This is Part One of our series on how to choose stones, exploring the key characteristics and stability of chemical compositions that influence stone selection, ensuring optimal performance and durability for your projects.
The chemical composition determines the chemical reactivity of the stone. Stones primarily composed of carbonate minerals, known as carbonate stones, have relatively poor chemical stability, while those mainly composed of silicate minerals exhibit strong chemical stability.
Carbonate stones
Carbonate minerals include calcite and dolomite, which form a major part of stones such as limestone, dolostone, dolomitic limestone, calcite marble, and dolomitic marble. For limestone and calcite marble such as Carrara marble, Thassos marble, and Galala marble, when they come into contact with dilute hydrochloric acid (5% HCl), a reaction occurs: CaCO3 + 2HCl → CaCl2 + H2O + CO2, causing fizzing and bubbling. In contrast, dolostone and marble such as Arabescato marble and Ajax react weakly with dilute hydrochloric acid, producing only small bubbles.
Carbonate stones are commonly characterized by their poor acid resistance. Calcium carbonate (CaCO3), the primary component of these minerals, can react with water and carbon dioxide to form calcium bicarbonate (Ca(HCO3)2), which is soluble in water. This reaction is particularly relevant when limestone and marble are used as exterior cladding materials for buildings, as these stones are exposed to atmospheric precipitation. As a result, some calcium carbonate dissolves and reacts with carbon dioxide, as well as rainwater that may contain other components. The resulting calcium bicarbonate is then washed away by the rain. Over time, this process can lead to a loss of luster and the development of an uneven surface on the stone.
Although dolostone contains less calcium carbonate content and has slightly better chemical stability than calcitic stone, and exhibits only weak effervescence when exposed to dilute hydrochloric acid, the calcium carbonate within it will inevitably react with water and carbon dioxide. In more severe cases, the cementation of the rock can be compromised, potentially resulting in a granular, weathered appearance.
Silicate stones
Silicate stones, which contain SiO2, exhibit very high chemical stability and are less likely to react with other substances. Silicate minerals exhibit various crystalline forms, including isolated, chain, sheet, and framework structures. Each crystallization form displays distinct growth habits, leading to significant differences in the development and characteristics of the minerals.
Chain silicate minerals develop crystals predominantly in one direction, often forming columnar, needle-like, or fibrous shapes. Sheet silicate minerals, on the other hand, develop crystals in two directions, resulting in plate-like, flaky, or scaly forms, such as mica, graphite, and molybdenite. Framework silicate minerals develop crystals in three directions, typically forming granular or prismatic crystals, with examples including quartz and feldspar.
One-dimensional and two-dimensional minerals tend to exhibit strong selectivity in their cleavage development and physical properties, making them less stable in nature and more susceptible to weathering. In contrast, three-dimensional minerals show less pronounced selectivity in their cleavage development, with physical properties that are relatively uniform in all directions. This results in greater stability and less susceptibility to weathering.
Conclusion
In conclusion, carbonate stones have relatively poor chemical stability and are best suited for common indoor environments. They should not be exposed directly to the outdoors, particularly to natural precipitation. On the other hand, silicate granite stones offer greater stability and strong resistance to chemical weathering, making them ideal for both indoor and outdoor use.
