Mosaicism
Mosaicism is the breaking of a crystal into many slightly misaligned domains by intense shock.
Mosaicism describes a crystal that, under the polarising microscope, no longer goes dark all at once but shows a patchwork of tiny domains, each extinguishing at a slightly different angle. The crystal has been broken internally by shock into a mosaic of blocks.
Mosaicism in olivine and pyroxene is a key diagnostic feature of high shock stages in chondrites. In the widely used scheme, weak mosaicism marks shock stage S4, at roughly 30 to 35 gigapascals, and strong mosaicism, often with plagioclase converted to maskelynite, marks S5, at roughly 45 to 55 gigapascals.
Identifying mosaicism requires a thin section and crossed polars. It is one of several features, including undulatory extinction, planar fractures and high-pressure minerals, that help researchers assign a shock stage. These classifications reveal how violent an asteroid's collisional history was and help sort meteorites into groups that may share a common break-up event.
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