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Figures 1-2. A small slice of iron meteorite, displaying an etched face (left) and a polished face (right). This iron displays, on the etched face, the Widmanstatten pattern of alloy intergrowths in the nickel-iron matrix, plus inclusions of both tawny sulphide (troilite) and of silicate inclusions. The latter are rather unusual in iron meteorites, though they occur in a number of examples, such as Kodaikanal, Landes and Miles. Sample weight 11.92 grams. The slice is perfectly square-cut, 24x24x3 mm, the nominal specific gravity 6.90 (less than a typical iron meteorite, which is composed largely of Ni-Fe alloys). Sample from Blaine Reed, Denver show, September 2015.
"Rock of the Month #173, posted for November 2015" ---
A "silicated iron" meteorite
from Russia.
The Maslyanino IAB iron
meteorite is a fine octahedrite with silicate inclusions (a "silicated iron" in common parlance).
A single mass of 26 kg was found in 1992 by agronomist V.G. Okel' in a wheat
field near Maslyanino village, Novosibirsk region, Siberia, Russia.
Maslyanino (Wlotzka, 1995, p.793) assays 12.43% Ni, 70 ppm Ge, 29 ppm Ga
and 1.0 ppm Ir. The main mass is in the Geological Museum of Novosibirsk
(Wlotzka, 1995).
I am not aware of any published work on this interesting iron, which is strange, given that
the TKW is 26 kg and material is widely available through meteorite dealers.
A video showing the etching of a thin, polished slice of
Maslyanino can be viewed on YouTube, courtesy of
Polandmet.
A minority of iron meteorites contain silicate inclusions, in addition to the
usual components such as Ni-Fe alloys, troilite, schreibersite and graphite.
The IIE irons are especially prone to being "silicated irons" (Delaney, 2000).
Some of the larger irons such as Campo del Cielo are largely inclusion-free,
but occasional silicated samples indicate that the parent meteoroid was
far from homogeneous. Twenty-seven examples are shown in the following table,
with a capricious selection of references, weighted toward those
with hand-specimen photographs.
Figure 3.
More slices of Maslyanino, showing nice etch.
Note the generally smooth, subrounded outlines of the
dark silicate inclusions.
Blaine Reed collection, Denver show, September 2015.
The genesis of the many classes and smaller grouplets of meteorites
has long been summarized as follows:
1) the chondrites are the consolidated detritus of the early
solar nebula, formed by condensation from very high temperatures and
modified by local remelting and cooling events,
at variable distances from the young Sun, while
2) irons, stony-irons and the igneous achondrites formed in
differentiated (i.e., melted and concentrically stratified)
parent bodies, yielding an asteroid like a small version
of the model for the Earth: iron core, silicate (achondrite) mantle,
with some core-mantle mixing to form a thin boundary layer of metal
and silicates (pallasites and mesosiderites).
Variations on such models probably account for most meteorites,
but the tremendous advances in meteorite research in the past 50 years
have led to a more sophisticated view.
In the case of the iron meteorites, much attention was paid
to chemical and textural classifications, beyond the earlier
textural and structural divisions into octahedrites, hexahedrites
and ataxites.
John Wasson, Ed Scott and colleagues put much effort into
analyses of select major, minor and trace elements
that could, in samples of circa 1 gram of metal,
provide insights into the evolution of the diversity of
compositions.
Earlier data on Ni, Ir, Ga and Ge were augmented
by a range of elements of differing degrees of
volatility and siderophile (iron-loving) affinity.
These included Cu and Co, As and Sb, W and Au.
Wasson et al. (1980)
reclassified some Ni-poor IAB irons as IIICD.
In contrast to the original concept of
an iron-rich melt, sinking to the core of a differentiated body
(the so-called magmatic irons), IAB and IIICD
irons did not form by fractional crystallization of metallic magma
but, perhaps, in the segregation of small pools of impact melt on the
surface of an unequilibrated parent body
(Scott and Wasson, 1975; Choi and Wasson, 1994).
The IIICD irons
contain inclusions with a mixture of silicates
and phosphates, as well as more reduced minerals
such as graphite and phosphides.
The assemblages are not in equilibrium,
and it is likely that
liquid immiscibility played a role in the
formation of these irons (McCoy et al., 1993a).
The IIICD irons Maltahohe, Carlton and
Dayton all contain silicate-bearing inclusions rich in troilite, graphite,
schreibersite and phosphates (including uncommon phases such as
brianite, panethite and chladniite (McCoy et al., 1993b).
