[meteorite-list] Geophysics, meteorites, and Electron microscopy-QUESTION

[Carl Agee]

Hi Shawn,

It's actually the electron microprobe that gives the quantitative
analyses needed for classification of stony meteorites. The data is
usually output in tables of oxide component concentrations in weight
percent, totalling hopefully, to approximately 100%. The microprobe
software lets you select which elements (expressed as oxides) to
analyze and assign the element to one of the probe's spectrometers.
The software will also give atomic concentrations to check for mineral
stoichiometry. For metals and sulfides the output is usually in
element percent. Calibration of the probe against known standards
(minerals or compounds similar to what are in your meteorite) precedes
the microprobe session and can take several hours. Once the probe is
calibrated, you are good to go on your first unknown meteorite that
would normally be in a polished thin section or polished epoxy-cast
probe mount. Because the calibration is time consuming it is
economical to do several samples in a single session. Sessions can go
on for hours, and you can even set up a collection of x-y coordinates
and let the auto-feature of the probe analyze different spots all
night long. In the morning a stack of data would be be waiting for
your perusal and evaluation.

Of course there are other tests and analyses that contribute to a high
quality meteorite classification, which include visual textural
information from both polarizing microscope and SEM observation (e.g.
shock effects, percent mineral make-up, weathering), and of course
macroscopic characteristics seen in the hand sample. Oxygen isotopes
are also great to have, especially as a "court of appeals" for
borderline cases or for anomalous meteorites that don't fall into
clear-cut geochemical groups. There are many other techniques such as
isotopic age dating, cosmic ray exposure, bulk trace element analyses,
and so on that enhance the characterization of a stony meteorite, but
the electron microprobe is the work horse for most classification
data.

Someone who has done work on Ensisheim (LL6) and Saint-Séverin (LL6)
could give a better answer that I can about the their subtle
differences and whether the microprobe can distinguish between them,
but my guess would be yes, especially when combined with SEM imaging.

The length of time for classification from start to finish, including
the write-up is variable. The initial ID or categorization of an
unknown doesn't have to take long, that's what emerges in the first
hour or so of microprobe analysis. But often, for a water-tight,
authoritative classification, a second probe session may be required.
Plus a lot of the time is spent puuzzling through the data and
narrowing down the possibilities. It's like detective work, and
personally I find it immensely engaging.

Hope this helps,

Carl Agee


--
Carl B. Agee
Director and Curator, Institute of Meteoritics
Professor, Earth and Planetary Sciences
MSC03 2050
University of New Mexico
Albuquerque NM 87131-1126

Tel: (505) 750-7172
Fax: (505) 277-3577
Email: agee at unm.edu
http://epswww.unm.edu/iom/pers/agee.html

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On Wed, Sep 7, 2011 at 4:47 PM, Shawn Alan <photophlow at yahoo.com> wrote:
> Hello Carl and Listers
>
> Great post on scanning electron microscope (SEM) process, now does this process also ID the whole classification of the meteorite which is used for classifying meteorite for the Meteoritcal Bulletin database? I know alot of school are discontinuing classifying meteorites is UNM also doing the same?
>
> Question if someone wanted this service to be done on a meteorite, how much would it cost, cause these days money talks and helps everyone out when it comes to classifying meteorites or confirming that the meteorite is the meteorite suggested to be. Also can this process determine the difference between Ensisheim (LL6) and Saint-Séverin (LL6) by the cosmic ray exposure or terrestrial age. These two meteorites look identical and some can fake it. Does SEM also cover that test or is that a different test? Lastly how long does it take for you to classify a new meteorite from start to finish if its a stony meteorite?
>
>
> Shawn Alan
> IMCA 1633
> eBaystore
> http://shop.ebay.com/photophlow/m.html
>
>
>
>
> [meteorite-list] Geophysics, meteorites, and Electron microscopyCarl Agee agee at unm.edu
> Tue Sep 6 12:27:18 EDT 2011
>
>
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>
> A scanning electron microscope (SEM) is great for high magnification
> images that also contain information about the chemical composition of
> the different minerals in meteorites. SEM is also a quick way to do a
> qualitative analysis of a sample, say for example, detecting nickel in
> iron meteorites with energy dispersive spectroscopy (EDS).
>
> My instrument of choice for definitive ID of stony meteorites is the
> electron microprobe. This also has SEM imaging capability. In less the
> hour, on a calibrated electron microprobe, I can make a definitive ID,
> although unequilbrated chondrites can take more time to narrow down
> the possibilities. Some of the key geochemical quantities that help in
> categorizing are, Fe/Mn of olivines and pyroxenes, the fayalite and
> ferrosilite content of the olivines and pyroxenes, the
> anorthite-albite content of plagioclse, and a few other parameters,
> plus the percent mineralogy and other textural characteristics.
>
> So for a simple example lunar olivines usually have higher Fe/Mn than
> terrestrial basalt olivines. Check out this page's second figure for
> an overview of Fe/Mn versus plagioclase content of planetary basalts:
> http://www.imca.cc/mars/martian-meteorites.htm
>
> Carl Agee
>
> --
> Carl B. Agee
> Director and Curator, Institute of Meteoritics
> Professor, Earth and Planetary Sciences
> MSC03 2050
> University of New Mexico
> Albuquerque NM 87131-1126
>
> Tel: (505) 750-7172
> Fax: (505) 277-3577
> Email: agee at unm.edu
> http://epswww.unm.edu/iom/pers/agee.html
>
> --------------------------------l----------------------------------------------
> Geophysics, meteorites, and Electron microscopy
>
> Barb and Jake Baker bakers5acres at frontiernet.net
> Tue Sep 6 10:50:46 EDT 2011
>
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> ________________________________
> Could someone tell me, in Microscopy 101 language: How is an electron
> microscope used in the study of meteorites? Using electron microscopy -
> what differences are apparent between meteorites and terrestrial rocks? For
> instance what are the microscopy differences between lunar basalt and
> terrestrial basalt?
>
> Thanks
>
>
>
>
>
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