about Triolite inclusions & CAIs in Gibeon & Allende
by Frank Stroik
To answer your question about troilite, it is a form of FeS that has less oxygen bonded to it's crystal lattice. When minerals form, they will tend to incorporate other elements in their crystal structure. So when we see the chemical formula for pyrite(FeS), it should say 48%Fe,50% S, and 2% other elements. Actually troilite has such a designation, it is a subscript attached to the chemical formula of pyrite, x-1. x-1 lets us know that the troilite is less "contaminated" with other elements(x being the number of oxygen found in the lattice, and therby, after completing the equation, we get the number of sulfur atoms in the crystal lattice). It is a bit more complicated than I have stated, but I think you get the idea.
Troilite is a dull brassy yellow, and is commonly rounded in appearence. In Gibeon, however it is oval, to elliptical, which indicates a settling out from molten material. What you described in your previous comunication read as if it was a troilite bleb. It is hard to find troilite in Gibeon these days, as it appears that the meteorite was segregated into parts. I mean, could it be that there are troilite rich areas in Gibeon, and that these have been found sequentialy(troilite rich--> troilite poor fragments)? That, I think, is an interesting question to ask.
As for Allende inclusions, there are many. It seems that the dealer you delt with indicated a very special inclusion, but did not want to tell you. How odd. In any case I will try to let you discover all the inclusions you can in the polished face. The Allende meteorite contains many inclusion including, but not limited to: Chondrules, CAI's, Troilite rimmed Chondrules, Opaque inclusions, and CM2 inclusions. New inclusions are being found all the time, but these are found using SEM, and the likes, becuase they are small and difficult to decipher .
Chondrules in Allende show the normal chondritic pattern displayed in all chondrites, but the chondrules are perhaps a little larger than "ordinary" chondrites. One inclusion to look for is troilite rimmed chondrules. At sometime before Allende accreeted, the chondrules to be, condensed a fine vapor of FeS around them. This occured during nebular processes, and may predate rocky body formation in our solar system. They look like any other chondrules, except they have a fine golden dust like quality around the outer edge of the chondrule. This inclusion is not rare, but it is not common either. You may just find one on your cut face, whereas you see a lot more standard chondrules.
CAI's stands for Calcium Aluminum rich inclusions. These are white, irregular aggregates that formed about 500 million years before the solar system began to collapse. They are star dust in the most literal sense of the phrase. They are rather common in the matrix of Allende, and I suspect you will find two or three in your sample(if you have'nt all ready).
Opaque inclusions are just black rectangular masses in the matrix of Allende. They are extremely fine grained, and difficult to get a mineral composition from. I have not encountered any truly enlighting article that describe these inclusions, but that doeas not mean that one is not out there.
CM2 inclusions are in affect another carbonaceous chondrite acreeted in the Allende matrix. They are of the same basic properties of the Murray, Murchison, and Mighei(sp?) CM2 carbonaceous chondrites that have fallen to Earth. These inclusions are rectangular, and have extremely small chondrules in them. Perhaps that is their defining characteristic. Look for an area dominated by small chondrules, and see if it is bounded sharply by boarders, if so, it is a CM2 inclusion.
I hope this helps you in deciphering the properties of the Gibeon, and Allende meteorites.
Department of Geology and Geophysics
The University of Wyoming