[Rockhounds] Pyrrhotite

[R. Peter Richards]

If it is really the hexagonal version of pyrrhotite (and I have no  
reason to doubt it), the stacked texture cannot be due to twinning,  
because the inherent symmetry already contains the mirror plane that  
would be the twin plane.  I.e. by "twinning" it in this direction,  
you do absolutely nothing to change how it would grow in the absence  
of twinning.  So it fails to meet the definition of twinning, which  
is a symmetry operation absent from the crystal system of the  
mineral, and relates two (or more) parts of a crystal in a manner  
different than that resulting from normal crystal growth. (My  
paraphrase of the concept of twinning).

Muscovite is often twinned, but it is not hexagonal.  And in fact the  
pseudo-c-axis is not at right angles to the other two axes, so  
twinning can occur along this pseudo-axis and (nearly) perpendicular  
to the perfect cleavage, creating minor zig-zags.

Pete Richards


On Oct 4, 2009, at 1:54 PM, Lawrence Rush wrote:

> I have just received what I think is a very interesting mineral  
> specimen that I would like to share, and maybe get some additional  
> information about:
>
> This is a well-crystallized piece of pyrrhotite from Zacatecas,  
> Mexico. It is the hexagonal form (Fe10S11), and has an obvious  
> "stacked" appearance, similar to the way muscovite grows. I am not  
> sure if this reflects a twinning feature or growth lines, maybe  
> someone can help there.
>
> At any rate, I am very pleased to add this one to my permanent  
> collection, and thought you might like to see it, too.
>
> Larry
>
> See it at :  http://www.connroxminerals.com/Temp3.html
>
>
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___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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