Planar deformation features
Sets of very thin, straight, parallel lamellae of amorphous material in minerals such as quartz, formed by shock and diagnostic of impact.
Planar deformation features (PDFs) are narrow lamellae, usually less than about 2 micrometres thick and spaced a few micrometres apart, that cross mineral grains along specific crystallographic planes. In fresh material they consist of amorphous (glassy) silica; in older rocks they are often annealed and decorated with tiny fluid inclusions. In quartz they form at shock pressures of roughly 10 to 35 GPa, and commonly occur in more than one orientation within a grain.
The orientations are diagnostic and pressure-dependent. Those parallel to the {10-13} rhombohedral planes (ω) dominate at moderate pressures, {10-12} (π) at higher pressures, and basal PDFs, parallel to (0001), are often mechanical Brazil twins. Measurement uses a universal stage on a polarising microscope, or transmission electron microscopy, with results plotted as frequency histograms of angles to the c-axis. PDFs must be distinguished from tectonic deformation lamellae, which are curved, wider, more widely spaced and typically in a single set.
Multiple-set PDFs in quartz are among the main criteria for confirming terrestrial impact structures. They occur in shocked grains from the K–Pg boundary layer and in rocks from Chicxulub, Vredefort and Sudbury. Feldspar, zircon and other minerals develop comparable features. Their presence is recorded in descriptions of shocked crystalline rocks.
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