Shock Deformation of Quartz from Two Meteorite Craters

T. E. BUNCH ALVIN J. COHEN

SHOCK DEFORMATION OF QUARTZ FROM TWO METEORITE CRATERS

Abstract: Quartz, in association with coesite and fracturing under shock loading and is similar to silica glass from two meteorite-impact sites, ex- ruptures developed in quartz by static and shock- hibits prominent cleavage. Individual quartz grains loading experiments. The degree of cleavage de- have a shattered appearance and are largely co- velopment and the texture of the shattered quartz herent with little or no rotation of cleaved seg- is different from quartz deformed by other geo- ments. Cleavage was probably induced by brittle logic processes.

during the propagation of a crack, it is a ductile Introduction fracture. If the fracture is characterized by lack During a petrographic investigation of rock of gross plastic deformation, it is termed a materials from meteorite craters it was found brittle fracture. Parting in crystals is separation that quartz displayed certain textures distinct along planes that are not true cleavage planes. from quartz of other geologic environments. Cleavage is the fracturing of a mineral body Quartz in association with coesite and silica along crystallographic planes, usually of low glass from two meteorite craters, Meteor bond density, so that the observable break is a Crater, Arizona, and Wabar Crater, Saudi smooth planar surface. Cleavage is always con- Arabia, shows well-developed cleavage. Milton sistent with the crystal symmetry and takes and others (1962, Astrogeologic Studies Semi- place only parallel to atomic planes. Although annual Prog. Rept.) described regular frac- cleavage does not normally occur in quartz, a tures, termed either parting or cleavage, in number of occurrences of cleavage in quartz deformed rocks from the Scooter high-explo- have been reported. sive cratering experiment. Englund and Roen Fairbairn (1939, p. 359-364) calculated the (1962) reported intensely deformed quartz that crystallographic planes in quartz that involved displayed parallel fracture patterns from the the least breaking of Si-O bonds. From these Middlesboro Basin, Kentucky, cryptoexplosion calculations he snowed qualitatively that the structure. This discussion illustrates the ex- best cleavage directions in order of decreasing tensive development of cleavage in quartz. facility should be parallel to r {1011}, z {01 Tl}, It is difficult to find a widely accepted usage wflOTOj, T{0001}, a{\m], *{1121}, and of the terms fracture, parting, and cleavage. *{5151}. Bloss and Gibbs (1963, p. 835-836) In describing breaks in a solid body that has determined the number of Si-O bonds which been under an unknown stress system (i.e. de- would have to be broken per unit area for sets formation by natural processes, not by experimental devices), it is of particular interest of rational planes in the (0001) : (10TO) and to determine if these breaks are controlled by (0001) : (1120) zones. Of the first set of planes the crystal structure or if they occur at random. rflOTlj cuts the fewest bonds; in the second In a geological sense, the three above terms can set £{1122} cuts the fewest bonds, with r be used to describe a break in a mineral that cutting fewer than £. has undergone stress. In the present work, ruptures in quartz that The following is an account of the way these have resulted from shock deformation and terms will be used in this paper. Fracture or which parallel reported cleavage directions in rupture is the separation or fragmentation of a quartz are, in order of decreasing abundance, solid body into two or more parts under the r or z, c, m or a, and £. We refer to these action of stress. If fracture occurs with ap- crystallographically controlled ruptures as preciable plastic deformation prior to and cleavages.

Geological Society of America Bulletin, v. 75, p. 1263-1266, 2 pis., December 1964 1263

