It is calcium carbonate. Rabbits are unique. Their bodies do not regulate blood calcium levels as well as other animals. As a result, calcium is absorbed readily and may reach very high levels in the blood.
Calcium is removed from the blood and excreted as calcium carbonate in the urine. Over time, calcium carbonate builds up and becomes the chalky white film you see.
Rabbits require from 0.6 – 1.3% calcium in the diet, depending on age and life stage. Calcium is the mineral your rabbit needs most to develop teeth and bones. It also has other functions in the body. Purina formulates its’ rabbit diets to contain a constant amount of calcium in the diet to insure proper development of strong bones and teeth.