Planar deformation features, orPDFs, are optically recognizable microscopic features ingrains ofsilicate minerals (usuallyquartz orfeldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain'scrystal structure.
PDFs are only produced by extreme shock compressions on the scale of meteor impacts. They are not found involcanic environments. Their presence therefore is a primary criterion for recognizing that animpact event has occurred.
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