Geoscience Reference
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isotopic ratio only at the K-T boundary, again strengthening the
possibility that the osmium and iridium came from space. 1 5
Spinel is a rare mineral that sometimes forms a variety of ruby.
Many of the K-T clays contained a nickel-rich variety of spinel pre-
viously found only in material worn off of meteorites. Furthermore,
the highest spinel abundances occurred at exactly the same place in
K-T sections as the iridium spike—at some locations each gram of
boundary rock contained more than 10,000 spinel spherules.
Until recently, diamonds occurred in nature only in rocks be-
lieved to have originally formed deep within the earth (where heat
and pressure are high), and that subsequently were elevated to the
surface. Within the last few decades they have been produced in
explosion experiments and found in meteorites, where the dia-
monds are so tiny as to be barely detectable. The hope that dia-
monds would also show up at terrestrial impact sites led Canadian
scientists David Carlisle and Dennis Braman to search the K-T
boundary clay in Alberta, where they immediately found them. 1 6
Now the story gets even more interesting. 1 7 As the Soviet Union
began to collapse, reports started to emerge that scientists there had
not only found diamonds at several of their impact sites, but in
numbers reaching into the millions. The most thoroughly tested
crater was the 3 5-million-year-old, 100-km-wide Popigai Crater in
Northern Siberia, which the Soviets probed with over 500 bore-
holes. Most of the diamonds there were tiny, but some were as large
as peas. (Although none are of gemstone quality, they may prove
useful for industrial purposes.) A British team searched the Ries
Crater and soon found diamonds by the billions in the melt rock,
which had been the source of the stone for the town hall and the
church in Nordlingen, the medieval German town located within
the crater. The citizens of that town, unbeknownst to them, had
been surrounded all their lives by innumerable diamonds formed 15
million years ago by a giant impact. The Ries diamonds occur in
association with silicon carbide, like diamond a rare and hard min-
eral. From this association and various other chemical indicators, the
British scientists concluded that the diamonds and silicon carbide
had not formed directly as a result of shock but rather had crystal-
lized in midair from the white-hot impact fireball. If they are cor-
rect, then diamonds should be found at other impact craters and
provide an excellent marker of impact. The search was immediately
extended, and by mid-1996 diamonds had been found in each of
the eight impact sites studied. No diamonds occur in rocks immedi-
ately above and below the K-T boundary—only right in it. The find-
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