MINERALOGICAL CHARACTERIZATION OF A FULGURITE FROM OUTER BANKS NORTH CAROLINA (U.S.A.): FORMATION CONDITIONS AND ENVIRONMENT

In 2021 several remarkable elongated sandy fulgurites were found by Prof. Edwin S. Bair on the beach in the Outer Banks, North Carolina (U.S.A.). The samples are formed by lightning striking the beach quartz-rich sand. Four samples analyzed show elongated hollow, glass-lined tubes of about 5-7 cm diameter surrounded by a fine-grained matrix with shell fragments encased in it; four samples are tubeless porous droplets with elongated and spherical shapes. The goal of the present research is to shed light on the genesis of fulgurites from Outer Banks, North Carolina.

X-ray powder diffraction (XRPD) was used to reveal the presence of residual and newly formed high-temperature and high-pressure minerals. XRPD pattern of hollow, glass-lined tubes showed the presence of quartz, plagioclase, clinochlore, cordierite, cristobalite, coesite, stishovite, and dolomite. XRPD pattern of the droplets showed the presence of quartz, plagioclase, clinochlore, cordierite, cristobalite, coesite, and dolomite. Dolomite is ascribed to the shell fragments embedded in the droplet matrix. The kinetics of quartz polymorphs (cristobalite, coesite, and stishovite) and cordierite formation depend on pressure and temperature. The coexistence of clinochlore, cordierite, coesite, and stishovite in the hollow, glass-lined tubes would indicate induced temperature above 1000 °C and pressure above 6 GPa if thermodynamic equilibrium was reached. The presence of cristobalite and coesite in the droplets would indicate induced temperatures above 1000 ºC and pressure above about 2.5 GPa if thermodynamic equilibrium were obtained.

The samples studied display the effects of impact-related processes owing to lightning impact on wet and dry beach fine-grained sand. The mineralogical features of the samples from Outer Banks, North Carolina provide insights into the physical-chemical effects occurring in rocks and sediments in response to shock-impact events such as a lightning strike.