Clay Mineralogy and Geochemistry Exercise #6

Construct mineral equilibrium diagrams (at 25° C and 1 atmosphere) for the following mineral assemblage: gibbsite, quartz, kaolinite, halloysite, microcline, muscovite and albite. Use the free energies of formation given in the table below. For each case below, calculate the stability boundaries between all mineral pairs and plot them on a diagram. Then determine which sections of the boundaries are stable and redraft a second diagram with the stable fields labeled. Show all your calculations and annotate your procedure

Case 1. Choose log aK+ / H+ as your abscissa and log aNa+ / H+ as your ordinate for the diagram. Assume the activity of the dissolved silica is fixed at quartz saturation (i.e., assume the solution is in equilibrium with quartz).

Case 2. Choose logaK+ / Na+ as your abscissa and log aH+ as your ordinate for the diagram. Assume the activity of the dissolved silica is fixed at quartz saturation .

Table 1: Free energies of formation in kJ mol-1, at 25° C, 1 atmosphere pressure.

Phase

DG

gibbsite

-1154.916

halloysite

-3780.713

kaolinite

-3799.364

quartz

-856.288

muscovite

-5600.670

microcline

-3742.330

albite

-3711.722

water

-237.140

Na+

-261.900

K+

-282.490

H4SiO4o

-1308.000