[meteorite-list] RFS Picture of the Day => Brenham slice with unusual inclusion

[bernd.pauli at paulinet.de]

Tett asked:

"How can you tell the difference between kamacite
 and taenite? Does the crystal structure give it
 away"?


Hello Mike and List,

As I am hardly ever *absolutely* sure I wrote: "If I am not mistaken here" ;-)

I'm sure that the crystal structure and the telltale color of the inclusion
would give it away if you could have a close look at it "in person" but
as I am too far away, my approach is one of "by inference". When there
are inclusions in iron meteorites (troilite, schreibersite, cohenite, etc.)
and when these inclusions have a rim or a seam of material surrounding
or enclosing them, then it is usually "swathing kamacite". Those among us
who are fortunate enough to own the Buchwald trilogy, will be able to
produce numerous pictures from Buchwald where one can clearly see this
phenomenon and then deduce by comparing several pictures and, of course,
Buchwald's description of these phenomena that it is what it is. One such
example is figure 153 on page 105 of Buchwald (Vol. 1). You can see a
picture of the Grant medium octahedrite and it shows several of these
"oriented schreibersite lamellae, each of which displays rims of swathing
kamacite". Another picture of the same iron is to be seen on page 107, a
textbook example of a "troilite nodule with ... well-developed swathing
kamacite".

Buchwald is such a precious treasure trove of information! When a list member
asked me about these "needle-like" inclusions a few days ago, I was able to
confirm (of course with the help of Buchwald!) that these inclusions are in
all probability so-called Brezina-lamellae and this again, by inference from 
information culled from Buchwald who writes that if phosphorus is abundant,
the angular plate-like crystals [of schreibersite] may be seen to be oriented.

And thus, once again by inference, (although I do not know how high the P
content in Brenham is), one may conclude that Brenham has an appreciable
amount (above ca. 0.4% P - according to Buchwald) of phosphorus. Other-
wise we wouldn't see those beautiful, long, needles of schreibersite with
their seams of swathing kamacite.

Cheers,

Bernd