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Geochemistry - Rock Analysis

 
 
Reply Wed 5 Dec, 2018 01:12 pm
Hello,
I'm a 3rd year Chemistry student currently working on lab project. The idea is to purchase meteorite fragments online, and using Inductively Coupled Plasma-Mass Spectrometry metal analysis to figure out their elemental concentrations/ratios.
So far it's been difficult to get rock to fully dissolve in solution (so far we tried powdering reguler earth rock and dissolving it in glacial acetic acid and aqua regia - to no avail).
So I have two questions:
Does anybody know of a reliable procedure to make solid rock fragments into ICP-MS ready samples?
And, once we do get our metal analysis, is there a good way to tell if the material is indeed extraterrestrial?
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farmerman
 
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Reply Wed 5 Dec, 2018 05:21 pm
@lirironen,
are you as familiar with Energy Dispersive X-Ray?. ICAP is a detructive test and there are several non destructive Xray and EDX (with flourescence spec) techniques used to focus in on metal ratios.
What if you have a chondrite?? They are so heterogeneous in composition you could be like the blind wise men touching the elephant.(Its a metal, stone and silicate mix but like a rice pudding you also gotta find the raisins and the cinnamon to analyse)
ICAP is ok but mteorites are arranged in crytal axes that are defined by their Widdmenstaten figuews. SO if you get one that hs rpidly cooled, the Ni and Fe form totally different bars in the "W" figures.

I think you need to learn some more about how many types of meteorites there are and how they present themselves.
A meteorite is cut and polished to show its structure (If its a metal Fe/Ni one). An energy dispersive x ray (EDAX) can be run acrocc a line on the cut surface and youd get back ratios of Ni/Fe along that line. You can then PLOT the ratios by running several lines and then modelling their relative yields of metal.
(Many meteorites will contain other metals too. By doing an ICAP, you may miss these (unless you take huge samples of the meteorite and take it down in Aqua regia and then dealing with the insolubles in another way (silica based-use HF)

An EDAX wont give you ppb levels but in an Fe/Ni meteorite, why are we doing ppb (or even PPM level analyses??)


farmerman
 
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Reply Wed 5 Dec, 2018 05:30 pm
@farmerman,
heres a widmenstaaten figure pattern. Its been cut and acic etched and polished. Were looking at different xls of diff iron and nickel valence states and therefore xl shapes.

  https://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Widmanst%C3%A4tten_pattern_Staunton_meteorite.jpg/220px-Widmanst%C3%A4tten_pattern_Staunton_meteorite.jpg                                Its a pic from a Wikipedia site on Crystal lattices in meteorites
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rosborne979
 
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Reply Thu 6 Dec, 2018 05:33 am
@lirironen,
You might want to analyze rocks which are purported to be meteorites and compare them to regular rocks and see if there is any reliable difference. You will need lots of samples if you go that route.
farmerman
 
  2  
Reply Thu 6 Dec, 2018 06:19 am
@rosborne979,
My concern is that, although we can tell meteorites fairly asily, we rally dont have to crush em up and destroy em to do any kind of assay.
We have tools and gizmos that are non-destructive. An ICAP requires we take a sample, grind it up, bake it, dissolve it and all the while knowing that, because of a particluar meteorites heterogeneity, that assay is only good for that little piece weve destroyed. We can use XRF and EDAX to do spot analyses from points all over the meteorite and then "add eme up, and do the stats"
It isnt like were looking for gold (unless we are), but even then we cant go out in space and collect the exact "Strata" from where the little bomb came from. Its not how geochemists work and I think we should be teaching the whole thought process instead of waiting for some senior project or grad school.

The instructor who assigned this lab problem (if the OP stated it correctly) is not rally thinking about anything but doing an exercise using the ICAP, I care more about the meteorite.



Silly.

rosborne979
 
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Reply Thu 6 Dec, 2018 10:51 am
@farmerman,
I get that. I would prefer not to crush meteorites either. But the OP may have access to lots of tiny little meteorites with little value. I don't know.

I just thought that from an academic and experiment perspective, it would be nice to compare lots of Meteorites to lots of non-Meteorites, to see what (if any) chemical difference emerged from the data.
farmerman
 
  1  
Reply Thu 6 Dec, 2018 11:17 am
@rosborne979,
we can do that from a satellite sporting a multispectral scanner. I hadda look up the name I was trying to think of, PALLASITES. These are a stony/Iron meteorite in which the matrix is often pure gem quality Olivine.
The olivine appears as blebs of this greenish glass interspaced among the metal. Beautiful. These kind record planetary and satellite collisions or collisions of another sort.

farmerman
 
  1  
Reply Thu 6 Dec, 2018 11:19 am
@farmerman,
I recall reading about how Victorian ENglish households would burn mummies in fireplaces as part of heating or making garden fertilizers. Now we kick ourselves for not knowing back then what we threw away just to be fashionable.
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