What is stishovite?

Stishovite (a high-density polymorph of quartz)

An excerpt from Vanders and Kerr (1967):

"The formation of natural stishovite is attributed to the transient high shock, pressure, and temperature accompanying the impact of a huge meteorite. Most of the natural stishovite from the highly siliceous Coconino Sandstone of Meteor Crater in Arizona has a needle-like habit, which suggests that it crystallized at temperatures above 900 degrees Centigrade......The thermal dependence of the crystal habit of stishovite may thus constitute a high temperature geologic gauge, which could indicate the limiting values for the peak temperatures that prevailed at the impact craters in highly siliceous rocks (Sclar et al., 1964)."

Depending on the temperature of formation and on the silica content of the target rock, stishovite (sometimes misspelled as stishivite) may have a bipyramidal habit, a granular to prismatic to tabular habit, or a needle-like habit; however, the crystals are very small, because the intense short duration of formation prevents the crystals from growing to a large size. The typical size for stishovite crystals ranges from fine sand (less common) to silt (more common). Considering that the density of stishovite is roughly twice that of quartz, it would appear that the mineral might be separated and concentrated from regular quartz sediment by traditional panning methods (although I don't know if this collection method has actually been tried).

PROPERTIES OF STISHOVITE Chemical Formula: SiO₂
Crystal structure: similar to rutile (although the stishovite crystals tend to be very small and very poorly formed)
Structural unit: SiO₆ octahedron
Crystal system: tetragonal 4/m 2/m 2/m
Density: 4.28
Index of Refraction: 1.81
Fracture: conchoidal
View an X-ray diffraction pattern for natural stishovite.

Here's another diffraction pattern of the mineral, created at an experimental pressure of 55 GPa (that's 55 billion Pascals pressure).

References:

Vanders, I., and P.F. Kerr. 1967. Mineral Recognition. John Wiley and Sons, New York. 316 pages.

Sclar, C.B., L.C. Carrison, and G.G. Cocks. 1964. Stishovite: thermal dependence of crystal habit. Science 144:833.