Meteorite or Meteor-wrong?

[Probeman]

I should have asked our ET experts this a long time ago, but does anyone have handy a short writeup (or be willing to write a short post), on what are useful chemical characteristics to look for (using an SEM and/or EPMA) in order to distinguish slag, native iron, NiFe meteorites and "meteor-wrongs" from each other?

Beyond the fact that NiFe meteorites have more Ni than Fe, I know I know little about such materials. For example, are there manufactured high alloy steels that are compositionally similar to natural specimens?  That is, what are the EPMA pitfalls in such investigations?

[Mike Spilde]

The majority of meteorites contain some metallic iron, so the first and most fundamental test is whether the suspect meteorite is magnetic. If it isn't, it very likely is NOT a meteorite. Keep in mind, though, that there is a slim chance that it could be a non-magnetic achrondite or even a Martian meteorite (very rare). The most common type of meteorite (and the most common meteor-wrong) brought in for identification is the iron meteorite, which is solid metallic iron. Other types such as stoney-irons will have Fe-metal blebs scattered throughout, but the rock is still magnetic.

All meteoritic iron will contain some amount of nickel, so that is the first thing to look for in the SEM or microprobe. Fe-Ni metal consists of 2 types: kamacite and taenite. Kamacite is the low-Ni variety and contains about 5-10% Ni whereas taenite is Ni-rich with 30-70% Ni. You should be able to find both types in a sample, so look around with the BSE at high contrast for subtle differences in atomic number in the iron metal. Another good test is to look for Cr and Mn.  Meteoritic iron contains very low levels of Cr and Mn, less than 0.02%. Since man-made Fe alloys usually contain some additives, if you find much Cr or Mn, it is definitely NOT a meteorite. If the suspect meteorite is porous or has vesicles, it likely is NOT a meteorite. And, if you find quartz, it is NOT a meteorite (although some Martian meteorites do contain silica).

You can look for unusual phases, such as schreibersite (Fe-Ni phosphide) inclusions in iron meteorites. If sulfides are present in stoney or stoney-irons, check the Fe-S ratio, since most meteoritic sulfides are troilite FeS, not FeS2 as in pyrite.

If you are dealing with stoney meteorites, they should contain olivine, pyroxene, and possibly plagioclase. Chondrites will contain small spherical bodies, resembling droplets, in a mineral matrix. The chrondules are distinctive and will consist of olivine, pyroxene, or both. Occasionally they display a barred texture. There are some distinct chemical signatures for meteoritic pyroxene, olivine, and feldspar, but that topic is too detailed to go into here. A good reference is Papike et al. (2003) Determination of planetary basalt parentage: A simple technique using the electron microprobe, American Mineralogist, 88, 469

[Probeman]

Hi Mike,
Thank-you, this is super helpful!

So, if someone brings in a specimen that is essentially pure Fe with zero Ni and just traces of P then that is not a meteorite?  I ask because I've heard the term "native iron". Is that something real and/or distinct from meteoritic iron?

[mhutson]

Regarding meteoritic iron:   With EMPA, all you are likely to pick up is nickel (4-30% for an iron meteorite, and typical metal in chondrites, stony-irons) and a trace of cobalt (few 10ths of a percent).   There are other trace elements (Ir, Ga, Ge, etc.), but not at levels picked up with EMPA.

While there is some manmade metal with nickel this high, those samples also have measurable amounts (with EMPA) of other material (such as manganese and chromium) which you don't find in such abundance in meteoritic iron.

We have had a few cases of metal with too much nickel - apparently bars of nickel ore having 50% iron/50% nickel were manufactured in Oregon.  But meteoritic iron never goes that high in nickel content. 

True "native iron" is caused by reduction of oxidized iron in rocks.  The best example is in Disko Island, where a basaltic magma interacted with carbon-rich shale and coal layers.  The iron was "reduced" from Fe2+ to Fe^o, and shows up as metal flakes and blobs in the basalt.

Melinda Hutson, curator, Cascadia Meteorite Laboratory, Portland State University

[mhutson]

Oops, forgot to say that in the "native metal", there is essentially no nickel -- making it easy to distinguish from meteoritic metal.

[John Donovan]

Oops, forgot to say that in the "native metal", there is essentially no nickel -- making it easy to distinguish from meteoritic metal.

Hi Melinda,
Note that you can use the "Modify" button after you click Post to make subsequent edits to your posts...

This is very helpful- and finally this is starting to fall into place for me. A complicated area for sure!

