Crystalline Silica

Is crystalline silica allowed in a limestone product?

By Jarod Rhoades

In the world of rocks, there is perhaps no other mineral with more names than silica. Quartz, jasper, amethyst, chert, flint, citrine, chalcedony, and tridymite are just a sampling of the many silica varieties, and those are just the crystalline structure forms. Silica, also known as silicon dioxide because of its molecular formula (SiO2), is the second most common mineral in the Earth’s crust. 

It is quite common to see a reference to crystalline silica on a mined product’s Safety Data Sheet (SDS) because of silica’s prevalence. However, crystalline silica’s appearance on the SDS does not necessarily require a separate review. Rarely is any mined mineral product completely pure. For example, limestone often contains significant impurities, including sands, clays, and other sediments. Many of these impurities are varieties of silica or contain silica, and can be structurally crystalline or amorphous. While there are no entries in Canada’s Permitted Substances List (PSL) for crystalline silica specifically, it does appear under many of its aliases in several places, including the listings for diatomaceous earth (PSL Tables 4.2, 5.2, 5.3, 6.5, 8.2), silicon dioxide (PSL Tables 6.3 and 6.5), and unprocessed mined minerals (PSL Table 4.2).

While it is typically undesirable to have large proportions of crystalline silica in a limestone product due to silica’s hardness and abrasive nature, it is often not realistic or cost-effective to attempt to remove it completely. Limestone deposits with very high amounts of quartz or silica sands would likely be passed over for more uniform ores. That said, the very high-purity limestone deposits are more commonly used in building materials and industrial or chemical applications, rather than as a soil liming agent, for example. Agricultural applications of limestone tend to have a crystalline silica percentage at or below approximately 1.5%, whereas high-purity deposits would be expected to contain almost no silica content whatsoever.

Supplementary crystalline silica could also be added to a limestone product’s formulation. OMRI separately reviews the silica to the relevant PSL table in cases like this. Materials like quartz sand could, for example, provide significant soil drainage benefits when used as an amendment. If the quartz sand meets the PSL Table 4.2 annotation for an unprocessed mined mineral, it is allowed. 

So, why would an SDS claim the presence of crystalline silica if it was not an intended constituent of the product? The U.S. Occupational Safety and Health Administration (OSHA) and the Canadian Centre for Occupational Health and Safety (CCOHS) define crystalline silica as a health hazard in the workplace. Prolonged and repeated exposure to extremely fine crystalline silica particles significantly increases the risk of silicosis, lung cancer, COPD, and kidney disease, so inclusion on a Safety Data Sheet is often required by law.

This article was originally published in the winter 2019 edition of the OMRI Materials Review Newsletter, and was revised in September 2025 by Bilingual Technical Research Analyst Jacky Castañeda.