Shock Lamellae Research Articles

Generation of shock lamellae and melting in rocks by lightning‐induced shock waves and electrical heating

AbstractThe very rapid energy release from impact events, such as those resulting from lightning strikes or meteorites, can drive a variety of physical and chemical processes which alter rocks and result in the formation of natural glasses (i.e., fulgurites and tektites). Fulgurite is the vitrified soil, sand, or rock resulting from lightning strikes. A thunderbolt is associated with air temperatures of up to 105 K, which can heat rocks to >2000 K within tens of microseconds. The rapid fusing and subsequent quenching of the surface of the rock leaves a distinctive, thin, garbled coating composed of a glassy to fine‐grained porous material. Previous studies on rock fulgurites found planar deformation features in quartz crystals within the target rock substrate, evidence of strong shock waves during fulgurite formation. In this paper, we simulated the shock pressure and temperature caused by an idealized lightning impact on rocks and compared the model results with observations on rock fulgurites from the literature. Our model results indicate that a lightning strike can cause >7 GPa pressure on the rock surface, generate a layer of fulgurite (of radius ∼9 cm), and leave a burned region (of radius ∼11 cm). The fulgurites found on rock surfaces share many features with sand fulgurites, but their spatial distribution is completely different, as sand fulgurites are hollow tube structures. Our study on rock fulgurites provides an indirect constraint on the energy of a lightning event and also demonstrates that the presence of shock features in rocks cannot be taken as unequivocal evidence for impact events.

Lightning-induced shock lamellae in quartz

Using transmission electron microscopy we show that planar deformation lamellae occur within quartz in the substrate of a rock fulgurite, i.e., a lightning-derived glass. These lamellae exist only in a narrow zone adjacent to the quartz/fulgurite boundary and are comparable to planar deformation features (“shock lamellae”) caused by hypervelocity impacts of extra-terrestrial objects. Our observations strongly suggest that the lamellae described here have been formed as a result of the fulgurite-producing lightning strike. This event must have generated a transient pressure pulse, whose magnitude, however, is uncertain at this stage.

Extensive soft-sediment deformation structures in the early Darriwilian (Middle Ordovician) shallow marine siliciclastic sediments formed on the Baltoscandian carbonate ramp, northwestern Estonia

Abstract The nearshore Middle Ordovician (early Darriwilian) deposits of the Baltoscandian Basin, formed on the flank of tectonically stable Fennoscandian Shield, comprise numerous soft-sediment deformation structures. The areal extent of the deformed interval extends up to 9000 km 2 from NW Estonia to SE Sweden. The deformation structures occur within a meter-scale sandstone horizon that includes load casts, flame structures, ball-and-pillow morphologies, up to 2 m wide sedimentary dykes, autoclastic breccias, sand volcanoes, and other related soft-sediment deformation structures. Evidently, these deformation structures formed during a single seismic event that caused liquefaction and fluidization of the water-saturated unconsolidated shallow marine sediments within almost the entire sandstone unit (Pakri Formation). Probably a unique catastrophic earthquake of magnitude 7 or higher occurred in this region penecontemporaneously with the deposition of these sediments. The ultimate cause of seismic event of such magnitude in the tectonically stable Baltoscandian Basin is unclear. However, deposition and deformation of the Pakri Formation fall within the Middle Ordovician meteoritic bombardment period which resulted from the disruption of a large parent body in the asteroid belt approximately 470 Ma ago. This in turn suggests a potential impact-origin for the single catastrophic earthquake in the tectonically stable study region. Although impact crater of suitable age is currently not known from the region, then previous studies have documented some impact metamorphic shock lamellae (i.e. PDFs) and abundant extraterrestrial chromite grains from the deformed sediments.

