Some of the most popular natural deodorants in that aisle are the "crystal" deodorant stones and sprays. But most people don't know that these crystal deodorant products contain aluminum.
The crystal deodorant stones are made from alum. The most widely used form of alum used in the personal care industry is potassium alum. The full chemical name of potassium alum is potassium aluminum sulfate.
The reason that most people try to avoid aluminum in deodorant is because of its possible link to Alzheimer's disease. There is some strong evidence of a link -- for instance, in 1988 a truck driver accidentally dumped 20 tons of aluminum sulfate in to a town's drinking water. Now, over 20 years later, they are finding a higher incidence of Alzheimer’s in the people of this town.
Incidentally, the article linked below notes that, “many doctors such as popular web guru and natural health expert Dr. Mercola suggest avoiding aluminum as much as possible.”