Diamonds in the Sky

by Peter Tyson

Glittering stars in the night sky aside, scientists have long known that there are diamonds in the heavens. In 1981, for example, when Smithsonian researchers tried to cut through a large iron meteorite that had crash-landed in the Allen Hills of Antarctica, the sawteeth on their blade got all chewed up. Subsequent x-rays showed that the stone was riddled with microscopic diamonds, the hardest substance known. The scientists theorized that the meteorite's diamonds were born during a cataclysmic collision out in the asteroid belt.

Can dying stars known as red giants spawn diamonds? Some scientists think so.

Other meteoritic diamonds apparently hail from deep space. In 1987, a team of researchers headed by Edward Anders and Roy Lewis of the University of Chicago reported the discovery of meteorite-embedded diamonds so miniscule that trillions could fit on the head of a pin. Unlike the Smithsonian diamonds, these microscopic crystals contain an isotopic mixture of xenon gas not found on Earth. "It seems necessary to invoke an extra-solar origin for the diamond," the scientists concluded in a paper published in Science (3/11/87), indicating a birth outside our solar system. Indeed, the team proposed that the lucent crystals formed in the atmosphere of a "red giant" or dying star before it collapsed and exploded billions of years ago. The supernova would have sent the diamond-studded material far out into space, where in the fullness of time some pieces eventually fell to Earth. If this scenario is correct, the researchers said, then interstellar dust may be peppered with tiny diamonds.

Still other diamonds are apparently created during the fiery instant when meteors and meteorites slam into Earth. In the 1960s, scientists discovered more microscopic diamonds in the remains of the vast Canyon Diablo meteorite, which formed Meteor Crater in Arizona. The diamonds are sand-grain-sized, only hundredths of an inch across. Other crater-related diamonds are larger. In the 35-mile-wide Popigari crater in Siberia, the result of a huge impact 35 million years ago, Russian researchers unearthed polycrystalline diamond clusters reaching nearly half an inch across. Many of these impact-spawned diamonds bear the cubic structure of ordinary, Earth-grown diamonds. But analysts studying the Canyon Diablo diamonds found that up to a third of them bore a hexagonal atomic structure never before seen in diamond. Mineralogists named the new hexagonal variant of diamond lonsdaleite after the British mineralogist Dame Kathleen Lonsdale, who helped advance the study of natural diamond crystals.

Microscopic diamonds appear to have formed in the fiery instant when the meteor that created Arizona's Canyon Diablo struck the Earth.

Today, more than 30 years after the discovery of the Canyon Diablo diamonds, scientists still debate how such mini-diamonds form. Some suspect they were wrought in the vacuum of space by vapor deposition, a process that specialists can use to make synthetic diamond here on Earth. Others maintain that carbon atoms (or, in a minority opinion, grains of meteoritic black graphite) within the hurtling meteorite itself transformed instantly into diamond during the extraordinary heat and shock of impact.

Whatever the origin of meteorite diamonds, some scientists believe they have found evidence that the colossal cloud of dust thought to be thrown up into the atmosphere in the wake of such impacts may spread newly formed diamond dust all around the world. In 1991, Canadian geologists David B. Carlisle and Dennis R. Braman reported finding Lilliputian diamonds embedded in a layer of sediment 65 million years old - right at the time when many scientists believe a giant meteor slammed into Earth and precipitated the extinction of the dinosaurs. Can these miniature diamonds, which are so fine-grained that the researchers deem them the result of a collision, serve as an indicator of this ancient catastrophe, much as the famous iridium layer has done? Scientists won't be able to say without further study, but the idea holds promise. (In the 1998 book The Nature of Diamonds, the geologist George Harlow and two Russian colleagues wrote simply, "This subject is very new, and many exciting discoveries have yet to be announced.")

The black diamonds known as carbonados got their name for their carbonized, or burnt, look.

Tracing black diamonds

Outer space may also be the birthplace of the mysterious black diamonds known as carbonados. From the Portuguese word for burned or carbonized, carbonados were first found in Brazil in the 1800s and have since turned up elsewhere, most notably in central Africa. Unlike the clear diamonds of engagement rings, which are single crystals, black diamond consists of aggregations of individual crystals, which lend the gem its dark color. The largest diamond ever found was a carbonado from Brazil; named Sergio, the stone weighed 3,167 carats. (One carat equals one-fifth of a gram.)

The origins of carbonados have long baffled scientists. Black diamonds don't adhere to the rules of diamond mineralogy, and they don't occur in the usual places where clear diamonds are found. Even so, scientists initially believed they must have been fashioned in the same conditions under which clear diamonds are thought to form. That is, they were crafted deep within the Earth, 100 to 300 miles down, when intense heat and pressure transformed carbon into diamonds, which volcanic eruptions then lofted to the surface. But that theory suffered a blow when scientists examined the carbon isotopes of black diamonds. (Isotopes are species of a chemical element that reside in the same place on the periodic table but have different atomic weights and physical properties.) Unlike clear diamonds, black diamonds feature ratios of the two most common carbon isotopes in the Earth's crust-carbon-12 and carbon-13-that characterize surface carbons rather than those found in the Earth's depths.

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