[meteorite-list] R Chondrites and Magnetism

Tue May 31 16:17:32 EDT 2005

Hola, Bernd, Tom, List, PS, I would like to add  to this, that the reason the 
pyrrhotite mineral is magnetic is due precisely to  excess of Sulfur 
(non-stoichiometric pyrrhotite).  Like troilite,  pyrrhotite loses its magnetism as it 
becomes saturated with iron (i.e. the net  formula).  The fact that some are 
not as magnetic as others could easily be  speculated to be because the 
pyrrhotite is closer to stoichiometric (one iron to  one sulfur).  Thus, it is the 
ABSENCE in the difficient iron Iron sulfide,  not the presence of iron that 
probably contributes to the greater  magnetism!!

A surprising result until you consider that when all the  "holes" that could 
have been filled by oxidized iron destroy the conditions for  magnetic moment 
cancellation as is the case in troilite or "Iron saturated"  pyrrhotite...

Saludos, Doug

Doug wrote:
Bernd, as you rectify  Tom's synthesis of  everything thus far, to "*Some* 
R-chondrites have  *j u s t* enough metal to be  attracted to a strong enough 
magnet" -  that's not what you meant to say, I  cannot help believing.  I had 

posted (and you also later cited) the  literature the paper describing  the 
magnetism to the pyrrhotite which is about  5% of the weight  percent of the 
material 
at least in the type specimen.   You even  quoted O. Richard Norton's book, 
which I will will also do, page   191:  "The pyrrhotite is magnetic giving 
the 
meteorites weak  magnetic  attraction."  The rest of Norton's comments and 
yours  
supports that there  is little or no reduced metal:

See once  again, Meteoritics V. 29, No. 2,  pp. 275-286 March 1994
"Mineralogy and  Chemistry of Rumuruti: The First  Meteorite Fall of the New 
R 
Chondrite  Group" by Schulze, H.; Bischoff, A.;  Palme, H.; Spettel, B.; 
Dreibus,  G.; Otto, J.

It is clear to me that with  a strong enough magnet  whatever that means 
funny 
things can happen, but I think  you were more  focussed on why the rock is 
attracted to a magnet in the  listmember  type tests (indeed why it is a 
magnet, 
too).  This  is due   according to the literature the pyrrhotite as best as I 
can tell, which  I  wouldn't call metal, but just a magnetic mineral, causing 
the 
debil  magnetism of  the rock as a whole.

Thus, careful, as the literature  does support the  high oxidation levels, 
but 
it does not support that  the magnetism some list  members are observing as 
being due to *j u s  t* enough metal, but rather  principally to the low 
content 
of the  magnetic pyrrhotite mineral, which is iron  "oxidized" by an excess 
of  
sulfur under oxidizing conditions.  Recal that  pyrrhotite is  magnetic but 
troilite isn't, and troilite is stochiometric FeS -  i.e.,  when an equal or 
excess of iron is present, and not the oxidative   reverse.  Of course you 
may have 
another source backing up the  magnetic  metal grains (which are in the trace 
level per 1994  paper)?  If I have  missed something, it would be nice to 
know  
your thinking!  How much  susceptibiulity the hypothetical free  metal 
contributes seems to be another  question with no support nor  quick and 
dirty magnet 
test.  I did notice  that you added "some"  which could mean one, but I don't 
get 
that that was your  thinking,  either...

And errrr... what's this about water in space   ;-)
Saludos, Doug

En un mensaje con fecha 05/31/2005 2:01:22  PM  Mexico Daylight Time, 
peregrineflier at npgcable.com  escribe:
"extensive aqueous  alteration on the R parent  body"

Water in space? That sounds like a whole  new subject! : )  

Good grief, Tom

Thanks,  Tom
peregrineflier  <><

----- Original Message ----- 
From:   <bernd.pauli at paulinet.de>
To:   <Meteorite-list at meteoritecentral.com>
Sent: Tuesday, May 31, 2005  11:48  AM
Subject: [meteorite-list] R Chondrites and  Magnetism

> Tom  concluded:
> 
> > So, most  R's have enough metal to be
>  > attracted to a powerful enough  magnet
> 
> Careful, please. I'd  like to add the little word  "just",
> leave out the word "most" and  replace it by  "some":
> 
> > Some R's have  j u s t   enough  metal to be
> > attracted to a powerful enough magnet  ...
>  
> This sounds better because almost all the iron is oxidized   and
> also FeNi (nickel-iron) is extremely rare (only some tens of   parts
> per million!)
> 
> In his Cambridge Encyclopedia,  O.R.  Norton writes on p. 116:
> 
> "There is almost no free  iron metal (a  few grains here and there) ..."
> 
> > ... if  I can find out why  R's have so little metal.
> 
> Oxidation  (water was the oxidizing  agent):
> 
> Step 1: extensive  aqueous alteration on the R parent  body
> Step 2: dehydration  (removal of water) + thermal  metamorphism
> 
> 
> Best  wishes,
> 
>  Bernd
> 
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