A reducing environment is required to preserve organic matter in sediments. Therefore, at 25 o C a neutral solution has a pH of 7. If the pH is less than 7 the solution is an acid and if it is greater than 7 is a base. Thus, carbonates will begin to dissolve in rain water whereas quartz will not. Caves are underground openings usually created in carbonate rocks which have interacted with slightly acidic waters.
Water is a remarkable compound.
A good picture of a water molecule is given by an ellipse with a positive charge at one end and a negative charge at the other. Negatively charged ions are surrounded by the positive end of the water molecules and vice versa. In general, olivine breaks down before pyroxene and plagioclase breaks down before alkali feldspar. For all practical purposes quartz does not react chemically at the surface of the Earth. The formation of clays is an important part of the soil forming processes.