[meteorite-list] Troilite inclusions

[Bernhard Rems]

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-----
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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 knollenförmigen Massen von sehr
wechselnder Größe auf; besonders charakteristisch sind rundliche bis
eiförmige, auch wohl linsenförmige Partien. Sie erreichen nicht gerade
selten Wallnussgrösse und sind aus Seeläsgen bis zu 9 cm, aus Magura
sogar bis zu 13 cm gross beschrieben worden; aus Cosby's Creek isolirte
Smith ein 200 g schweres Stück. In manchen Meteoreisen trifft man sie
ziemlich häufig 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 Seeläsgen
(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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