Identification of shocked quartz by scanning cathodoluminescence imaging

Abstract— Quartz grains subjected to high‐strain‐rate shock waves owing to meteorite or cometary impact on Earth's surface commonly display shock lamellae. These lamellae appear as remarkably straight, thin, planar features (microstructures) in sets within which lamellae are essentially parallel to each other and spaced ≤ 20 μm apart. Two or more intersecting sets are typically present. Shock lamellae are commonly recognized and identified by optical methods, by use of the transmission electron microscope (TEM), and by etching polished sections and subsequent examination with a scanning electron microscope (SEM) operated in the secondary electron mode. We present here a method for observing planar microstructures in shocked quartz by using a cathodoluminescence (CL) detector attached to a SEM. The method relies on the fact that planar microstructures in quartz arising as a result of shock display no CL whatever; thus, they show up as distinct, thin, black lines on otherwise luminescent quartz grains. We used scanning CL imaging to study shocked quartz from the Ries Crater, Germany, a well‐known impact crater of Miocene age. We demonstrate that shock‐produced planar microstructures are clearly displayed in SEM‐CL images and can be distinguished from microfractures generated by tectonism, and subsequently filled with quartz, and other similar features not related to impact events. The SEM‐CL method provides a powerful supplement to other methods of identifying shocked quartz. It commonly provides better spatial resolution than does standard optical methods, and does not require etching of quartz grains. Further, it is easier and faster to use than are TEM methods, although it is not capable of the fine‐scale defect analysis possible with TEM.

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vrede- fort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diAraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within diAerent quartz grains are systematically misori- entated such that they share a subparallelh101i direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-di- agnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably asso- ciated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations sug- gest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly re- sulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 lm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent oAsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles.

Researchers focus on Earth's response to hypervelocity impacts

Imagine witnessing the impact of a 10 km‐sized comet or asteroid onto a broad expanse of sea just off the Yucatan peninsula of Mexico. The bolide, a brilliant fireball traveling at speeds of 15 to 20 km/s, would melt about 20,000 km3 of rock and excavate a crater with a diameter of over 200 km within a few seconds, all due to the release of energy equivalent to 100 mega‐megatons of TNT. Now imagine, at a twelve order‐of‐magnitude smaller spatial scale, a perfectly ordered crystalline quartz lattice and the sudden compression felt by the constituent atoms within a matter of microseconds. The creation of lattice dislocations, electronic defects, shock lamellae, amorphization, and a myriad of other features characteristic of shock metamorphism, 65 million years later, is providing scientists with critical clues for unraveling the mysteries of the now famous Chicxulub impact structure.

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

Emplacement of Cretaceous-Tertiary Boundary Shocked Quartz from Chicxulub Crater

Observations on shocked quartz in Cretaceous-Tertiary (K-T) boundary sediments compellingly tied to Chicxulub crater raise three problems. First, in North America shocked quartz occurs above the main K-T ejecta layer. Second, shocked quartz is more abundant west than east of Chicxulub. Third, shocked quartz reached distances requiring initial velocities up to 8 kilometers per second, corresponding to shock pressures that would produce melt, not the moderate-pressure shock lamellae observed. Shock devolatilization and the expansion of carbon dioxide and water from impacted wet carbonate, producing a warm, accelerating fireball after the initial hot fireball of silicate vapor, may explain all three problems.

A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa

The origin of the Vredefort structure in South Africa is still debated. Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains, shatter cones, veins of pseudotachylite and occurrence of coesite and stishovite (high-pressure quartz polymorphs). A number of these characteristics is widely believed to support an impact origin. However, the planar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered as anomalous when compared with those from accepted impact structures. We have investigated by optical and transmission electron microscopy (TEM) the defect microstructures in quartz grains from different lithologies sampled at various places at the Vredefort structure. Whatever the locality, only thin mechanical Brazil twin lamellae in the basal plane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glass lamellae in rhombohedral planes 10−1 n with n = 1, 2, 3, and 4. At the scale of the optical microscope, Brazil twins in (0001) are easily detected in Vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them. Similar alignments of tiny fluid inclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their traces. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, but stishovite was not found, even in areas where its occurrence was previously reported. Finally, definite evidence of high-temperature annealing was observed in all the samples. These observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-shock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.

