Title page for ETD etd-07052007-135551

Type of Document Dissertation
Author Olsen, Amanda Albright
Author's Email Address amalbrig@vt.edu
URN etd-07052007-135551
Title Forsterite Dissolution Kinetics: Applications and Implications for Chemical Weathering
Degree PhD
Department Geosciences
Advisory Committee
Advisor Name Title
Rimstidt, james Donald Committee Chair
Bodnar, Robert J. Committee Member
Dove, Patricia M. Committee Member
Schreiber, Madeline E. Committee Member
  • mineral dissolution kinetics
  • chemical weathering
  • ionic strength
  • ligand-promoted dissolution
  • olivine
  • forsterite dissolution
Date of Defense 2007-06-21
Availability unrestricted

Silicate minerals are the most common mineral group in the earth's crust so it is not surprising that their weathering reactions dominate the chemistry of many earth surface processes. This project used forsterite as a model system to identify the important factors that affect silicate mineral dissolution rates and grain lifetimes in the weathering environment.

I determined an empirical rate law for forsterite dissolution of forsterite in oxalic acid solutions: based on a series of 124 semi-batch reactor experiments over a pH range of 0 to 7 and total oxalate concentrations between 0 and 0.35 m at 25°C. These experiments show that oxalate-promoted dissolution rates depend upon both oxalate concentration and pH. I propose a reaction mechanism in which a hydrogen ion and an oxalate ion are simultaneously present in the activated complex for the reaction that releases H4SiO4 into solution. By analogy, I propose that water acts as a ligand in the absence of oxalate.

I also ran 82 batch reactor experiments in magnesium and sodium sulfate and magnesium and potassium nitrate solutions. These experiments show that ionic strength up to 12 m, log mMg up to 4 m, and log mSO4 up to 3 m have no effect on forsterite dissolution rates. However, decreasing aH2O slows forsterite dissolution rates. The effect of decreasing dissolution rates with decreasing aH2O is consistent with the idea that water acts as a ligand that participates in the dissolution process. Forsterite dissolution rate data from previously published studies were combined with results from my experiments and regressed to produce rate laws at low and high pH. For pH < 5.05 or and for pH > 5.05 or I then developed a diagram that shows the effect rate-determining variables on the lifetime of olivine grains in weathering environments using these rate laws.

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