The formation of impact coesite

Area of Science:

• Geology
• Mineralogy
• Planetary Science

Background:

• Coesite in impact rocks was traditionally thought to form from amorphous silica during pressure release.
• Recent studies suggested a subsolidus quartz-to-coesite transformation pathway.

Purpose of the Study:

• To provide further evidence for coesite formation directly from quartz in impact ejecta.
• To investigate the crystallographic relationship and growth mechanism of coesite in impact settings.

Main Methods:

• Microscopic and crystallographic analysis of coesite-bearing impact ejecta from Kamil crater and the Australasian tektite strewn field.
• Examination of coesite crystal morphology, orientation relative to host quartz, and relation to shock features (PDFs).

Main Results:

• Discovery of sub-micrometric coesite crystals as nucleation seeds in Kamil ejecta.
• Observation of well-developed coesite crystals in Australasian samples, growing at the expense of quartz and postdating PDF deformation.
• Coesite (010) planes commonly parallel quartz {10-11} planes, indicating topotactic transformation.

Conclusions:

• Coesite can form directly from quartz through a subsolidus, topotactic transformation during impact events.
• Pre-existing and shock-induced discontinuities in target rocks facilitate this transformation.
• The proposed subsolidus transformation model is applicable to various quartz-bearing rocks and impact scales.