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Re:Geo vs Astro chemistry was Asteroid, Comet, etc. question

Hello List,  Troy

I have been away for 6 weeks and in catching up on my mail,  I don't think Troy
got an answer...so here goes.... His questions were short but my answers are
like the energizer bunny.

Without going into a yes, no, maybe answer I have the short version and the long 
version.

Troy wrote:
 Hi list, I am confused about something.  Given the vast different elements here
on earth, some 100+, I ask this question. I would expect that other
planets/asteroids/comets would be made up of those other elements or even some
new elements and not necessarily nickle/iron. 

 Q: Why is it that one of the main tests for meteorites is the nickle/iron
content in them?

A:  Iron and Nickel can rarely be found together on earth.... rarely is iron
found in "native" form but when it is, it is product of human industry.   
Matter of factly-- I don't think you will ever find native (pure)iron from a
natural source on earth.  Any time you find it it on the earth it will be from
human activities OR extra-terresterial.  

Iron-nickel is a "signature" for meteorites because it distinguishes it from
iron from human sources. On Earth, nickel can be found in native crystalline
form  but will not be found in a native mixture with iron.  When one smelts a
meteorite, the bulk of the nickel will separate.  If you kept the mixture hot
enough and long enough the two metals would completely unmix.  Ergo, when you
find an iron mass which contains metallic nickel it is virtually 99.9% of the
time from a meteorite.   Meteorites do contain virtually every other naturally
occurring element and in quantities similar to their "cosmic abundances" but
these quantities compared to the abundance of iron, silicon etc. are minuscule.

Q: Do they always have nickle/iron in them? If so,    Why?

A: All of the stony-common chondritic, irons, and stony irons, contain
native/crystalline iron and nickel in them.  I also believe some of the
carbonaceous chondrites do and there are occasional veins found in some of the
achondrites--like Norton County.  Iron (Fe) and nickel (Ni) were originally
formed in what we call large "cosmic abundances".  Because they were not blown 
far from the the intersolar system they are common components of meteoroids and
asteroids. Comets, on the other hand, are believed to have formed much further
out in the solar system and do not appear to have much iron in based on
spectography--not like we have in the inner solar system anyway.

 When the meteorite comes from a smaller parent body it appears as flakes
amongst the silicates. If the body was not large enough to retain enough heat to
melt internally and go through what we call "differentiation", the metal will be
found in small flakes or veins.  If the body were larger and it was able to
differentiate into a metallic core, the iron and nickel and other siderophile
(literally iron loving) elements would pool into the innermost regions before
cooling.   

Along comes a cosmic collision-- and depending how deeply the bodies are
ruptured-- the body can release surface and core material-- the  different
classes of meteorites are liberated for their long journey to earth.  Radiation
causes some short lived isotopes to form along the trip.  Measuring these can
give us both a cosmic age and a terresteral age for meteorites we find.


Long(er) Version:  The rest of this discussion addresses other means we have to
identify ages and origins of meteorites from a fundamental understainding of
elements, isotopes, decay products and minerals.


Troy also wrote:  
I would expect that other planets/asteroids/comets would be made up of those
other elements or even some new elements and not necessarily nickle/iron.....

Elton Writes:
I'd like to address some of your assumptions on astrochemistry. To do so I need
to talk about elements, isotopes , minerals, and decay products.   If you took
back all the matter in the solar system and remelted them you would have
something which chemically resembles the sun. If you allow it to settle, the
denser matter will move to the center and the lighter matter will move away from
the center.  You would start getting areas which chemically look like Jupiter,
and areas which look like the Earth's crust,  then the Earth's core,   etc and
so on.  

Depending on how much of a given element was in a given location when the solar
syatem congelled governs how much one will ultimately find in a given  body. 
Further, given the amount of local geophysical processes, the elements will
again sort themselves out more.  Irridium, for example, is fairly common in the
solar system,  but apparently rare in the earth's crust.  The reason being is
that the irridium here sank into the mantle and core when earth was still very
molten.  It is therefore rare on the crust but, in the portions that it should
be when we take ALL of it in to consideration.

Elements:  The elements (as defined by the number of protons within the atom's
nucleus) here on earth are the same ones we find everywhere else!  We even have
a few more here in that we have been able to "make" them in the lab when they
have been long since extinct in nature. They are generally "transuranic" meaning
that they are the elements higher than 92 Uranium.  Any given element has a
specific number of protons and electrons (unless they are in a plasma state
where the electrons float freely from the atom nucleus).  Any fewer or any more
protons then they would be a different element.  

