[meteorite-list] R Chondrites and Magnetism

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Tue May 31 16:17:43 2005
Message-ID: <bb.588219db.2fce205c_at_aol.com>

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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Received on Tue 31 May 2005 04:17:32 PM PDT