A chunk of meteorite that once belonged to the American Museum of Natural History is about to go on the auction block, with an estimate of between $1.1 million and $1.3 million. The rock, a 28-pound section of the Willamette Meteorite — the largest meteorite every discovered in America — will be offered at Bonhams on October 28, in what is being billed as the first-ever auction exclusively of meteorites.

The Willamette Meteorite, which weighs more than 15.5 tons and is believed to be billions of years old, has been a centerpiece of the Museum of Natural History’s collection since 1906. In 1998, the museum sliced a 28-pound section off the top and traded it to Daryll Pitt, the curator of the Macovich Collection, the largest private collection of meteorites, for a half-ounce piece of Mars, which is now on display in the Arthur Ross Hall of Meteorites. A spokesman for the museum, Steve Reichl, said yesterday that the piece of Mars is of vital importance to research.

The trade wasn’t publicly disclosed until 2002, when Mr. Pitt auctioned off a couple of smaller slivers of the rock. By then, the Museum of Natural History had gotten into a legal battle with the Confederated Tribes of the Grand Ronde, who claimed that the meteorite, which was discovered in Oregon’s Willamette Valley in 1902, was a sacred object and should be returned. The conflict ended in a settlement, giving the tribe has the right to hold a religious ceremony once a year in the Rose Center for Earth and Space.