Maskelynite
Plagioclase feldspar turned into glass by intense shock, while keeping the shape of the original crystals.
Maskelynite is a glass with the composition of plagioclase that retains the outlines of the crystals it replaced. It forms when plagioclase is subjected to shock pressures of tens of gigapascals during a hypervelocity impact. Whether it is a diaplectic glass, made by collapse of the crystal structure in the solid state, or a melt that quenched under pressure, has been debated for decades; evidence for both has been reported, and both may occur.
It was first described from the Martian meteorite Shergotty, which fell in India in 1865, and named after the British mineralogist Nevil Story-Maskelyne. Maskelynite is characteristic of shergottites, most of which experienced intense shock during ejection from Mars, and also occurs in shocked lunar rocks, eucrites and chondrites. In the shock-classification scheme for ordinary chondrites, conversion of plagioclase to maskelynite is a defining feature of shock stage S5.
Maskelynite is isotropic, so under crossed polarisers in a thin section it goes dark, while unshocked plagioclase shows twinning and interference colours. In hand specimens of shergottites such as Zagami it looks clear to milky and glassy, lacking the cleavage of feldspar.
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