Dietary minerals are the chemical elements require by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen which are present in common organic molecules. The term "mineral" is archaic, since the intent of the definition is to describe ions, not chemical compounds or actual minerals. Furthermore, once dissolved, so-called minerals do not exist as such, sodium chloride breaks down into sodium ions and chloride ions in aqueous solution. Some dietitians recommend that these heavier elements should be supplied by ingesting specific foods (that are enriched in the element(s) of interest), compounds, and sometimes including even minerals, such as calcium carbonate. Sometimes these "minerals" come from natural sources such as ground oyster shells. Sometimes minerals are added to the diet separately from food, such as mineral supplements, the most famous being iodine in "iodized salt." Dirt eating, called pica or geophagy is hypothesized to be a means of supplementing the diet with elements, but this has not been verified . The chemical composition of soils will vary depending on the location.