Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

Abstract

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

On the formation of diaplectic glass: Shock and thermal experiments with plagioclase of different chemical compositions

“…For example in Martian meteorites, the shock pressure and temperature conditions (Fritz et al 2005a, 2005b) were found to be highly consistent with a thermal history inferred from a variety of independent properties, including (1) the radiogenic 4 He inventory (Schwenzer et al 2008), (2) the magnetic properties of ALH84001 (Weiss et al 2000); (3) the recrystallization of plagioclase in a decimeter-sized rock that cooled in space (Fritz et al 2005a), (4) the formation of seifertite enclosed in maskelynite in the Shergotty meteorite (Sharp et al 1999;Bl€ aß 2013;Kubo et al 2015;Cernok et al 2017; this work), and (5) the formation and preservation of thermally unstable high pressure phases associated with shock-produced melt veins and pockets in some meteorites (Fritz et al 2017;Tomioka and Miyahara 2017;Bischoff et al 2019). The low temperature solid state formation of maskelynite does not reset the 40 Ar-39 Ar chronometer of rocks, as Ar diffusion requires a prolonged heating regime (Jessberger and Ostertag 1982;Fernandes et al 2009;Shuster et al 2010).…”

“…; Cernok et al. ; this work), and (5) the formation and preservation of thermally unstable high pressure phases associated with shock‐produced melt veins and pockets in some meteorites (Fritz et al. ; Tomioka and Miyahara ; Bischoff et al.…”

“…; Cernok et al. ). Thus, the presence of seifertite in maskelynite of the Shergotty meteorite is consistent with a low temperature formation at shock pressures of 30 GPa as deduced from the shock effects in the rock‐forming minerals, including maskelynite with a high RI (Stöffler et al.…”

“…In a high temperature environment (>1000 K), seifertite is stable at pressures higher than the stability field of stishovite; i.e., depending on the temperature, it requires pressures in excess of 60-80 GPa (Sharp et al 1999;Dubrovinsky et al 2001;El Goresy et al 2004). Lower temperatures inhibit the formation of stishovite, and seifertite can form from cristobalite at low temperatures (~480 K) and hydrostatic pressure of 25 GPa (Bl€ aß 2013;Kubo et al 2015;Cernok et al 2017). Thus, the presence of seifertite in maskelynite of the Shergotty meteorite is consistent with a low temperature formation at shock pressures of 30 GPa as deduced from the shock effects in the rock-forming minerals, including maskelynite with a high RI (St€ offler et al 1986;Fritz et al 2005a).…”

Additive-Manufacturing of 3D Glass-Ceramics down to Nanoscale Resolution

“…2c), a few pronounced peaks become apparent. They coincide with vibrational signatures of the cristobalite 18 and tetragonal zirconia 19 phases, which are formed at high temperatures 20,21 . Figure 3 shows X-ray diffraction (XRD) data of annealed powder samples.…”