 In our present understanding of the way elements are assembled in the proto
sun, we believe that there are no additional stable elements to be found over
what we already know of. There can not be any undiscovered natural elements
because we have filled all the slots in the periodic table, unless it is
something transuranic, very short lived, existing in nano moments within the
deep recesses of a star's nuclear furnace.  For our discussion about "signature"
elements we must confine it to the current list of natural elements.  So it can
not be "new" elements which we would rely on for identifying meteorites.

Isotopes: An element may have many different forms .  An isotope for a given
element is one whose nucleus contains a different number of neutrons yet retains
the same number of protons.  Sometimes the number of neutrons make for a stable
atom which will last nearly "forever".  In others, the number of neutrons make
the atom unstable.  When that unstability reaches a certain level we call that
isotope "radioactive".  A quantity of radioactive isotope will disintegrate at a
measurable rate or ratio.  The term "half life" comes from one of these
mesurements.  It is defined as amount of time it takes for half of a quantity of
atoms to decay to another element.  Some are very long others very short.       
  

An Isotope is "chemically" and "physiologically" identical to any other isotope
of the same element.  So water formed from any of the three isotopes of hydrogen
(e.g. deutritium and tritium) will still be water. Wherever one would find the
compound water (H2O), it behaves like water-- e.g. melting point, solution with
NaCl (sodium chloride aka table salt). Isotope ratios such as the Oxygen 16,17,
and 18 mix, are an important marker used in identifying rocks from the moon and
those found on earth.  The ratio of the element "oxygen" is generally the same
on the earth as on the moon--  but the "oxygen isotope ratio" found on the moon
is very different from that on the earth. 

Minerals:  Minerals are compounds of the elements which form under different
conditions of gravity, heat, pressure, hydration etc. in a succession of
predictable, constant, "forms".  Not always the same form even though the
chemical compound or formula remains constant.  Silicon Dioxide (SiO2) can come
in the forms of glass, quartz, opal, flint  etc.. it remains SiO2 but presents
in different "morphs".  For example, Coesite, a variation of SiO2, is only found
where quartz has been subjected to intense short pressures... such as a
meteorite impact.  The element carbon can also be a mineral and can occur in
such forms as graphite, diamond, and buckminsterfullerite.  There are several
minerals which are found in meteorites but which are rare or non existant here
on earth.

Decay Product:  Also referred to a "daughter element", a decay product forms
from an atom of one element, which is isotopically unstable, when it gives off a
portion of its nucleus to form one or more different atoms of another element. 
Unless the decaying atom is remixed somehow, as in melting, it remains locked in
the crystal lattice where the mother atom was.  Because this exists, we are able
to inventory atoms in a specimen and determine an "age" of that specimen.  We
calculate the ratios of the mother element to that of the daughter.  You have
likely heard of Carbon-14 dating, and potassium-argon dating etc.   All of these
derive from the ability to measure ratios of isotopes.

In the case of aluminum26, it is a very short lived isotope , around 750
thousand years.  It decays to a magnesium26.  Aluminum and magnesium aren't
chemically interchangeable like iron and nickel can be.  When we find a
magnesium atom in a crystal where there should be aluminum, we deduce that the
magnesium is a decay product of aluminum26.  We also then know that the
crystal/mineral  etc. has not been remelted since the formation of  solar
system.  The isotope Al26 was abundant in the early protostar, it was not
replenished after the sun formed, and it rapidly decayed.  Any magnesium found
"out of place"  must not have been disturbed since it formed.  From this we can
date some of the meteorites we find to be as old as our solar system.

Regards,
Elton Jones

Troy Bell  wrote:
> Subject: Asteroid, Comet, Meteor, Meteorite question
> Date: Fri, 19 Nov 1999 07:20:40 -0600
> 
> Hi list, I am confused about something.
> Given the vast different elements here on earth,
> some 100+, I ask this question.
> 
> Why is it that one of the main tests for meteorites
> is the nickle/iron content in them? I would expect that other
> planets/asteroids/comets would be made up of those other elements or even some 
new
> elements and not necessarily nickle/iron. Do they always have
> nickle/iron in them? If so,  Why?

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