Maskelynite

Maskelynite is a diaplectic glass - a glass formed in the solid state - produced when plagioclase feldspar is subjected to the intense shock pressures of a meteorite impact or found within meteorites themselves. It has the chemical composition of plagioclase but the disordered, amorphous structure of glass.

Crucially, maskelynite forms without melting and flowing; the original crystal is converted to glass in place, so it preserves the outline and even the cleavage cracks of the parent feldspar. This makes it a key marker of shock metamorphism.

It is named after the astronomer Nevil Maskelyne and is best known from Martian (shergottite) meteorites and terrestrial impact structures.

Maskelynite forms by shock metamorphism: when a hypervelocity impact drives a shock wave through rock, plagioclase crystals experience pressures of roughly 30 gigapascals or more. At these pressures the crystal lattice collapses into a dense glass without conventional melting, a process called solid-state vitrification.

It is found in shocked rocks at terrestrial impact craters and abundantly in stony meteorites, especially the Martian shergottites, where its presence is used to estimate the shock pressures the meteorite endured. Some maskelynite may also form by very rapid quenching of localized impact melt of feldspar composition.

Maskelynite appears as clear, colorless to pale brown isotropic glass occupying the shape of former feldspar grains, with hardness around 6 and a vitreous luster. Under the petrographic microscope it is diagnostic: it stays dark (isotropic) under crossed polarizers, unlike crystalline feldspar, yet retains the grain outlines and sometimes relict cleavage of plagioclase.

It can resemble ordinary volcanic or impact-melt glass, but maskelynite preserves original crystal boundaries because it formed in place without flowing. Its occurrence in shocked rocks and meteorites is the key contextual clue.

Maskelynite has no commercial or gem use; its importance is scientific. It is a vital indicator of shock metamorphism and is used by planetary scientists and impact geologists as a barometer for the pressures generated by meteorite impacts.

Its presence in Martian meteorites helped confirm their violent ejection from Mars and provides clues to impact processes on other planets. As a research material it is studied to understand how minerals respond to extreme transient pressures.