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The nomenclature of quartz crystallographic and nonglassy in hand specimen. The former is planes used in this paper is: extremely friable and has a sintered appearance. r—the positive rhombohedron {1011} It is composed predominantly of quartz with g—the negative rhombohedron {01T1} minor amounts of silica glass, iron oxides, and m—the unit prism {10TO} coesite. The association of silica glass, coesite, a—the second order prism {1120} and quartz is shown in Figure 1 of Plate 1. c—the basal pinacoid {0001} The slightly deformed sandstone from the rim £—the second order trigonal pyramid {1122} of the crater is grayish white and loosely s—the second order trigonal dipyramid cemented and is composed of quartz, iron {1121} oxides, and a trace of plagioclase. x—the trigonal trapezohedron {5151} Quartz grains in thin sections of the quartz- Thin sections from four different specimens glass-coesite material are highly fractured with were studied on a 5-axis universal stage with a little development of cataclastic texture (PI. 1, fig. 2). A total of 1758 observations of planar polarizing microscope. The orientation of the fractures were made on grains selected at cleavage planes and the c axis of each grain random. Of these observations, 685 are cleavage were plotted on a stereonet and the angles planes that are parallel to r or 2 and are the most between the c axis and the poles of the cleavage frequent in abundance. Other cleavage planes planes were then measured and used to de- observed in order of decreasing abundance are termine crystallographic directions. c (510), £ (215), m or a (156), and s (55). Unde- This method cannot distinguish between termined planar fractures total 137. Cleavage correlative forms such as r{ 10T1} and z{01Tl} in individual quartz grains can be seen in Fig- or mjlOTO} and a{1120}. Thus, the symbol r ures 1, 2, and 3 of Plate 2. Under crossed nicols is used to indicate the specific form and the extinction is extremely mottled. The extinc- correlative forms. tion pattern is unlike any observed by the writers in quartz from any other location, with Meteor Crater Quartz the exception of Wabar Crater. It is probably Meteor Crater, Arizona, is 183 m deep and the result of microfracturing or slight plastic 1220 m in diameter and is encircled by a rim flow. Curved fractures are structurally con- that rises 30-60 m above the surrounding trolled combinations of several cleavage planes. terrain. The exposed rocks in the crater range In Figure 3 of Plate 2, the dominant cleavages from Permian Coconino Sandstone to Triassic in the grain illustrated are r and c. The long Moenkopi Formation in age. Beneath the curved fractures are combinations of r and c raised rim lies a complex sequence of Quater- cleavages, predominantly, which form contin- nary alluvium that unconformably overlies uous curved fractures. Moenkopi and Kaibab strata. The alluvium is Quartz is slightly fractured and the extinc- composed, in part, of material from underlying tion pattern is somewhat mottled in the un- strata, meteorite fragments, and "fused rock" damaged sandstone, but it is comparable to (Shoemaker, 1960, p. 420-421). Rogers (1930) tectonically deformed quartz. In comparison described a "unique" occurrence of lechate- some of the cleaved quartz in the deformed lierite (silica glass) in the Meteor Crater specimens has experienced a small change in sandstone. Chao and others (1960, p. 220-222) shape. A few grains are elongated in the direc- reported the first natural occurrence of the tion of the c axis and show cleavage parallel to high-pressure SiC>2 polymorph, coesite, from the basal plane. Other changes in grain shape the sheared, partly fused, Coconino Sandstone are caused by fragmentation around the grain of Meteor Crater. Because the existence of exterior and by internal plastic flow after coesite indicates pressures in excess of 20 kb, fracturing. In general the change in grain shapes they suggested that the presence of coesite has not been extensive, nor is it a predominant affords a criterion for the recognition of impact feature in the majority of the grains studied. craters. Chao and others (1962, p. 419-421) The subhedral quartz in Figure 1 of Plate 1 still discovered the very high-pressure tetragonal retains several well-developed crystal faces. The SiC>2 polymorph, stishovite, in coesite-bearing behavior of the cleaved quartz during deforma- rocks from Meteor Crater. tion appears to have remained elastic until For the present investigation thin sections rupture occurred. In thin sections no evidence were made of the fractured, glassy, coesite- of shear faults have been observed, nor is there bearing Coconino Sandstone and also of Coco- any apparent recrystallization of quartz. nino Sandstone that appeared to be unfractured Silica glass occurs interstitially between

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 Figure 1. Photomicrograph of a quartz grain showing crystal faces r, m, and c with cleavages that are parallel to these faces. Coesite, iron oxide, and silica glass are indicated at the periphery of the grain. C axis direction (arrow) and cleavages indicated in the photograph. Plane-polarized light, X 80

Figure 2. Photomicrograph (plane-polarized light) of partially fused, shattered Coconino Sandstone. Dark intergranular areas are iron oxides and amorphous material, X 40

QUARTZ FROM COCONINO SANDSTONE, METEOR CRATER, ARIZONA BUNCH AND COHEN, PLATE 1 Geological Society of America Bulletin, volume 75 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 Figure 1. Photomicrograph (crossed nicols) of Coconino Sandstone quartz grain exhibiting r, r', m, and c cleavage. C axis direction (arrow) and cleavages indicated in inset, X 150

Figure 2. Photomicrograph (crossed nicols) of Wabar sandstone quartz grain exhibiting c, m, and { cleavage, X 150

Figure 3. Photomicrograph (plane-polarized light) of Coconino Sandstone quartz grain showing combination of r and c cleavages which combine to appear as curved fractures, X 130

