282 IMPACT STRUCTURES Figure 10 Diaplectic mineral glasses Photomicrograph of granite from the Mistastin complex impact structure with biotite (upper left), quartz (centre), and plagioclase (bottom and far right) Field of view 2.5 mm (A) Plane light (B) Crossed polars Quartz and plagioclase (black) are isotropic, while retaining original mineral shape; i.e., they are diaplectic glasses This requires approximately 40 GPa shock pressure (Figure 7) the same as the original mineral crystal and shows no evidence of fluid textures Maskelynite, the diaplectic form of plagioclase, is the most common example from terrestrial rocks; diaplectic glasses of quartz and of alkali feldspar also occur (Figure 10) High-Pressure Polymorphs Shock can result in the formation of metastable polymorphs, such as stishovite and coesite from quartz and diamond and lonsdaleite from graphite Coesite and diamond are also products of endogenic terrestrial geological processes, including high-grade metamorphism, but the paragenesis and the geological setting are completely different from that in impact structures In terrestrial impact structures, stishovite and coesite polymorphs occur in small or trace amounts, as very fine-grained aggregates, and are formed by partial transformation of the host quartz Figure 11 Photomicrographs of planar deformation features in quartz crystals: (A) nondecorated features, New Quebec impact structure, Canada; and (B) decorated features, Charlevoix impact structure, Canada Field of view mm crossed polars Planar Microstructures The most common documented shock-metamorphic effect is the occurrence of planar microstructures in tectosilicates, particularly quartz (Figure 11) The utility of planar microstructures in quartz is a function of the ubiquitous nature of the mineral and its stability, in the terrestrial environment, and the relative ease with which they can be documented optically Planar deformation features in quartz have various orientations and are produced under pressures of $10 to $35 GPa (Figure 7), with their crystallographic orientation providing a measure of the recorded shock pressures Shatter Cones Shatter cones (Figure 12) are the only known diagnostic shock effect that is megascopic in scale They Figure 12 Shatter cones in quartzites at the Sudbury impact structure, Canada are unusual, striated, and horse-tailed conical fractures ranging from millimetres to tens of metres in length Shatter cones are initiated most frequently in rocks that experienced moderately low shock pressures, 2–6 GPa, but have been observed in rocks