Lightning Strike can Reshape a Mineral’s Crystal Structure:

 

Researchers once believed only meteorites could do so. At a rock outcropping in southern France, a jagged fracture runs along the granite. The surface in and around the crevice is discolored black, as if wet or covered in algae. But the real explanation for the rock’s unusual features is more dramatic: a powerful bolt of lightning.

Using extremely high-resolution microscopy, Prof Gieré et al found that not only had the lightning melted the rock’s surface, resulting in a distinctive black “glaze,” but had transferred enough pressure to deform a thin layer of quartz crystals beneath the surface, resulting in distinct atomic-level structures called shock lamellae. Prior to this study, the only natural events known to create this type of lamellae were meteorite impacts.

Geologists have long known that lightning, through rapid increases in temperature as well as physical and chemical effects, can alter sediments. When it strikes sand, for example, lightning melts the grains, which fuse and form glass tubes known as fulgurites – which can also form when lightning strikes other materials, including rock and soil. The current study examined a rock fulgurite found near Les Pradals, France.

Under an optical microscope, they found that the outer black layer – the fulgurite itself – appeared shiny, “almost like a ceramic glaze,” + was porous, almost like a foam, due to the lightning’s heat vaporizing the rock’s surface. A chemical analysis showed elevated SO2 and Phosphorus pentoxide, which may have derived from lichen living on the rock’s surface at the time of the lightning strike.

Transmission electron microscope revealed the fulgurite lacked any crystalline structure, consistent with it representing a melt formed through the high heat from the lightning strike. But, in a layer adjacent to the fulgurite, slightly deeper in the rock, they spotted an unusual feature: a set of straight, parallel lines: shock lamellae. This occurs when the crystal structure of quartz or other minerals deform in response to a vast wave of pressure. The lamellae were only 3 micrometers wide, ie energy of the lightning bolt’s impact dissipated over that distance.

This characteristic deformation of crystals had previously only been seen in minerals from sites where meteorites struck. Shock lamellae are believed to form at pressures up to 10 gigapascals.

Gieré et al hope to study rock fulgurites from other sites to understand the physical and chemical effects of lightning bolts on rocks in greater detail. Another takeaway for geologists, rock climbers and hikers is to beware when they see the tell-tale shiny black glaze of a rock fulgurite, as it might indicate a site prone to lightning strikes. http://www.upenn.edu/pennnews/news/lightning-reshapes-rocks-atomic-level-penn-study-finds