QUARTZ FROM COCONINO SANDSTONE, METEOR CRATER, ARIZONA, AND WABAR SANDSTONE, WABAR CRATERS, SAUDI ARABIA BUNCH AND COHEN, PLATE 2 Geological Society of America Bulletin, volume 75 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/75/12/1263/3442551/i0016-7606-75-12-1263.pdf by guest on 01 October 2021 SHORT NOTES 1265

quartz grains. A few tongues of silica glass tically. In a simplified and idealized situation extend into disrupted quartz. The index of where a brittle solid is deformed under the refraction for the silica glass, as determined effects of shock waves the material is first com- by oil immersion methods, is 1.468 + 0.002. pressed to a higher density and then pulled The silica glass is presumably formed from apart. Chemical-explosion experiments in rock quartz by shock-induced "melting" (De Carli show that the fracturing is the result of rock and Jamieson, 1959, p. 1675). The glass may being pulled apart by tension waves (Duvall have originated as a direct solid state trans- and Atchison, 1957, p. 1). It is well known that formation without actual melting. fracture occurs under rapid rates of loading. As an example, hot, semifluid glass, which normally Wabar Craters Quartz deforms plastically, may be broken by brittle The Wabar Craters near Al Hadida in east- fracturing if it receives a sudden impact. central Arabia were discovered by Philby Deformation of quartz specimens from (1933). The largest crater, approximately 92 m Meteor and Wabar craters may be attributed in diameter and 12 m deep, occurs in a sand- to brittle fracturing along crystallographic stone of undetermined age. Glassy and cindery planes and slight plastic flow after fracture. masses of material are strewn over the surface, The fracturing of rock material by tension and the walls of the crater appear to be built waves would favor extensive cleavage develop- entirely of this material. Many fragments of ment in quartz. Cleavage is a tensile fracture meteoritic iron were found around the periph- that occurs normal to directions of minimum ery of the crater. Spencer (1933) described cohesion in a crystal. Initiation of a fracture various glasses, including lechatelierite, and along a cleavage direction probably originates shattered sandstone. at some pre-existing weakness or microcrack Wabar sandstone from the crater area is where there is little or no cohesive strength, almost identical in mineral composition and and it is then pulled apart by tensile stresses. textural appearance to the Coconino Sandstone The possibility of cleavage development by described previously. The sandstone is com- crack propagation and extension is illustrated posed almost entirely of quartz with interstitial in Figure 1 of Plate 1. In this case crack exten- silica glass, iron oxides, and extremely small sion (cleavage) has taken place along particular grains of coesite. crystallographic planes; r, m, and c. Cleavages The quartz grains are anhedral to subhedral along the rhombohedral crystallographic planes and are intensely shattered. Well-developed have been propagated from southeast to north- cleavages, r, m, and c, occur in practically all west in the photograph and terminate in the the grains, with subordinate cleavage on £ upper portion of the crystal. and possibly s appearing in a few grains. Most Christie and others (1964), by experimental of the ruptures are cleavages; a few irrational deformation of quartz, attempted to produce fractures occur in most grains. Under crossed plastic deformation by applying confining nicols the quartz grains exhibit mosaic extinc- pressures equivalent to those at depths of tion that may be attributed to rotation of 100 km in the earth's crust. Their specimens cleaved segments and/or slight plastic flow. failed by rupture along "faults" that were One grain is faulted parallel to r with offset of crystallographically oriented, and they suggest the intersected cleavage planes and slight that the faulting was initiated by small amounts plastic distortion of the material immediately of slip on planes parallel to c, r and z, and m bordering the fault surface. and a. The experiments demonstrated that extensive plastic deformation of quartz does Discussion not occur under these conditions of high con- Meteor and Wabar craters were created by fining pressure (27 to 30 kb) and room tem- hypervelocity impact of large iron meteorites perature. Christie and others further suggest with the earth's surface. Stress resulting from that the resultant textures might be similar this impact moved through the surrounding to deformed quartz at meteorite-impact sites. rocks in the form of a shock wave. Studies In recent experiments at high pressures of shock effects from experimental deformation (15-20 kb) and moderate temperatures (400°- of solids under impulsive loading (Duvall, 800° C) J. M. Christie and co-workers have 1963, p. 52) show that, at some sufficiently high produced fractures that developed parallel value of shock pressure, atomic bonds in a to deformation lamellae. The majority of the solid are not able to sustain the high stresses; lamellae are parallel to c, r, z, £ and £' (Christie, the material either fractures or it yields plas- personal communication, March 1964). Quartz

