Earth's most abundant mineral.

Earth's most abundant mineral

When the earth formed 4.7 billion yearsago, it was an unsorted conglomeration of cosmic material. Through heating and chemical differentiation, the compounds making up the planet sorted themselves into three main layers: the core, mantle and crust. By studying the compositions and temperatures of these layers today, scientists can begin to reconstruct the evolution of the earth as it has cooled.

One significant step toward that end istaken in a Feb. 6 SCIENCE paper by graduate student Elise Knittle and geophysicist Raymond Jeanloz, both at the University of California at Berkeley. With high-pressure experiments, Knittle and Jeanloz have shown that a mineral called magnesium silicate perovskite makes up most of the lower mantle. And since this region accounts for about two-thirds the bulk of the earth, the researchers believe that magnesium silicate perovskite is the "most abundant building block of the whole planet.'

Magnesium silicate perovskite was discoveredabout 10 years ago by L.-G. Liu at the Australian National University in Canberra. He showed that many uppermantle minerals convert to the perovskite under the pressure of a diamond anvil press and the heating of a laser. But at that time the technology had not advanced far enough for Liu to easily subject the perovskite to the very high pressures found in the lower mantle.

Now Jeanloz and Knittle are able tosynthesize the perovskite samples at pressures of the lower mantle: from 240,000 to 1.3 million atmospheres, which is more than four times greater than what had been available to Liu. What's more, Jeanloz and Knittle became the first to study directly the crystal structure of the perovskite using X-ray diffraction while their samples were under these high pressures.

"The guts of our discovery are thatonce this perovskite is made in the mantle, there's no new high-pressure phase that forms at greater depths [or pressures] as far as we can tell, because we've covered the entire pressure range . . . of the mantle,' says Jeanloz. In other words, the perovskite retains its composition and structure under mantle pressures.

The researchers characterized some ofthe mineral's properties--including its density, compressibility and melting temperature --under mantle conditions. "This gives us a real handle for the first time on the long-term behavior of the deep earth,' notes Jeanloz.

Since the lower mantle is a solid, themelting temperature of the perovskite gives the researchers an idea of the highest possible temperature of the lower mantle. Jeanloz says they hope to obtain a more exact estimate of the temperature by comparing the seismologically determined densities of the mantle with additional laboratory studies of the perovskite density as a function of pressure and temperature.

Jeanloz says perovskite crystals withthe same structure, but with compositions different from the mantle material, have been of intense interest to scientists studying superconductivity (SN: 1/10/87, p.23). Now, in addition to the technological significance of perovskites, he notes, "we're saying they have a geologic importance as well.'