Impact Origin of the Avak Structure, Arctic Alaska, and Genesis of the Barrow Gas Fields (1)

Geophysical and subsurface geologic data suggest that the Avak structure, which underlies the Arctic Coastal Plain 12 km southeast of Barrow, Alaska, is a hypervelocity meteorite or comet impact structure. The structure is a roughly circular area of uplifted, chaotically deformed Upper Triassic to Lower Cretaceous sedimentary rocks 8 km in diameter that is bounded by a ring of anastomosing, inwardly dipping, listric normal faults 12 km in diameter. A zone of gently outward-dipping sedimentary country rocks forms a discontinuous ring of within the peripheral ring of normal faults. Beyond these anticlines, the sedimentary rocks are almost flat-lying. Basement consists of strongly deformed Ordovician and Silurian argillite. Density and acoustic impedance con rasts between the argillite and the overlying strata produce gravity and seismic-reflection signatures that define a ring of anticlines around the disturbed zone and a structural high surrounded by an annular structural low at its center. In the adjacent Barrow gas fields, the tops of the informally named Neocomian unit and the gas-producing Lower Jurassic Barrow sand (local usage) lie at average subsea depths of 488 m and 670 m, respectively. In the Avak 1 well, drilled on the central high, the pebble shale and the Barrow sand lie near the surface, documenting more than 500 m of relative uplift at the high. The cores in this well have steep dips (30-90 degrees), mixed breccia with Franklinian argillite clasts 10 and 90 m above basement, quartz grains with shock mosaicism and multiple sets of shock lamellae, oriented concussion fractures in sand-size quartz grains, and shatter cones resembling those found in the peripheral zones of well-documented impact structures. In addition, above-background levels o fractured quartz grains in Barrow sand were found as far as 19 km beyond the rim of the Avak structure. Data concerning the age of the Avak structure are not definitive. If submarine landslide deposits in the upper part of the Aptian and Albian Torok Formation, in the subsurface 200 km to the east, were triggered by the Avak event, then the Avak meteorite struck a submerged marine shelf about 100 + or - 5 Ma. However, the impact features found at Avak (shatter cones, concussion fractures, shock lamellae and shock mosaicism in quartz grains, and widespread cataclasis) characterize the distal zones of meteorite impact structures. Fused rocks, plastic deformation, and shock-metamorphic minerals found in more proximal zones of impact structures are apparently missing. These observations, and the lack of Avak ejecta in cuttings and cores from the Torok Formation and Nanushuk Group (Albian to middle Cenomanian) in surrounding test wells, indicate that the impact event postdated these beds. In this case, the Avak meteorite struck a Late Cretaceous or Tertiary marine shelf or coastal plain between the Cenomanian (ca. 95 Ma), and deposition of the basal beds of the overlying late Pliocene and Quaternary Gubik Formation (ca. 3 Ma).

25 years of the impact‐volcanic controversy: Is there anything new under the Sun or inside the Earth?

An interesting historical perspective on the current controversy about the cause of the Cretaceous‐Tertiary (K‐T) extinction has been provided by more than 2 decades of debate on an earlier but equivalent question: Are the circular, highly deformed cryptoexplosion structures, and the shock‐metamorphic features they contain, astroblemes formed by meteorite impact, or are they formed by internal volcanic processes (cryptovolcanism)? During the last 25 years of this debate, numerous studies have reinforced the view that shock‐metamorphic features uniquely indicate shock pressures of from about 5 GPa to &gt; 50 GPa; no generally accepted shock‐metamorphic features have been found in unquestioned volcanic structures; and no internal process has been demonstrated to produce the required shock pressures. This history therefore supports an impact origin for both cryptoexplosion structures and the K‐T event. Future discoveries required to make the cryptovolcanic theory a respectable alternative include undisputed shock features, for example, shatter cones, shock lamellae, stishovite, lechatelierite, in a volcanic structure; high Ir contents (&gt;1 ppb) in volcanic ejecta layers; and a rigorous and testable theory for producing high shock pressures by internal processes.