The winonaites (such as Pontlyfni and Mount
Morris (Wisconsin)) are achondrites that contain
relict chondrules. Oxygen isotopes in winonaites
match values in IAB and IIICD irons.
Brecciation in an impact event is one obvious way in which
chondrite clasts and achondrite melt could be mixed together
(Benedix et al., 1998).
The ungrouped achondrite Dhofar 500 is a breccia that may
be related to winonaites and silicate-bearing irons
(Lorenz et al., 2003).
In the case of the fine-grained ataxite irons,
both IVA and IVB irons cooled
fast, presumably in relatively small parent bodies, the low volatile
contents perhaps the result of planetary outgassing. IVB is the
group of magmatic irons with the highest Ni content (16-18 wt.%)
and the lowest content of volatiles such as Ge
(Rasmussen et al., 1984).
The IVA irons may contain silicate inclusions.
Some of these are mixtures of pyroxenes and tridymite,
possible cumulates.
Coarse textures and mutual intergrowths of pyroxenes and
tridymite are consistent with slow crystallization in the core of the
IVA parent body. Some may have formed at high temperatures
near the core-mantle
boundary of a differentiated parent body
(Ulff-Moller et al., 1995).
The IIE irons
are non-magmatic irons which, unlike
magmatic irons, not only contain the inclusions but also fail to
show geochemical trends indicative of fractional crystallization of
a slow-cooled metallic melt.
Mont Dieu, the largest-known IIE iron, contains
cm-size silicate inclusions with relict chondrules,
in which oxygen isotopes are
consistent with origin as an H-chondrite.
The metal
and silicate inclusions may have formed
by impact on the H-chondrite parent body early in
solar system history.
According to the nature of their silicate assemblages, IIE irons are
classified as primitive (1) to differentiated (5), thus
(1) Netschaevo and Mont Dieu, with preserved, relict chondrules, (2)
Techado, (3) Watson, (4) Miles and
Weekeroo Station and (5) Elga, Colomera and Kodaikanal.
A separate relation by age of formation recognizes
old, circa 4500 Ma examples (Weekeroo Station, Miles, Colomera and
Techado) and a younger, 3600 Ma event (Netschaevo,
Kodaikanal and Watson,
see Van Roosbroek et al., 2015).
References
Benedix,GK, McCoy,TJ, Keil,K, Bogard,DD and Garrison,DH
(1998) A petrologic and isotopic study of winonaites: evidence for
early partial melting, brecciation, and metamorphism.
Geochimica et Cosmochimica Acta 62, 2535-2553.
Brandstatter,F, Ferriere,L and Koeberl,C (2013)
Meteoriten - Meteorites: Zeitzeugen der Entstehung des
Sonnensystems / Witnesses of the Origin of the Solar System.
Verlag des Naturhistorisches Museum, Vienna, 270pp. (in Engl.
and in Ger.).
Choi,B-G and Wasson,JT (1994) Formation of IAB and IIICD iron
meteorites. Lunar and Planetary Science 25, 255-256.
Delaney,JS (2000)
IIEs or not IIEs: reduction is the question.
Meteoritics & Planetary Science 35, A48.
Dodd,RT (1981)
Meteorites: a Petrologic-Chemical Synthesis.
Cambridge University Press, 368pp.
Grady,MM, Pratesi,G and Moggi-Cecchi,V (2014)
Atlas of Meteorites.
Cambridge University Press, 373pp.
Haag,RA (1997)
The Robert A. Haag Collection Field Guide of Meteorites.
12th Anniversary Edition, Tucson, AZ, 60pp.
Haag,RA (2003)
The Robert Haag Collection of Meteorites.
Robert Haag Meteorites, Tucson, AZ, private collection edition,
126pp.
Heritage Auctions (2013)
The Hoppel Collection of Fine Minerals, Auction 1 (session 1):
and session 2, Natural History.
Heritage Auctions, Dallas, TX, 2-part auction catalogue, available
as a 366pp., 38.1 MB pdf file: Hoppel (228pp.) and Natural
History (138pp.) (02 June).
Killgore,K, Killgore,M and Killgore,E (2002)
Southwest Meteorite Collection, a Pictorial Catalog.
Southwest Meteorite Press, 201pp.
Krot,AN, Keil,K, Goodrich,CA, Scott,ERD and Weisberg,MK (2005)
Classification of meteorites.
In `Meteorites, Comets, and Planets' (Davis,AM editor). Treatise
on Geochemistry volume 1 (Holland,HD and Turekian,KK
editors), Elsevier- Pergamon, Oxford, 737pp., 83-128.
