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Davisite and Grossmanite: Born with the Solar System

09.08.2010
When the solar system was born 4.5 billion years ago, davisite and grossmanite were there.

These minerals were two of the first solids to form when an interstellar gas cloud collapsed to form the sun. Found in the Allende meteorite, they now carry the names of Andrew Davis and Lawrence Grossman, professors in geophysical sciences at the University of Chicago, in honor of their pioneering contributions to cosmochemistry.

“I was somewhat shocked to hear about this,” Davis says. “It’s a considerable honor.”

Grossman avidly collected minerals as a child growing up in Toronto, and he still has a collection. “It’s very flattering to have a mineral named after you,” he says. But some are more flattering than others. “I’m really happy that it’s not one of those minerals found in bat guano,” Grossman jokes.

Also honoring Davis and Grossman are asteroids 6947 Andrewdavis and 4565 Grossman. Davis and Grossman are among at least 10 other UChicago professors who have had minerals named after them — a list that includes Nobel laureate Harold Urey and Joseph Iddings and Richard Penrose Jr., two founding members of the University’s former geology department. Such recognition is a tribute to the scientists’ innovative work, and a measure of the University’s influential research on the composition of the Earth and the cosmos.

Pyroxene minerals

The California Institute of Technology’s Chi Ma and George Rossman named davisite in the May-June 2009 issue of American Mineralogist, and then named grossmanite in the same journal’s October 2009 issue. Neither mineral is entirely new to the research community, but Ma and Rossman have determined their crystal structure and chemical composition, which the International Mineralogical Association requires as part of the classification and naming process.

Davisite and grossmanite are pyroxenes, which are found in many kinds of Earth rocks, moon rocks, and meteorites. “Pyroxene is a silicate mineral with a chain structure,” Grossman explains.

Pyroxene’s chemical composition can vary widely because cations—positively charged atoms—of iron, magnesium, titanium, and aluminum, among others, can all substitute within available sites within its crystal structure. “Depending on which cations go in, you have a different pyroxene mineral,” he says.

Grossmanite is a mineral that is closely linked to Grossman’s research. “I was calculating what minerals would condense out of a gas of solar composition,” such as the primordial disc of gas that formed the solar system. His calculations seemed to match the mineralogy found in parts of the 4.5 billion-year-old Allende meteorite, believed to have formed in the earliest years of the solar system.

In the mid-1980s a graduate student working under Grossman’s supervision, John Beckett, PhD’86, verified Grossman’s calculations by synthesizing titanium-rich fassaite in the laboratory. “It was difficult, but he succeeded, and no one’s done it since, by the way,” Grossman says.

It was difficult because that mineral, now called grossmanite, condenses from a gas with large amounts of hydrogen and carbon, but hardly any free oxygen. “You’re in an Earth laboratory. You’ve got oxygen all over the place,” Grossman says. “You’re trying to make a system that’s devoid of oxygen, so it requires extreme conditions.” Beckett’s research showed that titanium-rich fassaite forms in a gas identical to the sun’s chemical composition.

This result was “a startling and important Rosetta stone,” Grossman says. The work confirmed that parts of the Allende meteorite formed from the primordial solar gas. “They’re still the only things we know for sure in meteorites that formed in a gas of solar composition,” he says.

Primitive meteorites

Like grossmanite, davisite is found inside refractory inclusions—parts of meteorites with distinctive mineral properties. “They are objects that you find most commonly in carbonaceous chondrite meteorites, among the most primitive kinds of meteorites,” Davis says. “We used to think that these refractory inclusions actually directly condensed from the solar system. We

think that now they have more complicated histories.

“They may have condensed from the solar system, but since then they’ve been re-melted and cooled off again,” he says.

Davis says having the mineral named after him was a gratifying capstone to years of hard work.

“This is something that everybody in the field dreams about,” he says.

Steve Koppes | Newswise Science News
Further information:
http://www.uchicago.edu/

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