KINETICS OF AL-SI EXCHANGE IN LOW AND HIGH QUARTZ - CALCULATION OF ALDIFFUSION-COEFFICIENTS

Non-uniform distribution of Al between the three symmetrically equival ent Si sites in low quartz is converted to a random distribution by dr y or hydrothermal annealing above 400-degrees-C due to Al-Si exchange, as shown previously by EPR measurements. Rate equations for the Al-Si exchange are derived from the experimental results presented here. We show that the kinetics of the Al-Si exchange reaction are strongly in fluenced by the type of Si substitution, whether by Al+Na ([AlO4/Na]4- ) or Al+Li ([AlO4/Li]4-). Rate constants k and activation energies E o f the Al-Si exchange differ significantly under identical run conditio ns according to the [AlO4/Na]4- and [AlO4/Li]4- defects. Thus, it is c oncluded that Na and Li are involved in the rate determining step of t he Al-Si exchange reaction. The role of Na and Li is discussed from th e electrostatic and structural viewpoints. In the case of [AlO4/Na]4- defects, no significant effect of water pressure on the activation ene rgy E of the Al-Si exchange is observed, while the rate constant k dec reases with increasing water pressure. In the case of [AlO4/Li]4- defe cts, the activation energy of the Al-Si exchange in low quartz increas es from 278 +/- 20 KJ/mole (dry, in air) to 400 +/- 25 KJ/mole at 100 MPa water pressure, while k decreases. For high quartz, no effect of w ater pressure is observed with respect to E and k, and the activation energy E is drastically reduced compared with low quartz. From the exp erimental results obtained on samples from different growth sectors of the same crystal, it is concluded that a vacancy mechanism is respons ible for the Al-Si exchange. Based on the ''random walk'' theory, equa tions are derived which allow calculation of the diffusion coefficient s (D(parallel-to c) and D(parallel-to c) of Al from rate constants (k) of the Al-Si exchange reaction. The calculated diffusion coefficients are in the range 10(-24) to 10(-27) m2S-1. With the method described here, diffusion coefficients can be estimated in temperature ranges wh ere conventional methods fail (e.g. diffusion of radioactive tracers o r measurements of electrical conductivity).