[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Cliftonite - only in meteorites?
Dieter Heinlein schrieb:
> Recently I got a rock sample that was analysed at the Freiburg Uni-
> versity: it consists of (nearly pure) CLIFTONITE! Has anyone of you
> an idea if cliftonite (a mineral that first has been discovered in
> meteorites!).
For the record:
These IA irons contain cliftonite:
Magura, Sargiin Gobi, Seligman, Linwood, Odessa, Smithville,Youndegin,
Otasawian, Sargiin Gobi.
1) Buchwald, Handbook of Iron Meteorites, Vol. 1, pp. 109-110:
Cliftonitic graphite - i.e. graphite crystals that, in sections, display
cubic, cubo-octahedral and pseudo-hexagonal outlines (Figure 166*) has
been observed by Haidinger (1846) and was named cliftonite by Fletcher
(1887c).
The cliftonite is apparently not a pseudomorph after diamond but rather
a solid state precipitate of C in kamacite, in which the external faces
of the growing graphite are governed by the cubic structure of kamacite
and thus assume cubic outlines.
Brett & Higgins (1967) demonstrated that graphite aggregates of cubic
morphology could, in fact, be grown in the laboratory by annealing
Fe-Ni-C alloys for about 1000 hours at one atmosphere and 550° C.
*Figure 166, caption: Seligman (U.S.N.M. no. 1761). Graphite occurs in
the kamacite phase as cliftonitic units with fine, oriented extinction.
Figure 167, caption: Seligman (U.S.N.M. no. 1761). The cliftonitic
graphite may also assume square outlines.
2) Buchwald, Handbook of Iron Meteorites, Vol. 2, p. 775: The [Linwood]
graphite is normally microcrystalline, but all border zones against
kamcite and troilite are composed of well-developed, cliftonitic
crystals, 50-200 µ across. Cliftonite also occurs as angular units of
the same size in the adjacent, swathing kamacite rims.
3) Buchwald, Handbook of Iron Meteorites, Vol. 2, p. 793: Haidinger
(1846) observed cubic graphite crystals [in Magura] with truncated edges
... Brezina
(1889) discussed Fletcher's description (1887c) of graphite crystals in
Youndegin and added his own observations on Magura in support of the
diamond pseudomorph theory... Only in recent years, corroberated by the
observations of, e.g. Lipschutz & Anders (1961), Brett & Higgins (1967)
and Buchwald & Wasson (1968) does Fletcher's view again appear to win
support: cliftonitic graphite is not a pseudomorph but probably a
solid-state precipitation of carbon taking place at long time annealing
above 500° C.
4) Buchwald, Handbook of Iron Meteorites, Vol. 3, p. 941: The graphite
[in Odessa] shows horsetail extinction or, locally along the edges, is
developed as
aggregates of 50 µ cliftonite crystals. Individual cliftonite crystals,
50-100 µ across, occur in the troilite, in the kamacite and in the
schreibersite.
5) Buchwald, Handbook of Iron Meteorites, Vol. 3, p. 1101: Graphite [in
Seligman] is further present as unusually well developed cliftonite
crystals with fine, oriented extinction. A square cliftonite crystal,
completely embedded in cohenite measured 200 µ across. Several square,
hexagonal or octagonal crystals in the kamacite varied from 200-800 µ in
size.
6) Buchwald, Handbook of Iron Meteorites, Vol. 3, p. 1149: Some of the
[Smithville] graphite has crystallized as perfect cliftonite crystals,
30-150 µ across. Most of them are cubes, judging from the hexagonal and
cuboid forms, appearing on sections; and they are uniformly oriented
within the same kamacite grain. They are also found in the schreibersite
and in the troilite-schreibersite interphase.
7) Buchwald, Handbook of Iron Meteorites, Vol. 3, p. 1355: Cliftonite
[in Youndegin] occurs as well developed large crystal aggregates,
100-600 µ across. In exceptional cases they attain 1 mm size. They are
situated in the immediate surroundings of troilite, either in kamacite
or overgrown with later precipitates of schreibersite and cohenite.
8) Buchwald, Handbook of Iron Meteorites, Vol. 3, p. 1409: The
[Otasawian] graphite shows cliftonitic development in many places;
particularly good crystals , 0.2-0.3 mm in size, are to be found in the
complex transition zones between troilite, cohenite and schreibersite.
The cohenite itself has not started decomposition to graphite.
As for cliftonite in Sargiin Gobi, see:
A.Bischoff et al. (1996) Meteorites from Mongolia (Meteoritics 31-1,
1996, 152-157):
" ... The graphite shows horsetail extinction and the troilite shows a
polycrystalline texture indicative of recrystallization. Within the
kamacite numerous 50 - 100 µm sized cliftonite crystals were found (
i.e. graphite with pseudo-hexagonal to cubic morphology; cf. Fig. 1576,
Buchwald, 1975)..."
I hope I didn't freak anybody out! :-)
Best wishes, Bernd
P.S.1: Thank you, Martin, for those friendly lines!
P.S.2: With regard to Martin's DaG 313, someone wrote he would have a
closer look at his Richfield specimen. In Meteoritics 31-6, 1996, pp.
925-926, there is a report on the Richfield LL3 chondrite by A.E. Rubin.
References: