Shocked quartz
Shocked quartz is a form of the mineral quartz (SiO₂) that has undergone shock metamorphism, characterized by distinctive planar deformation features (PDFs) formed under extreme pressures of 5–30 GPa and associated high strain rates, typically resulting from hypervelocity meteorite impacts.[1] These PDFs appear as sets of closely spaced, straight lamellae or planes within the crystal lattice, often oriented along specific crystallographic directions such as {10\bar{1}3} or {10\bar{1}2}, and are visible under a polarizing microscope as reduced birefringence bands.[2] The formation of shocked quartz occurs during the compression stage of an impact event, where shock waves propagate through the target rock at velocities exceeding 3–8 km/s, inducing solid-state amorphization or phase transformations without complete melting.[1] At lower shock pressures (around 5–10 GPa), initial PDFs develop as mechanical twins or Brazil twinning along basal planes, while higher pressures (~30 GPa and above) produce multiple PDF sets and partial conversion to diaplectic glass (a dense, amorphous silica).[2] Even more intense conditions (>30 GPa) can lead to the formation of high-pressure polymorphs like coesite and stishovite embedded within the quartz or associated glass, distinguishing these features from tectonic deformation, which produces curvilinear or irregular fractures.[2] In geological contexts, shocked quartz serves as a primary diagnostic indicator for confirming terrestrial impact structures, with grains often found in impact breccias, ejecta deposits, and distal layers such as those at the Cretaceous-Paleogene boundary.[2] Its presence allows scientists to estimate shock pressures, reconstruct crater sizes, and trace the distribution of impact ejecta over continental scales, as PDFs remain stable unless subjected to prolonged high temperatures (>900°C) that cause recrystallization.[1] Experimental simulations using high-explosive or gas-gun techniques have replicated these features, confirming their hypervelocity origin and aiding in the identification of over 190 confirmed impact sites worldwide.[2]Definition and Properties
Definition
Shocked quartz is α-quartz (SiO₂), the common low-temperature polymorph of silica, that has undergone shock metamorphism, leading to a deformed internal crystalline structure while preserving its overall macroscopic form and composition.[3][4] Shock metamorphism refers to the rapid transformation of minerals under intense, dynamic pressures generated by hypervelocity impacts or explosions, which induce mechanical deformation without substantial heating or chemical changes.[5][6] The term "shocked quartz" originates from these shock waves, with the earliest scientific usage appearing in 1963. This mineral was first recognized in the 1960s as a key diagnostic indicator of extraterrestrial impacts, distinguishing impact events from other geological processes.[7] Its primary microscopic evidence consists of planar deformation features.[3]Characteristic Features
Shocked quartz exhibits distinctive planar deformation features (PDFs), which manifest as sets of parallel lamellae, also known as shock lamellae, embedded within the crystal lattice of quartz grains. These features are narrow, typically 50-500 nm in width, and consist of amorphosed quartz (diaplectic glass) that forms under high-strain-rate conditions exceeding 12 GPa. The lamellae are spaced 2-10 micrometers apart and appear as crystallographically controlled planes that traverse the grain without significant offsets or bending.[8] PDFs in shocked quartz can be categorized into several types based on their internal structure and post-formation alterations: amorphous PDFs, decorated PDFs, and fluid inclusion planes. Amorphous PDFs are thin lamellae of disordered silica that lack luminescence in cathodoluminescence imaging due to their non-crystalline nature and remain unaltered in fresh impact materials. Decorated PDFs, in contrast, are healed versions containing high dislocation densities and fluid inclusions, exhibiting red luminescence from non-bridging oxygen hole centers formed during beam damage. Fluid inclusion planes represent traces of original amorphous PDFs that have recrystallized with water-assisted fluid inclusions along the deformation planes, also showing characteristic red luminescence. These types arise from varying degrees of shock-induced amorphization and subsequent healing processes.[9] The orientations of PDFs are precisely aligned with specific crystallographic planes, commonly {0001} (basal), {10\overline{1}1}, and {10\overline{1}3} in Miller-Bravais indices, reflecting the directional nature of shock wave propagation. A key hallmark distinguishing shocked quartz from tectonically deformed quartz is the presence of multiple PDF sets—up to 5-6 or more per grain—oriented in different directions, whereas tectonic deformation typically produces only single sets of broader, curved lamellae without glass infill. This multiplicity arises because shocked quartz develops up to 10 sets in highly shocked grains, with the number and diversity increasing with shock pressure. PDFs result from intracrystalline slip along these low-index crystallographic planes during the compressive phase of shock loading, enabling the mineral to accommodate extreme strain rates without fracturing.[10][8][11]Physical and Optical Properties
Shocked quartz retains the chemical composition of unshocked quartz, consisting of silicon dioxide (SiO₂), as the shock process does not alter its elemental makeup.[11] The physical properties of shocked quartz are similar to those of unshocked quartz in mildly to moderately shocked samples, including a hardness of 7 on the Mohs scale and a density of approximately 2.65 g/cm³.[12] However, in highly shocked specimens approaching diaplectic glass formation, density can decrease to ~2.2 g/cm³ due to structural disordering.[11][13] Macroscopically, shocked quartz grains frequently exhibit fracturing or brecciation, but individual crystals remain transparent and colorless, preserving the vitreous luster of quartz.[11] Optically, shocked quartz under polarized light microscopy reveals undulatory extinction and patchy birefringence, indicative of internal lattice strain from shock deformation.[11] The refractive indices (n_ω = 1.544 and n_ε = 1.553 for unshocked quartz) are slightly reduced in shocked samples, with birefringence decreasing from 0.009 in unshocked quartz to 0.006–0.001 or near zero in heavily shocked grains.[11][12]| Property | Unshocked Quartz | Shocked Quartz |
|---|---|---|
| Chemical Composition | SiO₂ | SiO₂ |
| Hardness (Mohs) | 7 | 7 |
| Density (g/cm³) | 2.65 | 2.65 (reduced to ~2.2 in high shock) |
| Birefringence | 0.009 | 0.001–0.006 (near 0 in heavy shock) |
| Refractive Indices | n_ω = 1.544, n_ε = 1.553 | Slightly decreased (e.g., 1.463–1.478 in isotropized) |