An Article In Meteorite-Times Magazine
by Tom Phillips and Norbert Classen

 

NWA 4483 Lunar Feldspathic Granulitic Impacticite


The thin section was (Once again) a loan from Jeff Hodges. He recently posted his meteorite thin section list. Check it out, you won't believe your eyes. http://www.meteoritethinsectiongallery.com/index.html and this month Norbert Classen didn't just have a sample of the subject meteorite, he had the exact slice the thin section material was taken from. That's real cool!


This is Norbert's slice and his explanation of the material.

"This is one of my favorite lunar meteorites: NWA 4483 is a unique lunar granulitic breccia, paired with NWA 3163, and NWA 4881. Together they comprise a whopping TKW of 2448 grams, and they obviously all stem from one single naturally broken stone that has been sold to different dealers at different times - hence the three NWA numbers for one and the same lunar. A total of 208 grams in form of 12 small to medium sized fragments is comprised under the designation NWA 4483 - fragments that had been purchased in Morocco by Stefan Ralew and Martin Altmann of Chladni's Heirs. Most of these fragments are partly covered by a thick, greenish, partly translucent, and bulgy fusion crust - a unique feature that hasn't been observed on any other lunar meteorite. Tony Irving classified this lunar as a fine grained recrystallized breccia composed of larger plagioclase grains (converted mainly to maskelynite) poikilitically enclosing very small grains of low-Ca pyroxene (some with very fine augite exsolution lamellae), olivine, Ti-chromite, ilmenite, troilite, and metal. That means that most of this lunar breccia has been shock-converted to isotropic glass (maskelynite), something that can be witnessed in Tom's marvelous thin section photos.

My own collection sample has been cut from one of the larger fragments of NWA 4483, a 33.9g stone, and represents a partly crusted full slice of this fragment. It weighs in at 3.01g with dimensions of about 26x24x2mm. Personally, I like these a bit more "sturdy" slices, and prefer them to the more vulnerable wafer-thin slices that are often sold in the planetary sector. Such a 2mm slice as this one can be easily polished, and Stefan Ralew did a great job with that, making this sample even more impressive. It's hard to photograph the greenish hue of this lunar's fusion crust, and you actually have to see this material in person to get an impression of this unique, unearthly fusion crust."


When looking at Norbert's slice I think beautiful material but tough to work with". Look at this whole thin section image. The majority of the matrix is glass and when I say glass I mean Isotropic glass. This transparent glass causes bright and dark spot exposure problems as well as crating a focus nightmare. A standard thin section is 30 microns thick. With most material, focus is on the material surface. At the higher than typical magnifications I work with, 30 microns is a deep space to focus within. Particularly when there are many focus points suspended in glass!


Isotropic minerals are those which do not alter the vibration direction of light as the light passes through it. What that means to a micrograph is, in full cross polarized light, the glass will be black contrasted by bright spots where the direction of light vibration is altered by specks of other minerals. It is nearly impossible to get a good image.

This leads me to explain why I use wave plates in my images. I do so not simply to brighten and alter colors but rather, to find a sweet spot where the light is balanced enough to get a good image. I have received emails explaining that my shots would be more useful in classification and mineral identification if I would not use a wave plate. To this I say it is the action of light in sample rotation that tells the story and one image could not convey much information anyway. Further the material has already been tested and classified. I am looking for the visually pleasing and in many instances, I will be able to show structures not readily seen in standard cross polarized light.

Using wave plates ads an additional dimension to polarizer adjustment. It is not the easy way to get an image. To give a visual example I have included three comparison shot sets. The first two are field of view (FOV) 0.95 mm and the last is FOV 0.25 mm. The first image in each set is utilizing wave plates and the second is taken in full cross polarized light. I tried to take the best image possible with these settings. Please note that this effect is present in nearly all material to some extent (Which is why I use wave plates so frequently) but is greatly exaggerated by the unique isotropic properties of this sample.



FOV 0.95 mm


FOV 0.95 mm


FOV 0.25 mm


For a great article on the effects of full and quarter wave plate use with beautiful and colorful examples please read Brian Johnston's microscope article. http://www.microscopy-uk.org.uk/mag/artjan05/bjcomp.html  

The remaining images were all taken with the aid of a full wave plate.

One thing I noticed about this thin section was the material alteration at the crust. I'm not attempting to explain it but it appears a fine band of material was changed by the heat of entry.

Crust FOV 0.95 mm


Crust FOV 0.62 mm


Crust FOV 0.40 mm


Crust FOV 0.25 mm


Crust FOV 0.16 mm


The following are mostly showing off the incredible glass in this meteorite. All were utilizing 1/4 or full wave plates.

FOV 0.95 mm


FOV 0.62 mm


FOV 0.40 mm


FOV 0.25 mm


FOV 0.16 mm



Be sure to visit Norbert Classen's site at:

Planetary Meteorites

 

Tom Phillips can be reached by email at:
STARSANDSCOPES@aol.com

Meteorite Photos and Images
The Tom Phillips Microscopic Meteorite Photography and Gallery