Planar fractures
Planar fractures are sets of parallel cracks in crystals formed by moderate shock.
Planar fractures are parallel cracks that cut through a crystal along specific crystallographic directions. They form when a shock wave passes through rock at moderate pressure, and they are easiest to see in olivine under the microscope.
In the standard shock classification of chondrites, planar fractures in olivine together with undulatory extinction mark shock stage S3, corresponding to peak pressures of roughly 15 to 20 gigapascals. Opaque shock veins and small melt pockets often appear at this stage too. More intense shock produces mosaicism.
Planar fractures should not be confused with planar deformation features in quartz, which are finer, closely spaced lamellae of altered material and are a hallmark of impact craters on Earth. Both, however, are evidence that a rock experienced a sudden high-pressure event of a kind volcanoes and normal tectonic forces cannot produce.
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