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)