Reactions and kinetics Lecture 8

Diffusion

The rate of diffusion depends on the nature (partly, on the molecular weight) of the chemical diffusing. In this demonstration, the blue dye, methylene blue, has a higher molecular weight than the compound potassium permanganate. Smaller, lighter molecules diffuse faster than larger, heavier ones, and the potassium permanganate solution (on the right) has diffused further from the well than the methylene blue (on the left).

The rate of diffusion depends on the characteristics of the medium through which the substance diffuses. Crystals of potassium permanganate were placed in distilled water (left) and on an agar gel (right). The agar gel is still about 98% water, but the gel forms a crisscrossing network of carbohydrate that the potassium permanganate ions must work through. Since the ions must move up, down, forward, and even backwards to diffuse, it takes much longer for them to move away from the source.  

Fig. 25-13a. Chemical zoning profiles across a garnet from the Tauern Window. After Spear (1989)

Fig. 25-13a. Conventional P-T diagram (pressure increases upward) showing three modeled “clockwise” P-T-t paths computed from the profiles using the method of Selverstone et al. (1984) J. Petrol., 25, 501-531 and Spear (1989). After Spear (1989) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.

Garnet reaction rim around pyroxene, Holsnoy, Norway

80499 (17.3 cm) eclogite facies metagabbro, Western Alps

80496-6 (18.8 cm)

80496-6 detail 1 (3.7 cm) J. Wheeler, Nov 2005

J. Wheeler, Nov 2005

J. Wheeler, Nov 2005

80496 summary J. Wheeler, Nov 2005

2. Exsolution Figure 6-16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, 489-511. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Covered in Mineralogy and previously in Petrology

Big kyanite, Pyrenees