[meteorite-list] Thin Sections rusting/tests

From: Alexander Seidel <ase_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:29 2004
Message-ID: <3B4214C9.6C3896E9_at_planet-interkom.de>

Hi Michael,

thanks for a very interesting post. As a thin section lover for many
years I am pleased with the fact that much more sources are available
now as compared to even the recent past. This is a fascinating addition
and complement to any "hard-rock-collection" of meteorites.

Regarding the cover/non-cover discussion I would favor Frank Cressy`s
argument - a pro for the cover-slips (at least for the more common
material). Thin sections now available for public sale are mainly aimed
at the private collectors, who may at best have microscopes with a
polarizing filters device, but only very rarely (well, there may be a
few exceptions we all can think of) will have the very sophisticated lab
equipment used in the mineral departments of universities or other
professional institutes. Yes, cover-slips may prevent some of the more
professional work on thin sections to be done, but it won´t at all
prevent the delight that you can experience when viewing them in between
crossed polars. It will probably additionally help to seal against rust,
and it will surely be a good way to protect against damage done by often
moving the slide from the storage boxes to the scope stage and vice
versa. And, regarding the rare stuff: this is likely to be documented in
a somewhat faster process all the way from the initial research lab
through the NomCom to the researchers again as compared to the more
ordinary material, and there will probably be "enough" non-cover-slides
be available for the more detailed analysis following, which in almost
all cases can´t be made by private collectors.

Alex

 
> Hi Frank & all,
> Since this cover slip/no cover slip controversy began,
> I spend some time talking to my thin section maker about
> a variety of things, including the above questions you
> posed. I cannot speak for other makers, but the individual
> I use is the same person David New used and his process is
> as follows:
> 1) He cuts the material using isopropal alcohol
> 2) He grinds the material using oil
> 3) The whafer of material is attached to the slide (& those
> with cover slips have the cover slip attached, as well)
> with an epoxy which seals the material, making it
> impervious to rusting (due to deprivation of oxygen -
> which is REQUIRED for "oxidation" to occur)
> 4) For those without cover slips, he polishes with diamond
> pastes (with an oil base) as follows:
> 1st is paste of 6 micron diamonds followed by:
> paste of 3 micron diamonds followed by
> paste of 1micron diamonds
>
> (To give you an idea of how fine this is, I used to do lapidary
> work many years ago. Typical finish on a cabashaun was as follows:
> 350 grit
> 600 grit
> Oxide polish
> VERY VERY obsessive lapidaries would skip the oxide
> polish and go to:
> 1,200 grit
> 3,000 grit
> 25,000 grit
> 50,000 grit
> This would leave a slightly "deeper" polish. the grit size
> indicated the number of holes in a screen PER INCH that
> size of diamond would fit through.
> However, a MICRON is 1/1,000th of a MILIMETER -
> there are 576 cmm to the cubic inch).
> The lack of cover slip leaves the thin section exposed
> to possible damage during use and simply in removing and replacing it in
> its storage box. However, it also makes the
> slide an ideal specimen for precise testing with
> extremely expensive equipment (which does not nessessarily
> have to be owned to be used).
> As a result of the Pro/Con cover slip controversy, coupled with my
> discussion with my thin section maker I have decided
> I will have all very rare material made WITHOUT cover slips
> and the less rare material made WITH cover slips.
> The results of the survey were 2 to 1 IN FAVOR OF cover
> slips. HOWEVER, responses were less than 10 people! So, that
> did not carry as much weight with me as another part of the
> discussion I had with my TS maker, as follows:
> There are THREE primary tests conducted on thin sections
> that require no cover slip - the fourth test - and THE test MOST
> UTILIZED in analysis is observation of extinctions of colors as
> the crossed polarized light is set at various angles and CAN
> BE DONE WITH COVER SLIPS on the thin sections.
> What is particularly interesting is that the OTHER 3 tests
> can be done on ANY small sample with a properly polished
> surface - these tests do NOT require the sample be a thin
> section at all! However, thin sections are particularly
> convenient AND, in the case of very rare, and, therefore, expensive,
> material it makes sense to conserve the cost
> of duplication of specimens by simply excluding cover slips
> from the thin sections of these more rare materials.
> These 3 tests are as follows and involve ONLY the surface
> of the material - they do NOT require, or even utilize the translucency
> of a thin section:
> 1) - reflecting light microscope
> looks at the surface only to tell the
> difference between silicates & metalics
>
> 2) - scanning electron Microscope
> up to 30K power - surface examination
>
> 3) - electron microprobe
> up to 400 power - & electron beam that
> "excites the surface" of the material
> --------
> Within the next 36 hrs I will have the Lunite, NWA 482
> and the Winonaite, NWA 516 available in Thin Sections.
> Both will be without cover slips and prepared as described
> above.
> They will have a MUCH larger viewing area than the
> Lunar Thin Sections Steve had on eBay several days ago
> (not that those were at all "undesireable" {I lusted after
> each and every one, myself!} - just that these will be much
> larger) and will be priced lower per cubic mm of viewing
> area than those which sold at the auction.
> Best wishes to all, Michael
>
> --
> “Suppose you were an idiot... And suppose you were
> a member of Congress...But I repeat myself.”
> - Mark Twain
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>
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Received on Tue 03 Jul 2001 02:54:01 PM PDT