[meteorite-list] Re: Mg/Fe+Mg of Bulk Planet

From: Robert Beauford <wendirob_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:12 2004
Message-ID: <03c001c0a08f$1ff82ee0$da4897cc_at_wendirob>

Thank you Bernd, Frank, and Collin. This really helps.
I really appreciate you all taking the time to pass on your knowledge.
-Robert Beauford : )

> > I'm still trying to figure out the significance of
> > "Mg/Fe+Mg of Bulk Planet" in terms of planetary
> > materials.
>
> Hi All!
>
> I don't know if the "Mg/Fe+Mg of Bulk Planet" refers to the distinction
> between achondrite groups, but if it does, here is what I can offer:
>
> The Fe/(Fe+Mg) ratio allows meteoriticists to measure the degree of
> oxidation of mineral assemblages in metorites. When you plot Ca/Mg
> vs. Fe/(Fe+Mg), you can easily recognize that the achondrites divide
> themselves into distinct groups with clearly discernible gaps between
> the aubrites, the diogenites, the howardites, the eucrites, etc.
> Best wishes,
>
> Bernd
>
I'm not sure that they way they are using the Mg/Fe+Mg ratio in the that
passage is referring to degree of oxidation. The bulk ratio of a planet
would be constant regardless of the extent of oxidation or reduction, just
like the bulk weight % of Fe in a meteorite sample includes all Fe whether
contained in silicates, sulfides, or as free metal. They may be using the
Mg/Fe+Mg ratio just as they do isotopic ratios of other elements, like
samarium or oxygen. The ratio of Mg to total Fe and Mg may be distinctive
between parent bodies, just as oxygen isotope ratios show distinctions
between the Earth, the Moon, and Mars. Mg and Fe cations are also nearly
the same size and have the same charge, and so act very much the same in
silicate melts, hence the olivine and pyroxene solid solutions. By knowing
the bulk ratio of the planet, you have a reference number to compare various
rocks to see if some outcrops or formations are relatively enriched or
depleted in one element or another.

Just my thoughts.

Frank Prochaska

> If I understood correctly, Steven Singletary kindly explained that oxygen
> fugacity refers to the measured amount of oxygen available to react in
> various environments (earth, asteroids, lunar) in regards to the formation
> of various lithologies (rock groups/types). This leads me to a further
> question:
>
> Shearer et al, Planetary Materials V.36 Reviews in Mineralogy states:
> "The moon lacks significant water and the lithologies making up its crust
> crystalized at a very low oxygen fugacity. Therefore, all lunar
> lithologies thus far sampled are anhydrous (without water) and contain
> elements in reduced valence states." (I believe valence states is
potential
> reactivity or combining power of an element (due to extra or lacking
> electrons...?))
>
> What is the significance of a reduced valence state? More or less
reactive?
> And why do valence states correlate to water presence or oxygen fugacity
> during crystalization?

The valence state will reflect the oxidation potential , transition elements
tend to be variable valent depending on the oxidation potential &
temperature eg Fe FeO Fe3 04 Fe2 O3 classic examples .The oxidant can be any
radical O ,S, CO3, SO4 etc .
this property per se doesn't affect the water of crystallisation .
The text relates to 2 separate conditions dryness & low oxygen potential .
the stuff would be expected reactive in an earth atmos
all the best
Col
Received on Tue 27 Feb 2001 02:30:13 AM PST