Irons
As the late, great O. Richard Norton has written, “If achondrites come from the crust of another world, iron meteorites must be from its core” (Rocks, 215). As we can imagine, impacting crustal rock off of an asteroid means one type of event, but to impact mantle and core is another complete order of destruction.
Let's return to our illustration of a parent body and look at the core:
As you can see, to get to the mantle and core, this parent body must be shattered, completely. Imagine the intensity to break solid iron.
But as iron meteorites have shown and have been collected from around the world – this has been done, frequently.
Types of Nickle-Irons
To create these massively solid and beautiful meteorites, the irons had to have grown in a liquid state, a state where the parent body kept hot enough to where the heavier elements settled to the center (a core), the lighter mantle accumulated around it, and the surface crusted with the lightest elements which had floated up.
Apart from this settling into zones, the cooling and mixing of the elements - especially nickel and iron at the center - determined the type of core which was formed.
"The structure of an iron meteorite depends on the ratio of nickel to iron and the cooling rate of the planetary core where it originated" (Norton, 216).
So, let's look at this
From this mixing of nickel and iron and from how this mixture cooled, we have three types of iron meteorites. (This is the classic, older structural view of iron meteorites.)
- Hexahedrites
- Octahedrites
- Ataxites
Each has a distinctive structure based on the amount of nickel it contains.
Element |
% in the meteorite |
Nickel (Ni) |
< 5 to 50% |
Iron (Fe) |
50 to 95% |
These combinations are called nickel-iron alloys. And the two most important are named Kamacite and Taenite, minerals which have six-sided crystals (hexahedral).
Within the cooling center of a parent body, Taenite and Kamacite form and divide into three fields – based on that cooling:
During cooling |
% of nickel |
Alloy Structure |
Field 1 - In the melt |
30 |
Only Taenite |
Field 2 - In the melt |
7 to 13 |
Both Taenite and Kamacite in intergrown crystals |
Field 3 - In the melt |
4.5 to 6.5 |
All Taenite changes to Kamacite |
We can see these mixtures, and in the older classification system, the alloy's crystalline structures define the iron meteorite.
Hexahedrites
Since Kamacite is a nickel-iron mineral with a low nickel content, between 4.5 to 6.5 percent, its crystal system defines Hexahedrite meteorites. In other words, Hexahedrites consist of large cubic crystals of kamacite over 50 mm across, and they can be cleaved in three perpendicular directions along the faces of a hexahedron, hence their name. They display a pattern of fine lines called Neumann lines, after the German mineralogist Franz Ernst Neumann, who first studied them in 1848.
We can see this structure by using a specific process of cutting, polishing, and etching (with dilute nitric acid).
Twannberg Meteorite – Main mass of the 1984 find. © www.meteoritestudies.com
So, this is one type of iron that has made it to the Earth. Now let's look at the other end of the three fields: Those irons created nickel-rich.
Ataxites
The ataxites, also called silicated irons, contain Iron and a significant percentage of nickel (greater than 16%), forming a unique nickel-iron alloy.
It's a beautiful iron as you can see in this example.
This is the Chinga Meteorite, from the Chinga river bed in Tanna Tuva, Turvinskaya, Russia.
It was first discovered in 1911.
From the Glendale Community College Earth Science Image Archive: http://www.gc.maricopa.edu/earthsci/imagearchive
Ataxites do not exhibit Neumann lines or a Thomson pattern when cut, polished, and etched with nitric acid or ferric chloride.
It is the most nickel-rich of the Iron structural-groups.
So, from one extreme to other, let's now look at the middle field, the Octahedrites.
Octahedrites
The meteorites in this group consist of a striking intergrowth of the nickel-iron minerals, kamacite and taenite. When polished and etched with acid they reveal the Thomson or Widmanstätten pattern.
A Side Trip
Widmanstätten patterns, called also Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite and taenite called plessite can be found.
Go here for a more detailed explanation:
http://en.wikipedia.org/wiki/Widmanst%C3%A4tten_pattern
Here's an example of the Thomson structures.
From the New England Meteoritical Services.
This formation has only been found in meteorites because of the millennia long cooling time needed for its creation. As a result, this pattern can not be created in a laboratory.
This pattern plays a key role in identifying Octahedrite meteorites. Here's how:
The rate at which an asteroid core cools as well as the amount of taenite contained in the metal affects the thickness of the kamacite bands in the Thomson pattern.
This has lead to descriptive structural classifications such as coarsest, medium and fine.
