UPDATE (7/28/2012): After receiving my new microscope, I was able to view the area at much higher magnification. I very carefully used an X-Acto knife to gently scrape the area and the film causing the rainbow does come off. There appears to be a thin film of something on the specimen (reminds me of cutting oil) that may have gotten baked on when the specimen was dried after cutting. Still a gorgeous specimen and wonderful to examine under the microscope.
ORIGINAL POST (4/24/2012): I received, just today, two Lodranite specimens: an 11.42g end-cut of NWA 4478 and a 9.71g end-cut of NWA 6075. Both specimens are beautiful and the NWA 6075 exhibits an interesting shimmer effect. However, when I looked at the face of the NWA 4478, I was shocked to see what can only be described as a rainbow about 2mm x 3mm (see images below).
Now the first thing that came to mind is that some oil was trapped in the structure and created the thin-film physical effect (think oil slick). On further inspection, there were no other areas that displayed the same effect though there are similar areas where oil could get trapped. I remembered seeing something similar on the www.meteorites.com.au website in an Al Mahbas specimen (click here), but this looks distinctly different.
Oil contaminant or a genuine unusual feature in this lodranite – help me solve the mystery.