Possible shock-induced crystallization of skeletal quartz from supercritical SiO (sub 2) -H (sub 2) O fluid; a case study of impact melt from Kamil impact crater, Egypt

Since its discovery, the Kamil crater (Egypt) has been considered a natural laboratory for studying small-scale impact cratering. We report on a previously unknown shock-related phenomenon observed in impact melt masses from Kamil; that is, the shock-triggered formation of skeletal quartz aggregates from silica-rich fluids. These aggregates are unshocked and characterized by crystallographically oriented lamellar voids and rounded vesicles. The distribution of the aggregates can be correlated with former H (sub 2) O- and impurity-rich heterogeneities in precursor quartz; i.e., fluid inclusions. The heterogeneities acted as hot spots for local melting. Due to the presence of H (sub 2) O and the high impact pressure and temperature, the formation of a localized supercritical fluid is plausible. Below the upper critical end point of the SiO (sub 2) -H (sub 2) O system (temperature <1100 degrees C and pressure <1 GPa), SiO (sub 2) melt and H (sub 2) O fluid become immiscible, leading to the rapid and complete crystallization of skeletal quartz.