[meteorite-list] Troilite inclusions

From: Zelimir Gabelica <Z.Gabelica_at_meteoritecentral.com>
Date: Tue Jan 4 12:13:08 2005
Message-ID: <5.0.2.1.2.20050104175153.02097860_at_pop.univ-mulhouse.fr>

Bernhard Rems wrote:

"The mineral troilite is actually stoichiometric pyrrhotite. Pyrrhotite
ranges in composition from Fe7S8 through to FeS. Most terrestrial pyrrhotite
lack some iron. The troilite end-member (FeS) occurs mainly in meteorites
and lunar samples"

I fully agree. Indeed, after a rapid checking through some textbooks on
hand, I did not find any terrestrial occurrence for troilite.
Does anyone know for some ?

Bernhard added:

"Fe7S8 is monoclinic (pseudohexagonal), whilst Troilite (FeS) is hexagonal.
Non-stoichiometric compositions are due to missing iron atoms, and not the
replacement of iron atoms with sulphur".

This is also true.
More generally, non stoichiometric compounds, often generated through some
(relatively) rapid crystallization process (at least certainly far more
rapid on Earth than what the mateorite magma would require for coolong and
crystallize in Space), thus generating all kind of defects.
The electroneutrality of the resulting phase can be then achieved through
some redistribution of charges that often involve electrons (located on
interstitial positions in the crystal or directly (re)located on the empty
crystallographic positions that become available if an element or ion is
missing in the structure). Similarly positive "holes" can fill equivalent
positions.

In many cases, it is easier (thermodynamically speaking) for the system to
get neutralized through some change of the oxydation state of some adjacent
cation (anion). For example when some Fe(2+) cations are missing, some
other that are located nearby can become (3+) (oxidized) so that there is
less Fe(2+) than S(2-) as a whole but the system remains neutral.
I have in mind the case very similar to FeS, namely FeO, that exists as non
stoichiometric phase Fe(0.94)O. The deficiency in Fe2+ is here sometimes
attributed to the presence of some Fe3+ ions (replacing Fe2+ and thereby
distorting the structure a bit because of their slightly smaller size).

I wonder whether in the case of Pyrrhotite, a similar phenomenon is not
likely to occur, thus whether some Fe2+ are also perhaps replaced by Fe3+.
That could perhaps explain why Pyrrhotite is the stable phase found on
Earth (oxidative atmosphere).
Conversely, in space (absence of oxygen and, hence, less (or no)
possibilities for Fe2+ to get oxidized to Fe3+ ), the system both has TIME
and CONDITIONS to stabilize all the Fe as Fe(2+) perfectly compensating
the S(2-) anions in the structure and thereby generate troilite instead of
pyrrhotite.