Absence of shock-metamorphic effects in the Bushveld Complex, South Africa: results of an intensive search

An extensive search was made for distinctive shock-metamorphic effects in order to test the theory that the Bushveld Complex (BC) was formed by the impact of large extraterrestrial objects. Field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock-metamorphic effects as shatter cones, unusual fragmental or melt breccias, and shock lamellae in quartz. These are: (1) pre-BC quartzite xenoliths in the Rooiberg Group, a thick sequence of acid lavas (rhyolites and rhyodacites) and related rocks, proposed by earlier workers as a possible crater-filling impact melt; and (2) the locally observable contact between the Rooiberg Group and the underlying pre-BC basement (Pretoria Group), a contact possibly corresponding to the original crater floor in established impact structures. More than 320 samples of pre-BC quartzite were collected and examined petrographically for shock-metamorphic effects. No megascopic shock-metamorphic effects (shatter cones or unusual breccias) were observed in any outcrop. In thin section, no petrographic shock effects (e.g., shock lamellae hi quartz and kink bands in biotite) were observed in any sample. The quartzites from both the basement (80 samples) and the Rooiberg xenoliths (245 samples) are virtually undeformed. Cataclastic textures are rare, and individual quartz grains show only normal metamorphic deformation lamellae (Böhm lamellae) and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures. Although some of these parallel sets superficially resemble so-called “decorated” shock lamellae, universal stage measurements of their orientation demonstrate that they are totally different. The absence of both megascopic and microscopic shock-metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the BC are not correct and that the BC is not an impact structure. This conclusion also agrees with: (1)the complex stratigraphy now recognized in the lower Rooiberg Group, which is not consistent with an impact melt origin, (2) the absence in the Rooiberg Group of any of the distinctive melt-rich fragmentai breccias which dominate the crater-filling deposits in established large impact structures, and (3) the undeformed character of the basement rocks below the Rooiberg Group and the apparently concordant contact between them.

Dynamic deformation of quartz and feldspar: clues to causes of some natural crises

Indications of natural catastrophes and crises are preserved in some geologic strutures and in the stratigraphic record, emphasizing the dynamic nature of the Earth's evolution. The crises may result either from exogenic or endogenic energy sources. Distinguishing between these sources is a complex, multidisciplinary problem in the Earth sciences. Shock deformation by impact has been well-studied both in shock recovery experiments and in known impactites, but only recently has the possibility of generation of shock waves by high-temperature internal explosions been seriously considered. An optically based reconaissance survey of microstructures in quartz and feldspar in some explosive silicic volcanics suggests that they may have been induced by dynamic deformation at high temperature; in some respects the character of the microstructures differs greatly from those induced by room temperature shock, whereas in others the differences are subtle. Results from recent shock recovery experiments on quartzite preheated to 440 ° and shocked to 28 GPa support this contention. For two classic Cretaceous/Tertiary sections at Gubbio, Italy, it is shown that shock mosaicism and shock lamellae occur over a 4 m interval, bisected by the boundary, in a pattern approximately correlatable with an independently established iridium enrichment. These data are incompatible with any single event and are most plausibly interpreted as reflecting a period (∼ 0.5 my) of intense, probably episodic, volcanism with a culmination at the K/T transition. Multiple dynamic events may also account for macroscopic and microscopic structures developed during the evolution of the Vredefort structure, South Africa.

