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 varieties of silica that can be found on the Earth’s surface, and those are just the crystalline forms. Silica (known as SiO2 to chemists – silicon dioxide) is the second most common mineral in the crust of the planet.

It is quite common to see a reference to crystalline silica on the Safety Data Sheet (SDS) for a mined product because of its prevalence. However, the appearance on the SDS does not necessarily require a separate review of this material. Rarely is any mined mineral product completely pure, and in the case of something like limestone there are often significant impurities, including sands, clays and other sediments, many of which include silica. 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, silicon dioxide, and under the general mined minerals allowances.

While it’s 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 being 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.

Additional crystalline silica could feasibly be added to a limestone product formulation as well, since materials like quartz sand could, for example, provide significant soil drainage benefits when used as an amendment. In cases such as this, OMRI evaluates the manufacturing process for the added ingredient separately from the limestone product and, if it meets the PSL Table 4.2 annotation for an unprocessed mined mineral, it is allowed. However, there are potentially synthetic versions of crystalline silica, or synthetic additives like flow agents, that could be included in the material. These materials would all be reviewed on a case-by-case basis to the relevant PSL table.

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), as well as 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.