Determination of the amount of Peroxy in granite rock using the Seebeck Effect

Abstract:

We present a series of laboratory experiments which provide evidence for peroxy defects in granite, coupled with a determination of the peroxy defect concentration. When peroxy defects are activated they become defect electrons (positive holes) in the oxygen anion sub-lattice. This in essence converts the granite sample to a p-type semiconductor. Our preliminary results of the thermoelectric (Seebeck) effect for granite show that positive charge carriers are being generated (positive gradient: see Figure) in the granite sample and that the concentration of peroxy defects in the granite sample is 1137 ± 20 ppm. The Seebeck coefficient (α) is the gradient between the voltage (V) and the temperature (T), such that α = V /T . One end of the granite sample was placed in a furnace and heated. At 300ºC the peroxy defect spins decouple, while at 430ºC the peroxy defects dissociate, producing positive holes. When the positive holes are activated their mobility increases and they move towards the cool end of the granite sample through diffusion. This induces a potential difference linked to a thermal gradient between the two ends of the sample. We then fitted the coefficients of two first order polynomials and a point of inflection using a Markov Chain Monte Carlo algorithm. This was done to statistically estimate the uncertainties in the coefficients from a Bayesian statistical analysis. The best fit and corresponding standard
error of the reflection point was found to be 426 ± 5ºC. This is in excellent agreement to values, around 430ºC, found in the literature. We then find α = 18.50 ± 0.18 μV K⁻¹ above 426 ± 5ºC, which equates to a carrier concentration of 1.16 × 10²⁰ cm⁻³ compared to the carrier concentrations of heavily doped semiconductors, which are on the order of 10²¹ cm⁻³. This then gives a peroxy defect concentration of 1137 ± 20 ppm.