| How does mineral composition influence weathering of stones ? |
Binder
Stones are composed of various binders spread in-homogeneously within the stone. These binders keep the stone together increasing the strength of the stone and protecting it from deteriorating quickly. Although binders bind the minerals of the stone, it binds in chunks. The boundaries of these bound chunks can be weak and gradually form cracks within the structure of the stone. The strength of the binders also determine how quickly the minerals will deteriorate. Weak binders will dissolve leaving pitted surfaces. These pits will form open passages into the structure of the stone for external agents. Therefore, lack of quality binder leaves the stone exposed to fast mechanical and chemical deterioration.
Impurities
Some minerals in the composition are impurities like iron, silica, mica etc. These impurities react with external factors such as rain and air to weaken the stone further and deteriorate the structure of the stone. For example, iron content present in certain stones can react with water and air to form rust which makes the stone weak. If stones contain sulphates such as limestones, these can also oxidize, dissolve the calcium resulting in pitting and flaking. This oxidation process also renders the stone weak against further chemical and physical weathering. Other soft impurities such as Mica can be spread through out the stone and has a very loose bonding with the structure of the stone. During polishing, mica from the surface pops out and they too leave pits on the surface.
Porosity
Porosity is determined by the total volume of all the pores in the stone. The value or the amount of porosity can affect the rate of deterioration. Lower the rate of porosity, slower the rate of deterioration. Porous stones are prone to ingress of stains, acidic materials, water and other chemicals. They also prone to physical weathering due to loose structure and bonding.
Packing Density
Packing density is directly correlated to the porosity of the stone and the strength of the minerals. Stones with high packing density such as granite have comparatively lower porosity than a loosely packed stone such as limestones.
Hardness
Softness or hardness of minerals, impurities and binder determines the hardness of the stone. But hardness of the stone does not necessarily guarantee higher resistance to weathering. In spite of hardness, stones can still remain porous which leave them extremely vulnerable to water, salt and other types of chemical weathering. Hardness of the stone is determined by the mean average hardness of the minerals in the stone and the packing density.
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