Gonze, "Ab initio calculation of the thermodynamic properties and atomic temperature factors of SiO2 [alpha]-quartz and stishovite
," Physical Review B, vol.
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.
The perovskite is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) with nonmetallic stishovite
(Si[O.sub.2]) and perovskite at the pressure 140 GPa .
In one part of the sample, the team found a bit of quasicrystal cocooned within a grain of stishovite
. 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.
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.
In nature, the polymorphs of silica are quartz; Cristobalite; Tridymite, Coesite; Stishovite
; lechatelerite and opal .
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.
leave the solution of cosmological problems to those learned in the infallible sacred texts.") 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.