Involves complex chemical reactions that alter the internal structure of minerals by removing and or adding elements through dissolution, oxidation, hydration, and hydrolysis.
Water, in all of its occurrences (rain, fog, raising ground moisture, ocean, etc.) and its chemical composition is the most important element in this process. It acts as a solvent and also as a chemical reactant. Carbon dioxide (CO2) and oxygen are also important agents along with climatic conditions.
Dissolution: This is a process whereby some minerals or part of the chemical composition of a mineral can be dissolved in water. When an acidic element is added to the water it increases its ability to dissolve minerals more readily, particularly those containing calcium, magnesium, sodium and potassium. The atmosphere is a mixture of gases and one of these
natural occurring gases is carbon dioxide (CO2). CO2 is also produced by the combustion of fuel and released into the atmosphere. When CO2 gas is dissolved in water it is converted to carbonic acid, and a carbonated solution is formed. Just a small amount of acid increases the corrosive effects of water. Limestone and marble are particularly subject to this process. In exterior installations, they can develop a petted surface and if they have a polished finish they will lose it. Sandstone that contains calcite is also affected. The calcite can be leached from the stone and may result in a flaking or spilling of the surface and in some incidences may cause the stone to crumble.
Rain is naturally acidic. This is the primary reason why polished marble and limestone in a exterior installation will not hold a polished finish.
Oxidation: Oxidation, also referred to as "rusting" occurs when oxygen assisted by moisture combines with iron-bearing minerals. Oxidation is accelerated by moisture and high temperatures. It is an important process in the alteration of iron and magnesium rich minerals. Mineralogical iron occurs in three states; metallic, ferrous (Iron II), ferric (Iron III). During the oxidation process Iron II is converted to Iron III resulting in color changes and a weakening of the mineral structure. Ferrous magnesium silicate minerals that undergo oxidation are responsible for the appearance of rust spots on some granite-type stones. When marble or limestone contains sulfide minerals such as pyrite, and it undergoes oxidation, the Iron II in this composition will produce rust spots and the sulfur is converted to sulfuric acid which can dissolve the calcium producing pitting. Iron oxide minerals are common in sandstone and over time may develop rust spots. During the oxidation process an increase in volume of the mineral structure can occur, usually making it softer and weaker and rendering it more vulnerable to other elements of the chemical weathering processes.
Hydration: Hydration is the chemical addition of water molecules to a mineral. This process frequently produces a new mineral compound. In addition, the size of the mineral structure is increased causing stress and developing zones of weakness. This can increase the overall permeability of the stone and lead to spilling, pitting and flaking. Overall color changes can take place as well.
Hydrolysis: This is the dissolution and alteration process that affects silicate minerals in granite-type stones. As with the dissolution process the addition of a acidic ingredient is an important element in this process. Feldspars are an important silicate mineral group in granite and granite-type stones, with their general formulas containing potassium, sodium and calcium. These soluble elements are leached out during the chemical reaction with acidly charged waters and carried away. As the process proceeds permeability increases exposing more mineral surface to the process, the mineral grains will eventually weaken leading to spilling and crumbling. Once this process has started it is irreversible.
