|MadSci Network: Chemistry|
The best way to think about the relationship between chemistry and geology is to remember that minerals are, after all, nothing more than chemicals. They may be very widespread and exist in complex mixtures (which we call rocks), but they are still chemicals. Since they are chemicals, they are subject to all the rules and formulas that govern other chemicals. Reaction rates, equilibria, balanced equations ... all the normal things you'd learn in chemistry also apply to minerals.
Geochemists are scientist who use the rules of chemistry to help understand geology. By knowing how the various minerals interact with the environment and with each other, and by understanding the influence of geologic temperatures and pressures on those interactions, geochemists can predict what reactions and transformations will occur. This is especially important in what is called economic geology - predicting where one is likely to find valuable substances such as petroleum or ores.
One thing that geochemists often have to take into account that is a little different from some other chemists is that some geochemistry proceeds very slowly so that it can take millions of years for some reactions to take place. All the principles and formulas are exactly the same - only the time scale is longer.
A good website to explore geochemistry is: http://www.geo.cornell.edu/geology/classes/Geochemweblinks.HTML
Try the links in the MadSci Library for more information on Chemistry.