Here is a response from Alex Ruzicka that was sent in response to an email fro me- I hope Alex doesn't mind me posting it here:

"How to distinguish iron meteorites from slag or other non-meteorites:
 
(1) Start with hand specimens.  We provide a fair amount of information in our website for things to consider (http://meteorites.pdx.edu ,  http://meteorites.pdx.edu/possible-met.htm ).  For irons the regmaglypts ("thumbprints") are typically prominent.  They are so distinctive one can identify them in images of rocks on Mars, and other evidence (composition, IR radiance) is consistent with these identifications.
 
(2) If you don't have a hand specimen, textures & mineralogy are diagnostic.  Look for FeNi alloys such as kamacite and taenite to make up most of the rock, with anywhere from ~4-25% bulk Ni.  Usually the kamacite and taenite are intergrown in characteristic ways, forming a Widmanstatten texture in octahedrites, or parallel kamacite plates in hexahedrites or spindles in ataxites.  A common accessory mineral is schreibersite.  Silicates can be present in some iron meteorites but are absent in most. 
 
Your email reminds me of a task we had a couple years ago, in which we had only cut pieces of some metal to work with.  They were picked up in southern Oregon supposedly.  In these chunks we didn't see a classic iron meteorite texture, but there wasn't much else besides metal of some sort, and one of the samples had a recrystallized texture that is known for some irons.  As soon as we analyzed it, though, we could tell they weren't irons, because one had essentially Ni-free metal and the other had over 50%. Then my colleague got a chance to look at the samples, and they were clearly not iron meteorites and instead some sort of melted slag material.  We would never have done the analyses had we had a chance to see the handspecimens-- these are diagnostic.  But we figured out quickly nonetheless with chemical analysis."

[John Donovan]

Here's some additional info Melinda sent me that might be useful:

The Elements of Steel Composition (percent by mass)
Cast Iron
Carbon 3.5%
Manganese .5%
Phosphorous .13%
Sulfur .13%
Silicon 1.2%
Cast iron contains high levels of carbon, which makes it a hard, brittle metal. Cast iron was commonly used throughout Europe to make church bells and, in colonial America, pots and pans.

Wrought Iron
Carbon .035%
Manganese .075%
Phosphorous .075%
Sulfur .1%
Silicon - .1%
Wrought iron is a strong, durable metal with a low carbon content. Items such as locks, bolts, tools, and fences are crafted out of this metal. Wrought iron bars were also sold and traded to be later converted into steel or cast iron.

Plain Steel
Carbon 1.35%
Manganese 1.65%
Phosphorous .04%
Sulfur .05%
Silicon .06%
During the early 20th century, new processes in steel production allowed steel to surpass iron as the most widely used structural metal. Its great strength and affordability allowed craftsmen to construct sturdier bridges and higher buildings.

High Strength Steel
Carbon .25%
Manganese 1.65%
Phosphorous .04%
Sulfur .05%
Silicon .12%
Nickel 2.5%
Chromium .8%
Adding alloys to steel yield higher strength, more wear-resistant metals. James Eads used alloy steel in the construction of a bridge across the Mississippi River -- the first steel bridge built in America.

Stainless Steel
Carbon .08%
Manganese 2%
Phosphorous .04%
Sulfur .03%
Silicon .75%
Nickel 8%
Chromium 18%
From spoons to blenders, cars to trains, stainless steel, with its sleek, shiny surface, can glorify even the most simple of gadgets. In addition to its aesthetic appeal, the light weight and strength of stainless make it ideal for transportation.

[John Donovan]

True "native iron" is caused by reduction of oxidized iron in rocks.  The best example is in Disko Island, where a basaltic magma interacted with carbon-rich shale and coal layers.  The iron was "reduced" from Fe2+ to Fe^o, and shows up as metal flakes and blobs in the basalt.

Here's the link:

http://link.springer.com/article/10.1007%2FBF00389387

[Probeman]

I'm attaching some additional papers that Alan Ruzicka sent me that look useful for meteorite analysis.

[Les Moore]

I once forked out some good money for a metallic meteorite and found I had been ripped off and the observations I used required no microanalyses at all.  The sample had evidence of cold work.  By this I could clearly see flow lines all around the sample where it had been tastefully dinged in a manner to con the gullible.

There were also ternary eutectic colonies in the material which would indicate it had solidifed from the melt. Its analysis could have been on spec but I doubt it would have these metallurgical features.  But then, I'm a metallurgist dabbling in the science of Geology - always a dangerous activty.