Limestones having an inorganic origin form when chemical changes or high water temperatures increase the water’s concentration of calcium carbonate to the point that it precipitates.
Travertine, the type of limestone commonly seen in caves, is an example (see Figure 2).
When travertine is deposited in caves, groundwater is the source of the calcium carbonate. As water droplets become exposed to the air in a cavern, some of the carbon dioxide dissolved in the water escapes, causing calcium carbonate to precipitate.
Another variety of inorganic limestone is oolitic limestone, a rock composed of small spherical grains called ooids. Ooids form in shallow marine waters as tiny “seed” particles (commonly small shell fragments) are moved back and forth with currents. As the grains are rolled about in the warm water, which is supersaturated with calcium carbonate, they become coated with layer upon layer of the chemical precipitate (Figure 1).