Acid-Base Properties of Minerals and Rocks

Algebraic manipulation shows that these expressions are related in the equation 

A few minerals produce acid when they contact water. These minerals can be described as salts of weak bases and strong acids. They chiefly result from weathering and oxidation of the pyrite or marcasite (FeS2) exposed in the mining of mineral deposits and coal. Such acid minerals, which are dominantly Fe sulfates and to a minor extent AP sulfates, typically form from the evaporation of pooled acid-mine waters or of the moisture in unsaturated mine wastes or spoils that contain the sulfides. Acidity is produced when they are dissolved by fresh runoff or recharge. For example 

More common minerals in such a setting are jarosite and alunite, which often dissolve incongruently to form their oxyhydroxides, releasing protons. The jarosite reaction produces significant acidity, whereas alunite is a very weak acid. For the reaction 

The pH at equilibrium with the aluminum sulfate salts on the other hand, is near neutral. For alunite in equilibrium with amorphous aluminum hydroxide, the reaction is 

Another common mineral under such conditions is basaluminite (Al4S04(OH)io) which has a solubility product of and dissolves according to the reaction 

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