Nat. Commun. 8, 15647 (2017)

Credit: GEORGE R. ROSSMAN, CALTECH

On Earth, the most common form of silicon dioxide is quartz. But in planetary dust, meteorites and lunar rocks, the mineral predominantly has a different structure, known as cristobalite (pictured). In meteorites, cristobalite comes mixed with yet another form of silica mineral called seifertite. But seifertite forms at very high pressures (over 80 GPa), implying a massive meteorite impact, whereas cristobalite requires much lower pressures. So the peculiar coexistence of the two structures in the same meteorite has intrigued researchers. And now, Ana Černok and colleagues have used high-pressure conditions and X-ray diffraction to understand how the two types of silica are formed.

Černok and co-workers found that when cristobalite is compressed uniformly around 11 GPa, it turns into a high-pressure phase called cristobalite X-I, which reverts back upon decompression. But when the pressure is uneven, cristobalite transforms into seifertite at a much lower pressure (50 GPa) than it would from ordinary silica. In meteorites, impact shock waves create areas with even and uneven compression where cristobalite and seifertite, respectively, can form. This also suggests that meteorites containing sefertite need not have necessarily suffered a dramatic impact.