[meteorite-list] Thin Sections rusting/tests

From: Michael Blood <mlblood_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:29 2004
Message-ID: <3B41C1C8.14C4_at_home.com>

Frank wrote (in part):
>... In my opinion, a potential problem lies here in that these commercia=
l manufacturers deal primarily in terrestrial rocks where rusting is seld=
em a problem. I would hope that the =

sellers of meteorite thin sections inform their manufacturers =

of potential rusting problems and have their thin sections prepared
using kerosene or whatever liquid is now used as a slurry/coolant for
"special" samples and not water.
I looked at my slides of recent falls and about half are stained =

with iron oxides around metal inclusions. Whether or not
the staining occurred prior to thin section manufacture or =

>after is unknown but suggests a potential problem to me....
---------
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 =

--
=93Suppose you were an idiot... And suppose you were =
a member of Congress...But I repeat myself.=94
- Mark Twain
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Received on Tue 03 Jul 2001 08:59:53 AM PDT