Zelimir



A 23:47 03/01/05 +0100, vous avez ?crit :
>What I have found so far:
>
>A non-magnetic form of iron sulfide (FeS) found in a variety of meteorites.
>
>Some troilite is thought to have formed at a 988oC eutectic point in Fe/S
>melts, where native Fe and troilite form simultaneously. Some troilite is
>devoid of native Fe, so this must have formed directly from a sulphur-rich
>silica melt. As well as late-stage crystallization, troilite may occur in
>solid rocks, where the partial pressure of sulphur increases rapidly. Some
>Apollo 16 rocks appear to show this, where native Fe has been "sulphurized"
>during shock metamorphism from meteoroid impacts. Primary troilite usually
>has less Ni and P, and more Co than that of troilite of meteoritic origin.
>
>The mineral troilite is actually stoichiometric pyrrhotite. Pyrrhotite
>ranges in composition from Fe7S8 through to FeS. Most terrestrial pyrrhotite
>lack some iron. The troilite end-member (FeS) occurs mainly in meteorites
>and lunar samples.
>
>Fe7S8 is monoclinic (pseudohexagonal), whilst Troilite (FeS) is hexagonal.
>Non-stoichiometric compositions are due to missing iron atoms, and not the
>replacement of iron atoms with sulphur.
>
>Ni, Co, Mn, and Cu can substitute for Fe. Such impurities have never been
>found above 1wt%, in lunar samples.
>
>Many specimens of pyrrhotite are ferromagnetic (i.e., capable of acting as a
>magnet). This effect decreases as the ideal composition of FeS is reached.
>Troilite is ideally antiferromagnetic.
>
>Pyrrhotite is decomposed by HCl (releasing H2S) whereas pyrite is not.
>Troilite is attacked more readily by dilute nitric acid than pyrrhotite.
>
>Both are opaque in thin section.
>
>
>Bernhard
>
>-----Original Message-----
>From: meteorite-list-bounces_at_meteoritecentral.com
>[mailto:meteorite-list-bounces_at_meteoritecentral.com] On Behalf Of
>bernd.pauli_at_paulinet.de
>Sent: Monday, January 03, 2005 11:43 PM
>To: Meteorite-list_at_meteoritecentral.com
>Subject: [meteorite-list] Troilite inclusions
>
>Norm wrote:
>
> > Norton doesn't answer the Troilite question anywhere that
> > I can see. He simply provides descriptive comments.
>
>Hello Norm, John, Steve, and List,
>
>Unfortunately O.R. Norton is not the only one. Even the famous
>Vagn Buchwald only comes up with a descriptive comment:
>
>"Troilite usually occurs as shapeless nodules, bars and dumbbells.
>The smaller ones may be bar-, diamond-, or plate-shaped."
>
>And:
>
>"At austentitic temperatures troilite, chromite and taenite were
>the only phases present in many iron meteorites. Therefore, when
>the taenite cooled and started to decompose, the first kamacite
>and schreibersite to precipitate formed heterogeneously upon the
>available troilite and chromite * n u c l e i *. That is why so
>many troilite and chromite inclusions are wrapped in successive
>sheets of various minerals. Beautiful examples are to be found in,
>e.g., Canyon Diablo, Coahuila, Sikhote-Alin, Cape York, Chupaderos
>and Wiley."
>
>BUCHWALD V.F. (1975) Handbook of Iron Meteorites, Volume 1, p. 107.
>
>I also tried to find something in Cohen's trilogy. Same result: descriptive
>comments - on page 192, vol.1, you can read in 19th century German (!):
>
>"Schwefeleisen tritt vorherrschend in knollenf?rmigen Massen von sehr
>wechselnder Gr??e auf; besonders charakteristisch sind rundliche bis
>eif?rmige, auch wohl linsenf?rmige Partien. Sie erreichen nicht gerade
>selten Wallnussgr?sse und sind aus Seel?sgen bis zu 9 cm, aus Magura
>sogar bis zu 13 cm gross beschrieben worden; aus Cosby's Creek isolirte
>Smith ein 200 g schweres St?ck. In manchen Meteoreisen trifft man sie
>ziemlich h?ufig von fast idealer Kugelform."
>
>Sulfurous iron is mainly found as globular lumps differing considerably in
>size
>with roundish or egg-shaped but also lenticular parts being especially
>prominent.
>Sometimes they even reach walnut-size and have been described in Seel?sgen
>(diameter up to 9 cm), in Magura (even up to 13 cm); Smith
>separated/isolated
>a piece weighing 200 grams from Cosby's Creek. In some meteoric irons you
>even find them quite frequently displaying an almost ideal/perfect globular
>shape.
>
>COHEN E. (1894) Meteoritenkunde, Heft 1: Untersuchungsmethoden und
>Charakteristik der Gemengtheile (Schweizerbart'sche Verlagshandlung,
>Stuttgart).
>
>
>Best wishes,
>
>Bernd
>
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Prof. Zelimir Gabelica
Universit? de Haute Alsace
ENSCMu, Lab. GSEC,
3, Rue A. Werner,
F-68093 Mulhouse Cedex, France
Tel: +33 (0)3 89 33 68 94
Fax: +33 (0)3 89 33 68 15
Received on Tue 04 Jan 2005 12:20:08 PM PST