke Expert

Thanks for trying Jolyon, the literature seems to be a hell of a confusing mess. Hopefully someone with some expertise in this field will be able to provide some insight.

6th Jan 2015 03:30 UTCFrank Keutsch Expert

I need to find a reference for this again, but I recall that the -XC just refers to the supper lattice size X times the c direction and that you can have both -6M (low-T) and -6H (high-T) . They both have the same formula.

Frank

7th Jan 2015 01:21 UTCReiner Mielke Expert

Hello Frank,

Mindat currently has a 6C (monoclinic) and a 6M (monoclinic) listed. So is 6C ( as noted in Mindat) the same as your 6H? Does the letter no longer represent the crystal system?

7th Jan 2015 02:21 UTCFrank Keutsch Expert

6C means that it is sixfold axis length in the C direction. This can be either monoclinic or hexagonal, but I think monoclinic is stable at normal temperature.

Frank

7th Jan 2015 11:11 UTCBen Grguric Expert

The literature is confusing because there are discrepancies between the structures of synthetic pyrrhotites and natural ones. There are a lot of papers on the pyrrhotites because they are structurally complex, and lots of experimental studies have been carried out since the 60's. A whole range of superstructures are generated in the compositional range Fe7S8 to Fe11S12 due to vacancy and Fe atom ordering arrangements in lab products. Some structures are quenched in when synthesised but don't occur in natural samples.

Mindat is I guess only concerned with the natural pyrrhotites.

Natural pyrrhotites include Fe7S8 (monoclinic 4C), Fe9S10 (hexagonal 5C), Fe10S11 (hexagonal 11C), Fe11S12 (hexagonal 6C), Fe9S10-Fe11S12 (hexagonal non-integral or incommensurate NC). On a very fine scale most natural pyrrhotites are actually an intergrowth of the different structural types. Probably the most stable with time are intergrowths of monoclinic 4C pyrrhotite and troilite.

A good reference is Lennie and Vaughan (1996) Special Publication of the Geochemical Society Vol. 5, pg. 117-131.

7th Jan 2015 11:43 UTCBen Grguric Expert

A further complicating factor is that the unit cell angular dimensions of 5C pyrrhotite are so close to 90 degrees (or 120 degrees) that when performing crystal structure refinements the choice of orthorhombic, hexagonal or monoclinic symmetry are apparently almost interchangeable. However the most recent refinement of some natural 5C pyrrhotites carried out by Lilies and de Villiers (2012) resulted in a best fit with a monoclinic cell (space group P2_1). Thus technically 5C pyrrhotites are monoclinic rather than hexagonal. Determination of this requires detailed single crystal diffraction work, not simply a run with a powder diffractometer.

7th Jan 2015 15:57 UTCReiner Mielke Expert

So Ben would you recommend then that Mindat just drop the M's and H's and just use C for everything? What to do with the 6M listed in Mindat currently? If that is changed to 6C then we will have two 6C's? I suppose to account for the old terminalogy that is still around we should list them as synonyms?

7th Jan 2015 23:16 UTCBen Grguric Expert

I think so. Most of my more recent references just use the #C to represent the cell repeats in the c-direction for the different natural polytypes. Practically though, most will simply characterise a sample as hexagonal dominant or monoclinic dominant pyrrhotite.

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