Moonrock tells of little-known lunar layers.

Moonrock tells of little-known lunar layer

"It's wonderful," says Ursula B. Marvin, "that after all these years one can still find a completely new rock type on the moon." According to Marvin, a space scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., the tiny fragment provides information about a little-sampled layer of the lunar crust.

Included among the samples brought back by the Apollo 15 astronauts in 1971, the unusual rock was pointed out to Marvin by Marilyn M. Lindstrom, who was working with the moon-rocks at the NASA Johnson Space Center in Houston. Marvin describes the rare bit, only about 1.5 millimeters across, as "white glass with two bright red spinels in it." Spinel is a crystalline mineral that exists in various colors, and although researchers might have broken up a larger chunk for different kinds of tests, Marvin says Lindstrom called this one "too small to split but too beautiful to pass up."

What makes it significant is that it contains tiny grains of a magnesium-rich mineral called cordierite, together with an olivine mineral called forsterite. According to J. William Carey of Harvard University, the maximum pressure at which this combination could form is equal to that about 30 miles below the lunar surface, about 6 miles above the moon's crust-mantle boundary. A few Apollo moonrocks appear to have formed at greater depths, and some came from closer to the surface, but the Apollo 15 sample represents an intermediate level. The high magnesium content suggests it came from deeper than about 16 miles, Marvin says, while the cordierite means it was probably no more than 30 miles deep.

The researchers report in the Feb. 17 SCIENCE that the rarity of this combination, called a cordierite-spinel troctolite, among the Apollo moonrock samples suggests it probably formed far enough down that only a major cataclysm, such as a meteorite impact, could have exposed it to view. The astronauts collected their samples near the moon's huge Imbrium basin, and the researchers suggest the cordierite bit was "excavated" by the same impact that formed Imbrium.