Shock Metamorphism of Deformed Quartz

Deformed, synthetic quartz containing a dislocation density of 2.9 ± 1.9 × 108/cm2 and abundant bubbles and small inclusions was shocked to peak pressures of 12 and 24 GPa. The resultant material was inhomogeneously deformed and extremely fractured. The 12 GPa sample contained large regions lacking apparent shock deformation, suggesting that the original microstructure of a quartz target may be distinguished in low-stress shocks with minimal annealing. No change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered. Generally the same types of microstructures were induced by shock of deformed quartz as by shock of relatively defect-free as-grown crystals. Glass-filled veins were abundant, especially at lower stresses, and contained angular fragments of quartz welded together. Microfaults formed on $${{\{ 10\bar 11\} } \mathord{\left/ {\vphantom {{\{ 10\bar 11\} } {\{ 01\bar 11\} }}} \right. \kern-\nulldelimiterspace} {\{ 01\bar 11\} }}$$ and $${{\{ 11\bar 22\} } \mathord{\left/ {\vphantom {{\{ 11\bar 22\} } {\{ 1\bar 212\} }}} \right. \kern-\nulldelimiterspace} {\{ 1\bar 212\} }}$$ , inclined close to 45° to the shock propagation direction. Curviplanar features occurred in groups, with contrast indicating Moire patterns, twins, and stacking faults or related structures; most were interpreted as fractures, possibly welded together with glass. Regions containing shock lamellae sets were present. Lamellae sets were uncommon at 12 GPa, but distributed every few microns at 24 GPa. Lamellae occurred in a spectrum of habits ranging from 35–1500 A in thickness, from 35 A upward in spacing, and from closely-packed parallel sets to networks of diverse orientations; some lamellae were not parallel-sided, but wedge-shaped with basal and $${{\{ 10\bar 13\} } \mathord{\left/ {\vphantom {{\{ 10\bar 13\} } {\{ 01\bar 13\} }}} \right. \kern-\nulldelimiterspace} {\{ 01\bar 13\} }}$$ edges, Thick lamellae were connected to glassy veins, and the wedge-shaped type generally narrowed away from veins; they also subdivided and merged along their length. Lamellae were dominantly basal at 12 GPa, and at 24 GPa on $${{\{ 10\bar 12\} } \mathord{\left/ {\vphantom {{\{ 10\bar 12\} } {\{ 01\bar 12\} }}} \right. \kern-\nulldelimiterspace} {\{ 01\bar 12\} }}$$ , with poles normal to the shock direction. We propose that they are not shear features, but rather glass-filled tensile fractures. Vitrification was widespread, especially at 24 GPa, apparently more so than in shock of as-grown material. This suggests that index of refraction is not an appropriate shock paleopiezometer, as it depends on the defect structure of the starting material. Neither lamella width nor spacing was correlated with shock stress; however, the criterion of multiple glass lamellae sets as indications of shock deformation and its intensity are consistent with our measurements. Dislocation density was lowered in lamellae-containing and glassy areas, possibly removed by nucleation of and absorption by lamellae. No high-pressure phases were observed. Based on the complete set of observed features, it appears that shock deformation in quartz is primarily brittle-melt deformation, with an important role played by hot, fluid silica.

Studies on the lattice distortion and substructures of shocked lamellae in naturally shocked quartz

One unshocked and 9 naturally shocked single quartz crystal grains with 1–6 sets of shock lamellae from the Ries, West Germany, and the Lake Lappajarvi, Finland, covering a range from unshocked quartz withNo = 1.544 to nearly completely glassy quartz withNo = 1.461 have been used for X-ray precession and Laue investigations. Four of the shocked grains have preliminarily been studied under a transmission electron microscope. It is found that quartz havingNo less than 1.539 shows intensive anisotropic cell expansion and lattice disordering which gradually increase asNo decreases. Shock-induced lattice distortion of quartz is clearly shown on both precession and Laue photographs. For the weakly shocked quartz (p < 200 kb) slight to pronounced spreading of spots is observed. When the pressure reaches 200 kb, both concentric spreading of spots having long ‘tails’ and concentric rings (powder pattern) are revealed on the same photograph, which means that besides a part of single crystal there also exist randomly oriented tiny ‘fragments’ of quartz in this shocked quartz grain. As pressure increases from 230 to 315 kb, more and more crystalline puases in the quartz grains have transformed from solid state into silica glass, and the concentric rings and the long ‘tails’ disappear and the spot spreading becomes slight again, but reflection intensities become much lower in comparison with those of weakly shocked quartz.

Impact Ejecta Horizon Within Late Precambrian Shales, Adelaide Geosyncline, South Australia

A solitary layer of shattered crustal rock fragments has been traced over a distance of 260 kilometers within folded 600-million-year-old Precambrian marine shales of the Adelaide Geosyncline, South Australia. The fragments consist entirely of acid to intermediate volcanics (approximately 1575 million years old) displaying shattered mineral grains, shock lamellae in quartz, and small shatter cones. Fragments reach 30 centimeters in diameter and show evidence of vertical fall emplacement. Available evidence points to derivation of the rock fragments from a distant hypervelocity impact into the Gawler Range Volcanics at Lake Acraman, approximately 300 kilometers west of the Adelaide Geosyncline.

Dynamic deformation of volcanic ejecta from the Toba caldera: Possible relevance to Cretaceous/Tertiary boundary phenomena

Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa. Strong dynamic deformation has resulted in intense kinking in biotite and, with increasing shock intensity, the development of plagioclase of planar features, shock mosaicism, incipient recrystallization, and possible partial melting. Microstructures in quartz indicative of strong shock deformation are rare, however, and many shock lamellae, if formed, may have healed during post-shock residence in the hot ignimbrite; they might be preserved in ash falls. Peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures, possibly including shock features observed near the Cretaceous/Tertiary boundary. 38 references, 3 figures.