Kamacite Crystal Band Widths of Octahedrites
Structural
Type |
Crystal Band Width |
% Of Nickel |
Standard
Symbol |
Chemical Groups
See New |
Coarsest |
greater than 3.3 mm |
5 to 9 |
Ogg |
IIAB, IIG |
Coarse |
1.3 to 3.3 mm |
6.5 to 8.5 |
Og |
IAB, IC, IIC, IIIAB, IIIE |
Medium |
0.5 to 1.3 mm |
7 to 13 |
Om |
IAB, IID, IIE, IIIAB, IIIF |
Fine |
0.2 to 0.5 mm |
7.5 to 13 |
Of |
IID, IIICD, IIIF, IVA |
Finest |
less than 0.2 mm |
17 to 18% |
Off |
IIC, IIICD |
Plessitic |
less than 0.2 mm kamacite spindles |
9 to 18% |
Opl |
IIC, IIF |
Examples
Here are a few samples of these Octahedrites:
Very Course Octahedrites
Image from www.astroday.net/NMNH.html
Notice the vary large bands
Another Very Course Example
The Sikhote-Alin shower of 1947 took place in a remote area of eastern Russia and was the largest single meteoritic event documented in modern times.
Image from http://www.chief-impactor.de/index.php?page=collection&cat=meteorite
Course Octahedrite
A 500g endcut from the Toluca iron meteorite .
Image from http://en.wikipedia.org/wiki/File:TolucaMeteorite.jpg
Medium Octahedrite
Henbury, Northern Territory, Australia, found in 1931, with a bandidth of 0.9 mm
Image from www.chief-impactor.de/index.php?page=collection&cat=meteorite
Fine Octahedrite
Muonionalusta.
Image from http://www.muonionalustameteorites.com/main.htm
Notes: The first Muonionalusta meteorite, from the north of the Artic circle in the uppermost region of Sweden, was discovered in 1906 by Viktor Mattila and his sister Amalia Mattila. In subsequent years other Muonionalustas have been found in the region as well. The largest specimen, weighing 15 kg, was found in 1946.
For more information about this meteorite, please visit the Muonionalusta site, run by Thomas Österberg and Daniel Svensson. Use the link above.
Another Fine Example
Local people were aware of this meteorite before 1836, but that year is recorded as its discovery date in Namibia, Africa. Known as Gibeon, its Thomson pattern measures 0.3 mm and also contains rare silica inclusions.
Image from www.rocksonfire.com
Plessitic Octahedrite
Taza (NWA 859), Morocco
Image from http://www.meteoritemarket.com/TAZ.htm
Note: Plessite is a fine mixture of kamacite and taenite. Plessite develops at low temperatures from retained taenite and fills the spaces between Thomson or Widmanstatten patterns. Descriptive names like finger plessite or net plessite are used for the different configurations.
Here's an example of Plessite structures in a Gibeon, which has very well developed Thomson figures. See below.
This part of the Gibeon shows kamacite bands and dark fields of cellular plessite and fine finger plessite in the remaining spaces.
This example from Gibeon shows plessite, dark, filling around the various bands.
Chemical Classification of Iron Meteorites
As we have discussed, scientists classify iron meteorites by structure. And this was true up until the mid-1960s.
Unfortunately, this classification did not tell how or if the different irons were related to each other. It also didn't help scientists understand how many parent bodies the irons represented.
Thus, a more refined method was developed.
Modern meteoritics now classify iron meteorites according to a chemical classification system using nickel and the trace elements gallium, germanium, and iridium, to define distinct chemical groups.
Other trace elements used to resolve groups are antimony, arsenic, cobalt, copper, gold, thallium, and tungsten.
The concentrations of the trace elements are plotted against the overall nickel content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group.
Fourteen groups, designated by Roman numbers and letters, such as "IAB", have been recognized so far, with each group comprising five or more members. It is believed that the iron meteorites of each chemical group share the same origin and formed on a common parent body.
However, you must keep in mind that over 15% of all iron meteorites don't fit easily into the existing classification scheme.
- These irons are designated as ungrouped and probably represent more than 50 different parent bodies.
- Also, we won't be able to identify these parent bodies because most of them must have been destroyed in order to become a source for the iron meteorites.
Here's the Chemical Groups:
IAB |
IC |
IIAB |
IIC |
IID |
IIE |
IIF |
|
|
|
|
|
|
|
IIIAB |
IIICD |
IIIE |
IIIF |
IVA |
IVB |
UNGR |
As we have discussed, most iron meteorites formed in the cores of small differentiated asteroids and were disrupted by devastating impacts shortly after their formation. They are true remnants of other worlds that once existed in the early solar system.
To learn more about the groups, here's a place to start:
http://www.haberer-meteorite.de/english/Chemical%20Classification.htm
But what is most important about this system is that it tells us that the irons currently known worldwide came from at least thirteen separate parent bodies. With the Ungrouped irons being unknown means they may represent more parent bodies. And so there is an interesting search going on..
But don't be confused, as you learn about this new system, you will come to learn that the fields are related with the structural classification. So, enjoy, your own search of learning and in being aware of the growing knowledge about the Irons. |
