An intriguing claim that human brain cells possess crystals of a highly magnetic mineral known as magnetite was described today by Dr. Joseph Kirschvink, a professor at the California Institute of Technology. The 38-year-old geobiologist said he believed that magnetite crystals enabled animals from bees to whales to navigate by using the earth's magnetic field. He said he doubted that they supported any sensory capability in humans, although he suspected that they might account for the possible influence of strong electromagnetic fields on human health. That magnetite, one of the hardest metals on earth, is synthesized by the human brain is sure to astound most scientists," Dr. Kirschvink said, but what it is doing there is a "total mystery." It might be a vestige from evolution and serve no purpose, he said. Or it could play a role in biology, explaining why electromagnetic fields have been associated with brain cancer and leukemia and why certain odd blips, called spin echoes, show up on magnetic resonance images of the brain. Each human brain on average contains seven billion particles of magnetite, weighing a total of one-millionth of an ounce. Magnetite interacts over a million times more strongly with external magnetic fields than any other biological material, Dr. Kirschvink said, including the iron in red blood cells. If only one cell in a million contains magnetite, he said, magnetic fields could exert an effect on the tissue.