The variation range of optical constants and distribution characteristics of shock lamellae in shock-metamorphosed quartz

Two moderately shocked rock samples collected from the Ries Crater, West Germany (granite—gneiss sample RC-647-29 and biotite-granite sample RP-627-55) and two weakly shocked pegmatite samples (Lj-711-12 and Lj-711-5) taken from Lake Lappajarvi, Finland, have been optically studied to establish the variation range of optical constants and distribution characteristics of shock lamellae in shocked quartz.It has been found that sample RC-647-29 contains shocked quartz grains with the average refractive index ranging from 1.4612 to 1.5331, and sample RP-627-55 from 1.5002 to 1.4669, i.e., they cover a wide range of shock pressures. As for the larger quartz grains in samples Lj-711-12 and Lj-711-5, the variation range of the average refractive indices are smaller than those of samples from the Ries Crater. Hence the estimation of degree of shock must est with the investigation of a set of representative shocked quartz crystals from a single shocked rock sample.The optical data on shocked quartz indicate that the degree of shock is highly independent of the number of shock lamellae sets and their orientations; the most sensitive optical indicator is the index of refraction.On the basis of TEM investigations of single crystal grains of shocked quartz differing in refractive index, three mechanisms of formation of shock lamellae have been established: host quartz crystals with lamellae having closely spaced dislocations; host quartz crystals with lamellae of randomly oriented fine grains of quartz; and host quartz crystals or their residual fragments with lamellae of silica glass.

ACHONDRITE ALHA77005: ALTERATION OF CHROMITE AND OLIVINE

The olivine crystals of the 77005 achondrite are brown except for colorless shock lamellae, mottled patches, and grains adjacent to pools of impact melt. Sporadic dark alteration patches in brown olivine and Cr‐rich spinel gave the following average electron‐microprobe analyses: (olivine) P2O5 0.9, SiO2 57.9, TiO2 0, Al2O3 0.7, Cr2O3 0.4, V2O3 0, Fe2O3 (assumed oxidation state) 17.0, MgO 1.6, CaO 0.2, Na2O 0, K2O 1.8, SO3 (assumed oxidation state) 9.2, Cl 0.1, sum 89.8 wt. %; (spinel) P2O53.5, SiO22.1, TiO2.2.2, Al2O32.1, Cr2O3 13.4, V2O3 0.8, Fe2O3 40.7, MgO 0.9, CaO 0.1, Na2O 0, K2O 2.0, SO3 11.1, Cl 0.1, sum 79.0 wt.%. Ion‐microprobe analyses revealed H in both. Rare orange patches in brown olivine from another area gave SiO2 33–35, FeO 30‐28, MgO 28–32, sum 93 wt. %. Thermal metamorphism under dry oxidizing conditions is discussed as a possible alternative to shock‐induced oxidation for generation of the brown olivine (McSween and Stöffler). Because alteration patches transgress shock lamellae, and because sulfatic alteration occurs in fusion crusts of Antarctic meteorites (Gibson et al., 1983), alteration of the 77005 achondrite at the Antarctic surface is preferred to a complex series of processes needed for pre‐terrestrial alteration.

Deformation in laboratory-shocked quartz

Single crystal quartz, shocked in the laboratory to 150–260 kbar pressures, was examined using optical, X-ray, electron microscope, and cathodoluminescence techniques. Results indicate that long range crystal order is maintained for pressures up to 150 kbar, and possibly as high as 260 kbar. Deformation occurred by production of glass intermingled in the crystal lattice, and by formation of planar elements (microfault zones, glassy plates, fractures, and shock lamellae) apparently created during initial loading. Microfault zones were formed in shear, fractures and lamellae in tension, and glassy plates in an intermediate stress regime. No dislocations attributable to shock were observed. Vitrification occurred as a sharp phase transition, with no transitional lattice distortion. Quartz shocked to 150 and 260 kbar was similar in appearance, and could not be definitely distinguished.