Here's what I found: http://www.fleascontrol.com/cat-eating-litter.htm

Why is my cat eating litter?

There is probably something wrong, so you need to take your cat to the vet if you see it eating litter. Cats will usually only do this because of a nutritional deficiency of some sort, which can be caused either by their diet or by a severe disease. Most litter contains a lot of minerals - but it's not good for the cat to eat it, as it is usually on balance worse for the cat, especially clay litter. Eating litter is a prime symptom of feline anemia, for example, because that disease causes iron deficiencies - you should check on that page to see if your cat has any of the other symptoms. Another potential disease is chronic renal failure in cats - a shutdown in the kidneys that sometimes causes cats to eat litter in response.

Cats also eat litter in response to nutritional deficiency in their diets. Are you trying to make your cat a vegetarian? If you don't get an approved vegetarian cat food, your cat could have a taurine deficiency - a dangerous condition that leads to blindness and even death in some cases. Are you not feeding your cat enough? Or not giving your cat regular cat food? Or feeding your cat dog food? Or feeding your cat only "people food"? Those are also common problems that can cause this.

You might also check to see if your cat has eaten poisonous plants. Sometimes that can cause them to eat litter as well.

There are links within that space for each of the problems if you go to the page - keep us posted!