3.3 h. Seifertite: A new natural very dense post-stishovite polymorph of silica (T.G. Sharp/Tempe, A. El Goresy/Mainz, L.S. Dubrovinsky, M. Chen/Guangzhou, B. Wopenka/St. Louis, P. Dera/Washington DC, C.T. Prewitt/Tucson, N.Z. Boctor/Washington DC and R.J. Hemley/Washington DC)

In the last two decades, considerable interest has been devoted to the response of silica to extreme pressure conditions using both diamond anvil high-pressure devices and First Principles Calculations. This interest stems from the possibility of the existence of dense free silica in Earth’s mantle. Silica is a major component of the Earth. Based on the chondritic model, SiO₂ makes up 50 wt.% of Earth’s bulk. Possible natural polymorphs of silica denser than stishovite are also important for understanding the behaviour of SiO₂ in the Earth's interior and during natural dynamic events on planetary surfaces. High-pressure diamond anvil cell (DAC) experiments revealed that silica undergoes several phase transitions to “post-stishovite” (polymorphs denser than stishovite) phases above 48 GPa. Stishovite inverts displacively above 48 GPa to a CaCl₂ I -structured-polymorph (space group Pnnm), a polymorph that contains silicon in distorted octahedrons. It has been shown experimentally and by First Principles Calculations that this polymorph is stable up to 78 GPa. Beyond that pressure the CaCl₂ polymorph transforms displacively to a denser species with the α-PbO₂-like structure with kinked chains of SiO₆octahedra (space group Pbcn or Pb2n).

The Shergotty and Zagami SNC (possibly Martian in origin) meteorites contain silica grains with lamellar structure previously misidentified as PDF in quartz. Silica occurs as large (150-900 µm) prismatic grains, rhombic-or triangular shaped cross sections typical of orthorhombic β-tridymite morphology. The morphology of the cross sections is also consistent with cristobalite. Every silica grain consists of mosaics of domains (10-60 µm in size Fig. 3.3-9) each displaying two orthogonal sets of lamellae that havedifferent brightness in Back Scattering Electron (BSE) mode in the Field-Emission Scanning Electron Microscope (FESEM). The brighter lamellae consist of a dense crystalline SiO₂ phase and the dark ones of dense SiO₂ glass (Fig. 3.3-10). Electron microprobe analyses with a defocused beam on the widest lamellae showed almost pure SiO₂ with minor concentrations in Na₂O (0.40 wt.%) and Al₂O₃ (1.14 wt.%). The lamellae are extremely sensitive to electron bombardment and are destroyed within a few seconds under a focused electron beam, despite the use of low sample current (< 5 nano-Amps). An attempt to determine the nature of the lamellae by laser microRaman spectroscopy failed due to the instantaneous vitrification under the focused laser beam, despite the use of low laser power.

Fig. 3.3-9: A back scattered electron (BSE) photomicrograph of a triangular dense silica grain from the Shergotty meteorite. The grain consists of numerous domains of a maximum diameter of 60 µm. Each domain displays an orthogonal pattern of bright (seifertite) and dark (dense SiO2 glass) lamellae.

Fig. 3.3-10: A BSE-SEM detail of the area within the white box in Fig. 3.3-9. White lamellae are seifertite and the black lamellae consist of dense SiO2 glass, probably formed by vetrification of another metastable post-stishovite silica plolymorph

X-ray diffraction and TEM investigations revealed that the dense crystalline silica in Shergotty is a new orthorhombic species (Space Group Pbcn or Pb2n) with the following cell parameters: a 4.097(1) Å, b 5.0462(9) Å, c 4.4946(8) Å, V 92.92 ų, Z = 4. Calculated density = 4.309 g/cm³ (with empirical formula) and 4.294 gm/cm³ (with pure SiO₂). High-pressure DAC experiments on cristobalite revealed a phase transformation above 40 GPa to the α-PbO₂-like structure. This strongly suggests that this species formed on the SNC parent body at peak-shock pressures slightly in excess of 40 GPa. The Commission of New Minerals and Mineral Names of IMA (International Mineralogical Association) approved the nomination of this very dense silica species after Friedrich Seifert (seifertite).