Archives for the Month of January 2008 on Geol 390
I do believe I have selected my project
I've selected my project for 390 and I am beginning to research it as of today. I'm working with Dr. Jim Van Orman, a geochemist with whom I've been working for as an assistant almost a year now.
Background:
Scientists are debating whether or not olivine (Mg,Fe)2(SiO4) is open to transfer of rare earth elements with the Earth's mantle. Research has been done to find diffusion constants, however some papers disagree. Two researchers have found diffusion coefficients differing by two orders of magnitude.
Katherine Crispin, a graduate student in the geology dept., has been doing similar research on trivalent cation diffusion in MgO.
I remember a presentation Katherine gave last semester regarding progress in her research. She mentioned the following: When trivalent cations are used as substitutions for Magnesium, a divalent cation, a Magnesium vacancy must exist per 2 trivalent ions in order to charge balance the mineral. The vacancy can be considered to have a negative charge due to the lonely electrons in the atoms surrounding it in the crystal structure which have nothing to bond to. Similarly, the trivalent ion can be considered to have a positive charge, as there are no extra electrons surrounding it in the lattice to balance it. She suggested an attraction between adjacent vacancies and trivalent cations may pull the cation into the vacancy, increasing it's rate of diffusion.
My Project:
I will investigate diffusion of Scandium in olivine. Scandium is a trivalent rare earth element with an atomic radius similar to that of Magnesium. It is possible that the same effect mentioned above may increase the rate of diffusion of Scandium in olivine, specifically Forsterite, the Magnesium end member.
Why:
Olivine makes up a large portion of the Earth's mantle. In addition, the use of Forsterite, as opposed to the iron end member or a solid solution of the two, makes the process easier as Iron can be trivalent.
Questions I Have:
Should a vacancy pull a scandium in, I imagine an adjacent vacancy is created. What would stop that new vacancy from pulling the Scandium back, creating a sort of see-saw effect, resulting in a no net change in diffusion rate?
Links:
Olivine (Wikipedia)
Magnesium (Wikipedia)
Scandium (Wikipedia)
Rare Earth Elements (Wikipedia)
My First Post
In class we've talked about geology's place in the heirarchy of sciences. Steve has also presented his research on constraints on the composition and shape of Mercury's core. The MESSENGER will be sending back data regarding new measurements, further allowing scientists to figure out the anatomy of Mercury.
I had the following questions:
1. In what way does Mercury's orbit oscillate in 88 day cycles?
2. "Further, Fe-S alloys are better
characterized at the relevant pressures and temperatures [Fei
et al., 1997; Hillgren et al., 2000] for Mercury’s interior
than other candidate compounds (e.g., Fe-C, Fe-Si)."
Why isn't Fe-Ni a possibility? If I remember correctly, Ni is the second
most abundant element in the Earth's core and Sulfur is significantly
less so.
3. It seems to me that this paper assumes that the core must account for atleast the lion's share of the high bulk density, as opposed to the
crust. Why can't the crust be of an abnormally dense composition? What
do we know about the crust of Mercury?
I looked into orbital libration a little bit and I found this, which I thought was interesting. It's an animation of the moon's librations.
http://www.youtube.com/watch?v=_3ryt9fBOBE