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Scientists have long suspected that at least some
diamonds found on Earth hail from the heavens.
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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.
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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.
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Microscopic diamonds appear to have formed in the fiery
instant when the meteor that created Arizona's Canyon Diablo struck the Earth.
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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.
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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.
Continue: A new theory of carbonado formation
The Science Behind the Sparkle |
Diamonds in the Sky
A Primer of Gemstones |
See Inside a Diamond
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