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from meteorite craters studied in this investiga- The most striking feature of the shocked tion has failed by crystallographically con- quartz is the apparently instantaneous brittle trolled fracture. Cleavage fracture with shear fracturing with very little movement of the faulting and plastic deformation were reported shattered grains. The gross deformation of the from the Holleford Crater, Ontario, Canada quartz is notably different from quartz de- (Bunch and Cohen, 1963). The quartz was formed by other processes; there is a definite described as being similar in textural ap- lack of plastic deformation associated with pearance to experimentally deformed quartz. fracture and extensive movement of grains or Future work involving the use of phase-con- fragments. A significant development of cleav- trast microscopy and the electron microscope, age in quartz may be a helpful criterion in might resolve the question of whether slip or recognizing structures created by meteorite brittle fracture is the mechanism responsible for impact or other shock mechanisms. producing textures of the type discussed here. The present work is a study of a few speci- Acknowledgments mens, and it is probable that quartz from We thank Dr. C. Oftedahl, Dr. J. M. various locations in a meteorite-impact site that Christie, Dr. G. deVries Klein, and Dr. W. H. has sustained shock deformation over a range Baur for critical review of the manuscript, of pressures and temperatures would show the and A. M. Reid for his assistance and helpful full spectrum of deformational behavior from suggestions. Specimens were provided by brittle fracture to plastic flow. A more exten- E. P. Henderson of the U. S. National Museum sive study of quartz-bearing rocks from these and F. T. Lowrey of Pittsburgh, Pa. This work meteorite craters, located as to exact depth and was supported by grants NsG-57-60, Supple- distance from the center of impact, should yield ment 1H52, and NsG-593 from the National a better understanding of the behavior of Aeronautics and Space Administration. quartz under shock deformation.

References Cited Bloss, F. D., and Gibbs, G. V., 1963, Cleavage in quartz: Am. Mineralogist, v. 48, p. 821-838 Bunch, T. E., and Cohen, Alvin J., 1963, Coesite and shocked quartz from Holleford Crater, Ontario, Canada: Science, v. 142, p. 379-381 Chao, E. C. T., Shoemaker, E. M., and Madsen, B. M., 1960, First natural occurrence of coesite: Science, v. 132, p. 220-222 Chao, E. C. T., Fahey, J. J., Littler, J., and Milton, D. J., 1962, Stishovite, SiC>2, a very high pressure new mineral from Meteor Crater, Arizona: Jour. Geophys. Research, v. 67, p. 419-421 Christie, J. M., Heard, H. C., and LaMori, P. N., 1964, Experimental deformation of quartz single crystals at 27 to 30 kilobars confining pressure and 24" C: Am. Jour. Sci., v. 262, p. 26-55 DeCarli, P. S., and Jamieson, J. C., 1959, Formation of an amorphous form of quartz under shock conditions: Jour. Chem. Phys., v. 31, p. 1675-1676 Duvall, G., 1963, Shock waves in solids: Internal. Sci. and Technol., v. 16, p. 45-54 Duvall, W. I., and Atchison, T. C., 1957, Rock breakage by explosives: U. S. Bur. Mines, Rept. Inv. 5356, p. 1-51 Englund, K. J., and Roen, J. B., 1962, Origin of the Middlesboro Basin, Kentucky: U. S. Geol. Survey Prof. Paper 450-E, p. E 20-22 Fairbairn, H. W., 1939, Correlation of quartz deformation with its crystal structure: Am. Mineralogist, v. 24, p. 351-368 Philby, H. St. J. B., 1933, Rub'al Khali: An account of exploration in the Great South Desert of Arabia: Geog. Jour. London, v. 81, p. 1-26 Rogers, A. F., 1930, A unique occurrence of lechatelierite or silica glass: Am. Jour. Sci., v. 19, p. 195-202 Shoemaker, E. M., 1960, Penetration mechanics of high velocity meteorites, illustrated by Meteor Crater, Arizona: 21st Internal. Geol. Cong., Copenhagen, Rept., pt. 18, p. 418-434 Spencer, L. J., 1933, Meteoric iron and silica-glass from the meteorite craters of Henbury and Wabar: Mineral Mag. v. 23, p. 387-404

DEPT. EARTH AND PLANETARY SCIENCES, UNIVERSITY OF PITTSBURGH, PITTSBURGH, PA. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 13, 1964

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