Ken wrote: "Hello Bernd" Hello Ken, nice to hear from you! As the comments below may be of general interest to other List members, I'll send my answers to your questions to the List.
"Do you have access to a microprobe or is it a learned skill, that by the colors and texture you deduce the minerals. How would I go about aquiring such skill." No, I don't have access to a microprobe, nor is it a learned skill. It's rather a combination or a mixture of these activities (and books that I own): - reading the articles by Tom Toffoli and O.R. Norton on thin sections in Joel Schiff's quarterly "Meteorite". - reading the entries in the Met.Bull. so I know what I can expect to see in my thin sections. - looking at the thin section photos that are published online in the Antarctic Meteorite Newsletters. See here: http://www-curator.jsc.nasa.gov/curator/antmet/amn/amn.htm As for books: - I have Marvin's book, too, and it is a pity there is no information on how to indentify the minerals. - I also have and can recommend: => NESSE W.D. (2004) Introduction to Optical Mineralogy (3rd Ed., Oxford Univ.Press, 348 pp.). => MacKENZIE W.S. and ADAMS A.E. (1995, 1996, 2000) A Color Atlas of Rocks and Minerals in Thin Section (John Wiley & Sons, New York-Toronto, 192 pp.). => Several books written in German on how to identify minerals in thin section with lots of color and b&w photos There are presently 112 thin sections in my collection and I sometimes spend hours studying one or several under my Russian MBS-10 microscope + Tobin Adapter for viewing them in cross- polarized light. I compare what I see to the crystal colors in the books, to the geometry (silhouettes) the crystals in the books show and draw my conclusions (sometimes probasbly wrong ones, of course, as I am not a learned mineralogist). But the more you look and compare the more experience you get. "How do you deduce:" 1. the vivid bluish pink crystals are zoned, calcic olivines; Those high relief, vivid colors are typical of olivines and if their rims are richer in iron than the main part of the crystals, the rims look bluish or vice versa. Another typical feature is "cleavage". Olivines rarely show good cleavage, whereas the pyroxenes do. That these olivines are calcic is a piece of information I culled from the entry in the Met.Bull. 2. the dazzling white crystals are anorthites; Here again the Met.Bull. says that the plagioclase is almost pure anorthite and that means it's white (cp. lunar anorthites!) 3. the light and medium brown, yellowish brown crystals are fassaitic pyroxenes; Typical color shades of pyroxenes under crossed polars - so-called first-order interference colors. Pyroxenes show show good and easily recognizable cleavages at 90°. 4. the upper half of the TS is dominated by vivid blue diopside crystals; Diopside is a clinopyroxene and looks blue under crossed polars. As it is a pyroxene, it does also show cleavages at 90°. 5. Numerous isotropic spinel crystals appear black with crossed polars. I am not quite sure about this and that's why I didn't want to write about it but as Matt asked ... No matter which direction I rotate the thin section, these dark areas remain dark under crossed polars. Thus you can deduce these minerals are isotropic (they show no double refraction). The Met.Bull. mentions (or will mention) "Cr-pleonaste" spinel: spinel is isotropic, and "pleonaste" means it contains a substantial amount of Fe2+. This high amount of Fe2+ may render the crystal almost opaque. But as I said before I am not absolutely sure if these black opaque areas are really spinels - especially because the entry in the Met.Bull. says that it occurs only as "subordinate Cr-pleonaste spinel". Best wishes from Germany, - 6.2°C (about 21°F) here, Brrrrrnd, sorry Bernd ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
