Reports

(For complete reports go to Files -> Reports.)

Abstracts

M.Sc. Project

Significant differences between the high temperature electrical conductivity of "natural" and synthetic olivine rocks, with and without iron, using Impedance Spectroscopy

The electrical conduction properties of olivine, the most abundant mineral in the Earths mantle, are beneficial to scientists studying deformation processes involving diffusive mass transfer, and deep geo-thermometry inferred from magneto-telluric methods. To date, studies have only concentrated on dry natural iron-bearing olivine and synthetic, polycrystalline, iron-free olivine (forsterite). This study will largely be focused on dry iron-bearing (Fe10%) synthetic olivine, to properly determine the magnitude of electrical conduction in upper mantle olivine in addition to better define the charge-transport mechanisms, which are seen in impure natural iron-bearing olivine. Using advanced sol-gel synthesis techniques of pure, fine-grained, iron-bearing olivine material with 10% fayalite / 90% forsterite composition and impedance spectroscopy, the effects of controlled atmosphere (redox conditions) on conductivity, at temperatures between 800°C and 1200°C, are further investigated. Experiments show that the conductivity of the iron-bearing material is similar in magnitude to the pure magnesiumolivine and for different grain sizes, with apparent activation energies at around 200 kJ/mole (800-1200) °C. This is a significant difference to the reported conductivities of natural olivine which show a spread in conductivity for two orders of magnitude. Natural (probably oxidized) olivine requires a mechanism involving Fe2+ to Fe3+ oxidation (small polaron formation) and magnesium vacancies, to explain the excess conduction and a 1/6 relation with the oxygen fugacity. Impedance spectroscopy and quantitative modelling of the defect population in our iron-bearing material shows no presence of the contribution of small polarons within the olivine stability field but is found to become one of the major charge carriers upon oxidation under normal atmospheric conditions. Instead under reducing conditions electrons and magnesium vacancies are the major charge carriers. Furthermore, evidence for reduced conduction via grain boundaries is found with higher oxygen fugacity. The apparent activation energy in the grain boundaries is at around 320 kJ/mol. There the most likely major charge carriers are iron and magnesium interstitials. Lastly, no apparent relation of the conductivity with increasing grain size was found. Most likely the conduction path of least resistance is through the bulk, or grain interiors.

B.Sc. Project

Grain boundary diffusion during active pressure solution in NaCl: Determination via resistivity and compaction experiments

Previous experiments have proven that pressure solution can indeed occur in granular salt at room temperature conditions and at fairly low stresses and that diffusion is the rate limiting process. However, at the microscopic scale the operating processes in the grain boundaries are still poorly understood because there are many unknown factors. It is also difficult to set up an experiment that takes into consideration all known factors, such as the geometry of grains, grain mineralogy, cementation, and solution chemistry in a grain boundary only up to 10 nanometres thick. This investigation has two objectives. 1) To verify that pressure solution is diffusion controlled and occurs in both granular salt and at a halite crystal glass contact for compaction experiments and an electrical resistivity experiment respectively. 2) To calculate Z* values for all experiments and compare them and determine which of the three factors, D, C, and S has the greatest influence on the rate limiting process, diffusion. Z* is the effective grain boundary diffusivity (m3/s) and is written as Z* = DCS, where D is the solute diffusion coefficient in the grain boundary (m2/s), C is the solubility of the diffusing species (m3/m3), and S is the average grain boundary fluid thickness, (m). The first set of experiments are one-dimensional compaction experiments performed on brine-saturated NaCl aggregates (grain size ~35 - 160 mm) at room temperature and with applied effective stresses in the range of 0.82 - 5.81 MPa. The other experiment is the measurement of resistivity in a single, annular halite-glass contact undergoing active pressure solution. The Z* values for the compaction experiments were calculated via a diffusion controlled strain rate equation and compaction displacement data whereas for the electrical resistivity experiment, Z* was calculated via the Nernst-Einstein equation and the resistivity measurements. Results from the compaction experiments obey the micromechanical theory; q n p e »ev- ×se ×d- where ev is the volumetric strain, se, the effective stress, and d, the grain size and q » 2 , n is slightly higher than 1, and p = 3 which proves that pressure solution in salt is diffusion dependent. The Z* values obtained lie in the range 3E-21 - 4E-19 m3/s and show for a grain size of 35 mm first a positive dependence on the average normal stress (sn) and at sn greater than 10 MPa, the dependence becomes inversely proportional. For grain sizes greater than 85 mm, the relationship is only inversely proportional. Results from the electrical resistivity experiment yield Z* values in the range of 2E-20 - 2E-16 m3/s with more data points towards the lower end and also shows an inverse dependence on sn. All results agree reasonably well with values from previous work on single contact and polycrystalline compaction experiments except for some Z* values in the electrical resistivity experiment at the upper end at 2E-16 m3/s. In addition, variation in Z* in the results could indicate changes in S, but this cannot be said with certainty.