Kurat,G, Zinner,E and Varela,ME (2007)
Trace element studies of silicate-rich inclusions in the Guin
(UNGR) and Kodaikanal (IIE) iron meteorites.
Meteoritics & Planetary Science 42, 1441-1463.
Kurat,G, Varela,ME, Zinner,E and Brandstatter,F (2010)
The Tucson ungrouped iron meteorite and its relationship to
chondrites.
Meteoritics & Planetary Science 45, 1982-2006.
Lorenz,CA, Ivanova,MA, Nazarov,MA, Mayeda,TK and
Clayton,RN (2003) A new primitive ungrouped achondrite,
Dhofar 500: links to winonaites and silicate inclusions from
IAB-IIICD irons. Meteoritics & Planetary Science 38, A30.
McCoy,TJ, Keil,K, Scott,ERD and Haack,H (1993a)
Genesis of the IIICD iron meteorites: evidence from
silicate-bearing inclusions. Meteoritics 28, 552-560.
McCoy,TJ, Steele,IM, Keil,K, Leonard,BF and Endress,M
(1993b) Chladniite: a new mineral honoring the father of
meteoritics. Meteoritics 28, 394.
McSween,HY (1999)
Meteorites and their Parent Bodies.
Cambridge University Press, 2nd edition, 310pp.
Mittlefehldt,DW (2005)
Achondrites.
In `Meteorites, Comets, and Planets' (Davis,AM editor). Treatise
on Geochemistry volume 1 (Holland,HD and Turekian,KK
editors), Elsevier- Pergamon, Oxford, 737pp., 291-324.
Olsen,EJ and Schwade,J (1998)
The silicate inclusions of the Ocotillo IAB iron meteorite.
Meteoritics & Planetary Science 33, 153-155 (1998).
Class Names and select references IAB Caddo County (Takeda et al., 2000), Campo del Cielo, Copiapo (Killgore et al., 2002), Kendall County (Grady et al., 2014), Landes (Mittlefehldt, 2005), Maslyanino, Ocotillo (Killgore et al., 2002), Pitts (McSween, 1999), Udei Station (Killgore et al., 2002; Haag, 2003), Zagora (Haag, 1997) IIE Colomera, Elga, Miles (Brandstatter et al., 2013), Kodaikanal (Dodd, 1981), Sombrerete, Tarahumara, Weekeroo Station IIICD Maltahohe, Carlton and Dayton (McCoy et al., 1993a,b) IVA Bishop Canyon (Krot et al., 2005), Sao Joao Nepomuceno, Steinbach UNGR Guin (Kurat et al., 2007), NWA 5549 (Heritage Auctions, 2013), NWA 6583, Tucson (Kurat et al., 2010) ![[240 kb]](mas-plus3.jpg)
Rasmussen,KL, Malvin,DJ, Buchwald,VF and Wasson,JT (1984) Compositional trends and cooling rates of group IVB iron meteorites. Geochimica et Cosmochimica Acta 48, 805-813.
Scott,ERD and Wasson,JT (1975) Classification and properties of iron meteorites. Rev.Geophys.Space Phys. 13 no.4, 527-546.
Takeda,H, Bogard,DD, Mittlefehldt,DW and Garrison,DH (2000) Mineralogy, petrology, chemistry, and 39Ar- 40Ar and exposure ages of the Caddo County IAB iron: evidence for early partial melt segregation of a gabbro area rich in plagioclase-diopside. Geochimica et Cosmochimica Acta 64, 1311-1327.
Ulff-Moller,F, Rasmussen,KL, Prinz,M, Palme,H, Spettel,B and Kallemeyn,GW (1995) Magmatic activity on the IVA parent body: evidence from silicate-bearing iron meteorites. Geochimica et Cosmochimica Acta 59, 4713-4728.
Van Roosbroek,N, Debaille,V, Pittarello,L, Goderis,S, Humayun,M, Hecht,L, Jourdan,F, Spicuzza,MJ, Vanhaecke,V and Claeys,P (2015) The formation of IIE iron meteorites investigated by the chondrule-bearing Mont Dieu meteorite. Meteoritics & Planetary Science 50, 1173-1196.
Wasson,JT, Willis,J, Wai,CM and Kracher,A (1980) Origin of iron meteorite groups IAB and IIICD. Z.Naturforsch. 35a, 781-795.
Wlotzka,F (1995) The Meteoritical Bulletin, No.78, 1995 November. Meteoritics 30, 792-796.
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