RockHoundBlog

Black Diamonds, Carbonados. “presolar” meteorites?

Filed under: Mineral of the day,regular postings — Gary January 16, 2007 @ 10:19 pm

carbonado

Long baffled by their origin, scientists now have evidence that these charcoal-colored gems formed in outer space.

Stephen Haggerty and Jozsef Garai, both of Florida International University, analyzed the hydrogen in black diamond samples using infrared-detection instruments at the Brookhaven National Laboratory and found that the quantity indicated that the mineral formed in a supernova explosion.

Also called carbonado diamonds, meaning “burned” or “carbonized” in Portuguese, black diamonds defy mineral-making rules and are neverfound in the world’s mining fields where the clear and classic variety typically resides.

Black Diamonds, or Carbonados, are black or coal colored diamonds sharing similar material properties to the more well-known white diamonds. Named carbonados by the Brazilians who first discovered them in 1840, black diamonds are found almost exclusively in Brazil and the Central African Republic. They are exceedingly rare — only about two and half metric tons of the rocks have been mined, usually from stream beds, since their discovery. The largest carbonado ever found measures 3,167 carats, 60 times larger than the largest clear diamond. Found in Brazil where it was named Sergio, the crystal weighs 1.4 pounds.

Use

Like white diamonds they are the hardest minerals on earth and can only be cut by other diamonds — it takes 6,000 pounds of pressure per square inch even to dent one. However, unlike white diamonds, carbonados are porous and agglutinated, meaning they’re comprised of clusters of crystals lurching every which way as opposed to the near uniformity of their clear counterparts. As a result they’ve got more cutting edges than other diamonds, and were used by the French in the 19th century to carve and sand hardwood for furniture. Carbonados were also used in the Suez Canal’s construction to break up other rocks. One current use for them are as super-hard drill bits and heads.

Origins

The rocks are a puzzle: scientists know what carbonados are but have only taunting clues where they came from. Though they are as hard as white diamonds and, like those diamonds, formed when ordinary carbon was subjected to immense heat and pressure, carbonados appear to be significantly younger — about 3.2 versus 3.5 billion years. Their estimated period of origin is out of sync with the volcanic evidence. They are also not found in conventional mines like white diamonds miles below the earth, but are found in relatively shallow areas, like steam beds.

The riddle of the carbonado has long been the subject of debate, and explanations include a theory that carbonados were born as the result of heat from natural radiation, the impact of rocky asteroids or exploding stars.

Professor Stephen Haggerty, a geologist at the University of Massachusetts, put forth a controversial theory recently that is gaining scientific ground. Haggerty proposed the carbonados of Africa and South America come from the same meteorite or meteorites. Billions of years ago distant exploding stars rich in carbon produced dense clumps of diamonds from the shock waves and pressure. Carbonados, he speculates, could be the resulting diamond aggregates, black diamond boulders that entered our solar system and plummeted to earth, fragmenting in the process. The idea of diamonds in meteorites isn’t new; it was first discovered a decade ago that tiny diamonds can form that way. But after comparing values for carbon isotopes’ radioactive decay in various ancient rocks and meteorite samples, Haggerty began to suspect that carbonados come from a new class of “presolar” meteorites, long-surviving fragments of which, sucked into our solar system by the sun’s gravitational pull, eventually fell to earth through the oxygen-poor atmosphere of the pre-Cambrian period.

“When you put all the pieces together,” says Haggerty — the apparent presence of a certain type of weak chemical bond, the isotopic frequencies of long-decaying carbon isotopes, and the rocks’ signs of intense heating and shock — it seems as if these black diamonds formed 2.6 to 3.8 billions years ago. “Beginning about 4.5 billion years ago the earth was pummeled by meteorites, and this lasted for 5 million years,” he says. “We don’t see the scars they left because of erosion — the evidence was destroyed on earth — but it is beautifully preserved on Mars and the moon. That’s because those atmospheres are low in oxygen, which would turn the meteorite fragments into immense fireballs. But nearly 4 billion years ago the earth’s atmosphere had little or no oxygen, and so the black diamonds could have rained down unimpeded.” So why would carbonados be found only in Brazil and central Africa? It Haggerty’s right about the diamonds’ origin, they fell to earth before an ancient block of the earth’s crust fractured into South American and Africa.

Not everyone is convinced. Haggerty’s model has been criticized because of the carbonados’ small size — what made the fragments splinter to such an extent? — and because some isotopic data don’t jibe with Haggerty’s presolar profile of the rocks. However, his theory is gaining ground among other scientists. Jan ’07 Astrophysical Journal Letters

1 Comment »

  1. http://six.pairlist.net/pipermail/meteorite-list/2007-February/032358.html

    [meteorite-list] Carbonados
    bernd.pauli at paulinet.de
    Tue Feb 27 15:30:08 EST 2007
    ————————————————-

    Elton wrote: “As far as I know the diamonds are only found in [Canyon Diablo / Barringer Crater]rim recoveries.”

    Hello Elton, Marcin, and List,

    Nininger was the one who first observed that diamonds were only found in fragments
    recovered from the rim. All these meteorites had a reheated, granulated matrix. Nininger
    suggested that the temperatures indicated were those required to form diamonds from
    graphite during the passage of shock waves produced in the meteorite upon collision
    with the Earth.

    Most carbonados are about 0.1-1.0 mm (some up to 2.2 mm) in diameter, a little porous,
    layered and polycrystalline. They usually occur in amorphous carbon-graphite-troilite nodules.

    Many of the carbonados are aggregates of diamond and a hexagonal polymorph of diamond
    called lonsdaleite.

    Reference:

    BUCHWALD V.F. (1975) Handbook of Iron Meteorites (Volume 2, Canyon Diablo, pp. 384-385).

    Best wishes,

    Bernd

    Comment by Insane4SpaceRocks — March 1, 2007 @ 1:30 pm

RSS feed for comments on this post. TrackBack URI

Leave a comment

You must be logged in to post a comment.