One of the goals that Paul and I have always considered very important in presenting Meteorite Times is to provide educational information to aid the student and beginning enthusiast. While I don’t think of myself as an educator this month I thought I would offer something a little more instructional.
There are a great many features to be found in iron meteorites. Inclusions, nodules, various kinds of metallic fields and crystals make up just some of what can be observed. How to tell what you are looking at can be a daunting task at first. So here is a brief introduction to what one might see in a slice of iron meteorite.
A good place to begin a discussion about minerals might be with a definition of mineral. A mineral is a naturally occurring substance that has an orderly arrangement of atoms. For the last part of that substitute “a crystalline structure.” At least that was the definition ages ago when I was working to be a geologist. And for our purposes here it will be a good definition. Graphite is a mineral made of carbon. Diamond and elemental carbon are also forms of carbon. Graphite and Diamond are minerals where coal which is made of carbon is not. The reason being that coal is not a crystal form of carbon with that orderly arrangement of atoms. Graphite has a sheet-like crystal structure with light bonding between the sheets. That is what makes it good for pencil leads. It is easy to rub off the sheets of graphite.
In meteorites and especially iron meteorites graphite is often found as nodules. Below are a couple pictures of graphite nodules. They are generally round or oval, dark gray to black in color and may have veins of metal or other substances running through them.
This is a 4 cm wide slice of a graphitenodule from a Canyon Diablo meteorite.
In this photograph the nodule has troilite the next mineral to be discuss on the left and graphite on the right.
This iron-sulfide mineral is commonly found in association with graphite in iron meteorites. Often surrounding a graphite nodule as a ring. But, it can be seen occurring by itself commonly also as nodules and blebs. It is characterized by its bronze color and by the fact that it will dissolve in weak acid and leave a brown stain on the slice. In the following photograph what appears to be rust is in fact this staining from a poor etching job. But the slice is a wonderful specimen and re-etching it myself is on my list of things to get done someday.
Brittle and silvery, schreibersite is seen in iron meteorites as needles that align with the crystal structure of the metallic portions of the meteorite. It is also seen as a sort of lacy border around troilite nodules. It is a iron-nickel-phosphorous mineral. Though it is very slivery white when first etched it tarnishes to a bronze color. Schreibersite as mentioned earlier is very brittle. Etched slices of meteorite containing schreibersite should not be rubbed as this will break off the acid exposed schreibersite crystals and needles.
This needle is about 20 mm long and as can be seen is aligned with the metallic crystals of the meteorite.
Here schreibersite can be seen enclosing a small graphite nodule with is about 4 mm in its short axis
Often mixed with schreibersite, cohenite is a iron-nickel-carbide mineral with a hardness of around 6. It too is brittle. And like schreibersite is silvery when fresh. Distinguishing the two can be difficult but they flourish in meteorites with differing nickel content. Cohenite appearing in iron meteorites of less than 7% nickel.
Small and often hard to find since they are usually mixed up with other minerals near or in graphite nodules diamond are also found in iron meteorites. First recognized in Canyon Diablo meteorites because they ruined the grinding wheels that were not nearly as hard diamonds are another mineral form of carbon. In Canyon Diablo fragments the diamonds are believed to have formed from shock pressure and heating during the impact of the asteroid with Earth. Generally they are black and opaque. I believe the crystal shown below from a small Canyon Diablo is a diamond or carbonado as they are often called. This one is well within the size range at about .7 mm. But it is also on the large size since many more are truly microscopic mixed in with nodules and not isolated as this one is. This crystal can not be lapped down on my diamond lap completely it stands in slight relief always. It is also not affected in any way by etching and will not produce a stain or take a copper coating as will cohenite.
The whole width of this photograph is about 10-12 mm so carbonado is quite small
Taenite and Kamacite:
These two nickel-iron minerals make up the metal portion of octahedrite family meteorites. Wider strips of kamasite will generally be separated by thinner strips of taenite. Hexahedrites are essential iron meteorites of solid kamacite, while ataxites are conversely iron meteorites of essentially pure taenite. These other two types of iron meteorites will not display a Widmanstätten Pattern when etched though they will often show other types of lines and feature that will distinguish them from man-made irons and steels.
The bulk alloy amount of original nickel in the parent body determined what kind of structure could form as the metal cooled. At high temperature taenite is formed, but as the temperature drops kamacite can form and continues to form removing nickel from the original taenite to make more kamacite till either the temperature drops to where no more diffusion of nickel can occur or all the kamacite that can form has formed. That is a real over simplification of a very complex process. But, the result is that if there is enough original nickel than both kamacite and taenite will be represented as portions of the crystalline structure. This structure is seen as a pattern when a polished slice is etched chemically. Named for one of the early investigators the Widmanstätten Pattern seen in octahedrite iron meteorites displays crystal sizes of over a wide range. These crystal sizes are again directly related to cooling time and original bulk alloy content of nickel. High bulk nickel content creates a fine pattern of small kamacite crystals, whereas lower bulk alloy nickel content results in the formation of larger kamacite crystals. Below are examples of how this range of size can appear in etched slices.
A coarse octahedrite
A medium octahedrite
A fine octahedrite
And now for some fun here is a very complex area of a meteorite containing nearly all the minerals and structures discussed. Take a look and see how many features you can identify.