There, Natalia Dubrovinskaia and colleagues managed to synthesize millimeter-sized transparent polycrystals and single crystals of stishovite
, a high-density form of silica (SiO2) usually found only in minute amounts near meteor-impact craters.
A naturally occurring, glassy compound, stishovite
forms only under pressures 100,000 times greater than those on the Earth's surface--during an asteroid-on-asteroid collision, for example.
Raman spectrum of natural and sysnthetic stishovite
Coesite and stishovite
were discovered in the Barringer Meteor Crater and the Ries Crater, in Germany, more than a decade before the Alvarez group advanced the theory about the K-T extinction.
Its atoms aggregate in forms as common as quartz crystals and as exotic as coesite and stishovite
, minerals formed by the intense pressures generated when extraterrestrial objects such as comets and asteroids strike Earth's surface (SN: 6/15/02, p.
Not until the discovery, several years later, that the "shock metamorphism" of large-body impacts (and nuclear explosions) can form two separate minerals, stishovite
and coesite, and that many suspected impact sites had high concentrations of both, did the hypothesis begin to win converts.
Some of these forms -- like stishovite
, a phase of quartz produced under extremely high pressure -- are "generally considered indisputable evidence [of an impact] when found near the surface," Melosh says.
The discovery of extraterrestrial amino acids (135: 356) and the rare mineral stishovite
(135: 132) in rocks from the Cretaceous-Tertiary Boundary provided further evidence that a large body collided with Earth 65 million years ago, causing the mass extinctions that occurred around that time.